Last post for MICR3350! This semester's been a wild ride!
We've come a long way in diagnosis and treatment of infectious diseases. Diseases that used to be major causes of death and disability are not as bad today, and one (smallpox) has been eradicated entirely (well, unless you count smallpox samples in labs). That being said, there's always room for improvement with regards to diagnosis, treatment, prevention and eradication.
Diagnosis
The quicker we diagnose something, the quicker we can give appropriate treatment. Therefore, we are always looking to find ways to diagnose diseases more rapidly.
Sepsis
Traditional diagnosis of sepsis ("a toxic inflammatory condition arising from the spread of bacteria or bacterial toxins from the focus of infection") relies on culture, which normally takes at least 24-48 hours. Possible rapid diagnostic tests that can be done instead include direct detection of biomarkers and point-of-care testing. Point-of-care testing might be done at a dedicated point-of-care lab that is closer than a central diagnostic lab, or it might be done at the bedside via a range of hand-held devices. For example, a portable "microscope" has been developed that sends light through a blood drop and can detect the interaction of light with blood components. Another possible device is the CD64 biochip which detects CD64 expression on neutrophils. The CD64 biochip can provide results after around 30 minutes.
Tuberculosis
Conventional TB diagnosis requires chest X-ray as well as sputum microscopy and culture. Since mycobacteria take a long time to grow, quicker tests, such as urine and breath tests, are being developed. Urine tests can be done to detect lipoarabinomannan (LAM), a component of the M. tuberculosis cell wall. These tests can give results in 25 minutes. Breath tests can look for metabolic products of M. tuberculosis, such as naphthalene derivatives, benzene and alkanes. Such breath tests can give results in six minutes- a far cry from the weeks required to culture M. tuberculosis!
Treatments
Unfortunately, not much has happened in the way of developing new antibiotics, despite new technologies for drug discovery, such as "iChips." One antibiotic that has been developed using the "iChip" is teixobactin, which was derived from soil microbes. Another relatively recent antibiotic is Baxdela, or delafloxacin, which is a fluoroquinolone.
Prevention
Public health campaigns and other preventative measures such as vaccines have helped to bring down the rate of infectious diseases. Unfortunately, there are many diseases that we don't have vaccines for yet, such as HIV/AIDS. HIV is highly mutable and very variable, making it difficult to produce a vaccine. There is also a lack of natural immunity to HIV that can be exploited with a vaccine. Other production issues for an HIV vaccine include a lack of an animal model, non-antigenicity of heat-killed HIV-1, and safety concerns with a live attenuated vaccine.
Eradication
As you most likely know, smallpox has been declared eradicated. Factors contributing towards smallpox eradication include an effective vaccine, good surveillance and the fact that humans are the only host. (If animals are also hosts, then you have to vaccinate all of the animals too, which can be tricky.) There are current global eradication programs for polio, malaria, Guinea worms and Yaws (caused by T. pallidum pertenue). Wild poliovirus type 2 has already been eradicated worldwide and type 3 hasn't been seen since 2012 (so it may be declared eradicated soon), but type 1 is still circulating in some countries, notably Pakistan, Nigeria and Afghanistan. The Democratic Republic of the Congo and Syria have also experienced outbreaks.
And that's it for MICR3350!! Good luck in the exam!
Showing posts with label MICR3350. Show all posts
Showing posts with label MICR3350. Show all posts
Tuesday, October 31, 2017
TB and HIV
Second last post for MICR3350!
Tuberculosis (TB)
TB has been around for a while (there are 5000-year-old mummies that have evidence of TB infection), but it wasn't always called TB. It used to be called phthisis pulmonalis or consumption, and may have been transmitted between animals and humans. The causative agent, M. tuberculosis, was first identified by Robert Koch, who was awarded the Nobel Prize in Physiology or Medicine for his discovery.
Presentation
M. tuberculosis is an aerobic, non-motile, rod-shaped bacterium. It has a unique cell wall structure, which makes it resistant to most typical stains, such as the Gram stain. The main defining feature of the M. tuberculosis cell wall is the presence of mycolic acids, which resist drying, acids, alcohol and lytic enzymes. M. tuberculosis cell walls also have a unique component called lipoarabinomannan (LAM), which has been investigated as a possible diagnostic sign.
The most common form of tuberculosis is pulmonary tuberculosis. Pulmonary TB is characterised by chronic cough, haemoptysis (coughing up blood), fever, night sweats, loss of appetite and weight loss. It can be diagnosed by the presence of "Ghon foci" in a lung X-ray, as well as granulomatous lesions and caseous necrosis (especially in active infection). Pulmonary TB can persist for months. Extra-pulmonary TB, which is more common in patients who are also HIV+, affects all organs and may cause meningitis and lymph node disease. "Potts Disease" is TB that affects the spine.
TB can be transmitted via respiratory droplet nuclei, which is why most infections are in the lungs. TB can grow within macrophages to evade the immune system, and from there it can spread via the blood to various organs of the body. Around 10% of infections will become symptomatic, but the remaining ~90% remain latent. It is estimated that around 1/3 of the world's population have latent TB. Only people with active TB can transmit the disease, and it is estimated that every active case results in around 10-15 transmissions per year.
Diagnosis
Specimens used for TB diagnosis include sputum and biopsy. Tests done include microscopy and culture, though culture of mycobacteria can take a long time. Therefore, molecular tests such as PCR and special blood tests such as the Quantiferon TB-Gold blood test (which detects cell-mediated immune response) are increasingly being used.
Treatment
Treatment of tuberculosis requires prolonged therapy (6 months or more). The first phase, the induction phase, lasts around 2 months. In this phase, patients are given rifampicin, isoniazid, pyrazinamide and sometimes ethambutol (if the organism is not sensitive to rifampicin and isoniazid). This cocktail of drugs results in a cure rate of over 90%. In the consolidation phase, which lasts for around 4 months, patients are given rifampicin and isoniazid. The consolidation phase may continue for over a year if patients present with meningitis.
Drug resistance
Unfotunately, TB is gaining resistance to many treatments. MDR-TB (multi-drug resistant TB) is resistant to at least rifampicin and isoniazid. XDR-TB (extensively drug resistant TB) is resistant to rifampicin, isoniazid, fluoroquinolones and at least one anti-TB injectable drug. Finally, XXDR-TB is totally drug resistant.
Human Immunodeficiency Virus (HIV)
HIV is a lentivirus of the retroviridae (retrovirus) family. As such, it is an RNA virus that uses reverse transcriptase to convert its genome into DNA and integrate into the host genome.
The two types of HIV are HIV-1 and HIV-2. HIV-1 is the most common cause, and is very similar to a chimpanzee retrovirus called SIVcpz. HIV-2 is much less common as it is less transmissible and less severe, and is largely confined to West Africa. It is similar to a virus in Sooty Mangabey monkeys called SIVsm. The most common forms of HIV transmission are sexual intercourse, intra-uterine, childbirth and sharing needles. Breastfeeding may also pose a risk, but there is still some controversy surrounding this.
As I have stated in other posts, HIV infects many cells, but particularly CD4+ cells, such as T-helper cells. CD4 falls by around 40-80 cells/μL per year, leading to impaired cell-mediated immunity and a higher risk of opportunistic infections. Opportunistic infections mainly appear when CD4+ cell count falls beneath 200 cells/μL, which normally happens around 6-10 years after diagnosis.
HIV is diagnosed primarily through detection of HIV antibodies or HIV p24 antigen via ELISA. These tests are then confirmed by a Western blot assay which has high specificity. The Western Blot used for HIV detects antibodies to all three major HIV gene products: gag, pol and env. Treatment response can be monitored by looking at CD4 count and at HIV RNA PCR. The goal for HIV RNA PCR is for an "undetectable viral load," or less than 40 copies/mL.
There are a range of treatments for HIV. These include NRTIs, NNRTIs, protease inhibitors, integrase inhibitors and entry inhibitors. However, I will not go into these drugs in further detail in this post.
TB and HIV
There is a profound interaction between TB and HIV. Over 25% of TB deaths are in patients who are HIV+, and around 80% of TB infections are co-infected with HIV. TB patients who are HIV+ have a 10% chance of TB reactivation every year, which is very different to HIV- patients who have a less than 5% risk of reactivation in their lifetime. HIV+ patients also have an increased risk of drug-resistant TB. Nevertheless, treatment for TB is the same in HIV+ and HIV- patients.
Tuberculosis (TB)
TB has been around for a while (there are 5000-year-old mummies that have evidence of TB infection), but it wasn't always called TB. It used to be called phthisis pulmonalis or consumption, and may have been transmitted between animals and humans. The causative agent, M. tuberculosis, was first identified by Robert Koch, who was awarded the Nobel Prize in Physiology or Medicine for his discovery.
Presentation
M. tuberculosis is an aerobic, non-motile, rod-shaped bacterium. It has a unique cell wall structure, which makes it resistant to most typical stains, such as the Gram stain. The main defining feature of the M. tuberculosis cell wall is the presence of mycolic acids, which resist drying, acids, alcohol and lytic enzymes. M. tuberculosis cell walls also have a unique component called lipoarabinomannan (LAM), which has been investigated as a possible diagnostic sign.
The most common form of tuberculosis is pulmonary tuberculosis. Pulmonary TB is characterised by chronic cough, haemoptysis (coughing up blood), fever, night sweats, loss of appetite and weight loss. It can be diagnosed by the presence of "Ghon foci" in a lung X-ray, as well as granulomatous lesions and caseous necrosis (especially in active infection). Pulmonary TB can persist for months. Extra-pulmonary TB, which is more common in patients who are also HIV+, affects all organs and may cause meningitis and lymph node disease. "Potts Disease" is TB that affects the spine.
TB can be transmitted via respiratory droplet nuclei, which is why most infections are in the lungs. TB can grow within macrophages to evade the immune system, and from there it can spread via the blood to various organs of the body. Around 10% of infections will become symptomatic, but the remaining ~90% remain latent. It is estimated that around 1/3 of the world's population have latent TB. Only people with active TB can transmit the disease, and it is estimated that every active case results in around 10-15 transmissions per year.
Diagnosis
Specimens used for TB diagnosis include sputum and biopsy. Tests done include microscopy and culture, though culture of mycobacteria can take a long time. Therefore, molecular tests such as PCR and special blood tests such as the Quantiferon TB-Gold blood test (which detects cell-mediated immune response) are increasingly being used.
Treatment
Treatment of tuberculosis requires prolonged therapy (6 months or more). The first phase, the induction phase, lasts around 2 months. In this phase, patients are given rifampicin, isoniazid, pyrazinamide and sometimes ethambutol (if the organism is not sensitive to rifampicin and isoniazid). This cocktail of drugs results in a cure rate of over 90%. In the consolidation phase, which lasts for around 4 months, patients are given rifampicin and isoniazid. The consolidation phase may continue for over a year if patients present with meningitis.
Drug resistance
Unfotunately, TB is gaining resistance to many treatments. MDR-TB (multi-drug resistant TB) is resistant to at least rifampicin and isoniazid. XDR-TB (extensively drug resistant TB) is resistant to rifampicin, isoniazid, fluoroquinolones and at least one anti-TB injectable drug. Finally, XXDR-TB is totally drug resistant.
Human Immunodeficiency Virus (HIV)
HIV is a lentivirus of the retroviridae (retrovirus) family. As such, it is an RNA virus that uses reverse transcriptase to convert its genome into DNA and integrate into the host genome.
The two types of HIV are HIV-1 and HIV-2. HIV-1 is the most common cause, and is very similar to a chimpanzee retrovirus called SIVcpz. HIV-2 is much less common as it is less transmissible and less severe, and is largely confined to West Africa. It is similar to a virus in Sooty Mangabey monkeys called SIVsm. The most common forms of HIV transmission are sexual intercourse, intra-uterine, childbirth and sharing needles. Breastfeeding may also pose a risk, but there is still some controversy surrounding this.
As I have stated in other posts, HIV infects many cells, but particularly CD4+ cells, such as T-helper cells. CD4 falls by around 40-80 cells/μL per year, leading to impaired cell-mediated immunity and a higher risk of opportunistic infections. Opportunistic infections mainly appear when CD4+ cell count falls beneath 200 cells/μL, which normally happens around 6-10 years after diagnosis.
HIV is diagnosed primarily through detection of HIV antibodies or HIV p24 antigen via ELISA. These tests are then confirmed by a Western blot assay which has high specificity. The Western Blot used for HIV detects antibodies to all three major HIV gene products: gag, pol and env. Treatment response can be monitored by looking at CD4 count and at HIV RNA PCR. The goal for HIV RNA PCR is for an "undetectable viral load," or less than 40 copies/mL.
There are a range of treatments for HIV. These include NRTIs, NNRTIs, protease inhibitors, integrase inhibitors and entry inhibitors. However, I will not go into these drugs in further detail in this post.
TB and HIV
There is a profound interaction between TB and HIV. Over 25% of TB deaths are in patients who are HIV+, and around 80% of TB infections are co-infected with HIV. TB patients who are HIV+ have a 10% chance of TB reactivation every year, which is very different to HIV- patients who have a less than 5% risk of reactivation in their lifetime. HIV+ patients also have an increased risk of drug-resistant TB. Nevertheless, treatment for TB is the same in HIV+ and HIV- patients.
Tuesday, October 24, 2017
Infections in the Immunocompromised
Recap the basics of the immune system
I took an entire unit on immunology last year, so you can read all about the immune system here. Otherwise, read on for the tl;dr version.
The main types of immunity are innate and adaptive immunity. Innate immunity includes anatomical barriers such as skin, the complement system, normal flora and cells such as NK cells, monocytes and neutrophils. Adaptive immunity is more specific, but needs to be primed by a previous encounter by that pathogen. Adaptive cells include the antibody-producing B-cells, as well as T-cells. T-cells come in two main flavours: the helper variety (CD4+), which mediate immune responses, and the cytotoxic variety (CD8+), which induce death of cells that have been infected by viruses.
T-helper cells can be further divided into different kinds of T-cells. In this post, we will only be looking at Th1 and Th2 cells. Th1 cells respond mainly to intracellular pathogens and viruses, and are characterised by IFNγ production. They activate macrophages and induce B-cells to make opsonising and complement-fixing antibodies, leading to "cell-mediated immunity." Th2 cells, on the other hand, respond to extracellular antigens. They are characterised by IL-4 release and activate B-cells, which make neutralising antibodies, leading to "humoral immunity."
Describe what happens when a patient’s immune system is compromised
If your immune system is compromised, you get sick more often (duh). The types of pathogens that you are more likely to get depends on the component of your immune system that is defective. If neutrophils are defective, you are more likely to get staphs, streps and some fungi such as Candida, if you have globulin defects, you are more likely to get encapsulated bacteria and Giardia, and if your skin is broken in some way (e.g. IV lines, catheters etc.) you are more likely to get S. aureus, S. pyogenes, Candida, and so on. Some immune defects, like a defect in cell-mediated immunity, predisposes you to infection from a range of different organisms.
Infections in the cancer patient (aka febrile neutropenia)
Neutropenia, a deficiency in neutrophils, may occur as a result of chemotherapy. Neutropenia can be mild (1000-1500 cells/μL), moderate (500-1000 cells/μL) or severe (less than 500 cells/μL). If a patient with severe neutropenia has a temperature of greater than 38.3°C, or a temperature of greater than 38°C for over an hour, they are considered to have neutropenic fever. Risk factors for neutropenic fever include severity and duration of neutropenia, cancer not in remission and mucositis (a side-effect of chemotherapy). Neutropenic fever, particularly neutropenic sepsis, is considered to be a medical emergency as infection can progress very quickly. Prompt empiric antibiotics are usually given as there is roughly 10% mortality per hour delay in antibiotic therapy.
Antibiotic therapy needs to be relatively broad-spectrum as a large proportion of patients (~1/2) will not have a pathogen identified. The most common bacteria in neutropenic fever are Gram-positive organisms such as S. aureus and S. epidermidis, but Gram-negative bacteria used to be more common. The pathogen associated with the highest rate of mortality is Pseudomonas, so antibiotic therapy must include cover for Pseudomonas. A commonly-used antibiotic for this purpose is Pipericillin-Tazobactam (tazocin).
If fever persists despite antibiotic therapy, other causative agents, such as fungi, should be considered. The main candidate is Candida, as risk factors for candidaemia include severe neutropenia, use of broad-spectrum antibiotics and mucocutaneous damage. Other common fungi include moulds such as Aspergillus, Fusarium and Zygomycetes, which are particularly prevalent following bone marrow transplant. These moulds usually cause pulmonary infections, but may also cause CNS or skin infections.
To diagnose the causative agent of neutropenic fever, blood cultures are usually done. Cultures are done from peripheral blood as well as from ports of IV catheters. Sputum, urine and other targeted samples may also be used. Chest X-rays and CT scans might be used to check for other signs of illness.
Neutropenic infection can be prevented in several different ways. Firstly, patients should be placed in a positive pressure room, free of flowers that might carry spores. A modified diet should be given, and prophylactic drugs may also be given if deemed necessary.
Infections in transplant patients
See previous post: Allograft-Transmissible Infections
Infections in splenectomy patients (sepsis)
The spleen is important for a variety of immune functions, including being the site of maturation of IgM memory B-cells, sequestering opsonised encapsulated bacteria, and modulating the effects of cytokines. Hence, removal of the spleen predisposes to a variety of infections, particularly by encapsulated bacteria such as S. pneumoniae, N. meningitidis and H. influenzae. Roughly 5% of splenectomy patients will experience an overwhelming post-splenectomy infection (OPSI) at some point, usually caused by S. pneumoniae. The greatest risk for OPSI is in the 6 months following splenectomy. Most deaths from OPSI occur within the first 24 hours of becoming unwell. OPSI can be treated with vaccination, lifelong antibiotics and extra emergency antibiotics.
The effects of acquired immune deficiency after infection by HIV
Since CD4+ cells are targeted by HIV, HIV severely weakens the immune system. Therefore, HIV/AIDS patients are at risk of many opportunistic infections. As their immune system gets weaker (CD4+ cell count decreases), they become at risk of a greater variety of infections. Here's a quick list of some opportunistic infections seen in HIV/AIDS patients:
I took an entire unit on immunology last year, so you can read all about the immune system here. Otherwise, read on for the tl;dr version.
The main types of immunity are innate and adaptive immunity. Innate immunity includes anatomical barriers such as skin, the complement system, normal flora and cells such as NK cells, monocytes and neutrophils. Adaptive immunity is more specific, but needs to be primed by a previous encounter by that pathogen. Adaptive cells include the antibody-producing B-cells, as well as T-cells. T-cells come in two main flavours: the helper variety (CD4+), which mediate immune responses, and the cytotoxic variety (CD8+), which induce death of cells that have been infected by viruses.
T-helper cells can be further divided into different kinds of T-cells. In this post, we will only be looking at Th1 and Th2 cells. Th1 cells respond mainly to intracellular pathogens and viruses, and are characterised by IFNγ production. They activate macrophages and induce B-cells to make opsonising and complement-fixing antibodies, leading to "cell-mediated immunity." Th2 cells, on the other hand, respond to extracellular antigens. They are characterised by IL-4 release and activate B-cells, which make neutralising antibodies, leading to "humoral immunity."
Describe what happens when a patient’s immune system is compromised
If your immune system is compromised, you get sick more often (duh). The types of pathogens that you are more likely to get depends on the component of your immune system that is defective. If neutrophils are defective, you are more likely to get staphs, streps and some fungi such as Candida, if you have globulin defects, you are more likely to get encapsulated bacteria and Giardia, and if your skin is broken in some way (e.g. IV lines, catheters etc.) you are more likely to get S. aureus, S. pyogenes, Candida, and so on. Some immune defects, like a defect in cell-mediated immunity, predisposes you to infection from a range of different organisms.
Infections in the cancer patient (aka febrile neutropenia)
Neutropenia, a deficiency in neutrophils, may occur as a result of chemotherapy. Neutropenia can be mild (1000-1500 cells/μL), moderate (500-1000 cells/μL) or severe (less than 500 cells/μL). If a patient with severe neutropenia has a temperature of greater than 38.3°C, or a temperature of greater than 38°C for over an hour, they are considered to have neutropenic fever. Risk factors for neutropenic fever include severity and duration of neutropenia, cancer not in remission and mucositis (a side-effect of chemotherapy). Neutropenic fever, particularly neutropenic sepsis, is considered to be a medical emergency as infection can progress very quickly. Prompt empiric antibiotics are usually given as there is roughly 10% mortality per hour delay in antibiotic therapy.
Antibiotic therapy needs to be relatively broad-spectrum as a large proportion of patients (~1/2) will not have a pathogen identified. The most common bacteria in neutropenic fever are Gram-positive organisms such as S. aureus and S. epidermidis, but Gram-negative bacteria used to be more common. The pathogen associated with the highest rate of mortality is Pseudomonas, so antibiotic therapy must include cover for Pseudomonas. A commonly-used antibiotic for this purpose is Pipericillin-Tazobactam (tazocin).
If fever persists despite antibiotic therapy, other causative agents, such as fungi, should be considered. The main candidate is Candida, as risk factors for candidaemia include severe neutropenia, use of broad-spectrum antibiotics and mucocutaneous damage. Other common fungi include moulds such as Aspergillus, Fusarium and Zygomycetes, which are particularly prevalent following bone marrow transplant. These moulds usually cause pulmonary infections, but may also cause CNS or skin infections.
To diagnose the causative agent of neutropenic fever, blood cultures are usually done. Cultures are done from peripheral blood as well as from ports of IV catheters. Sputum, urine and other targeted samples may also be used. Chest X-rays and CT scans might be used to check for other signs of illness.
Neutropenic infection can be prevented in several different ways. Firstly, patients should be placed in a positive pressure room, free of flowers that might carry spores. A modified diet should be given, and prophylactic drugs may also be given if deemed necessary.
Infections in transplant patients
See previous post: Allograft-Transmissible Infections
Infections in splenectomy patients (sepsis)
The spleen is important for a variety of immune functions, including being the site of maturation of IgM memory B-cells, sequestering opsonised encapsulated bacteria, and modulating the effects of cytokines. Hence, removal of the spleen predisposes to a variety of infections, particularly by encapsulated bacteria such as S. pneumoniae, N. meningitidis and H. influenzae. Roughly 5% of splenectomy patients will experience an overwhelming post-splenectomy infection (OPSI) at some point, usually caused by S. pneumoniae. The greatest risk for OPSI is in the 6 months following splenectomy. Most deaths from OPSI occur within the first 24 hours of becoming unwell. OPSI can be treated with vaccination, lifelong antibiotics and extra emergency antibiotics.
The effects of acquired immune deficiency after infection by HIV
Since CD4+ cells are targeted by HIV, HIV severely weakens the immune system. Therefore, HIV/AIDS patients are at risk of many opportunistic infections. As their immune system gets weaker (CD4+ cell count decreases), they become at risk of a greater variety of infections. Here's a quick list of some opportunistic infections seen in HIV/AIDS patients:
- Kaposi's sarcoma: Related to Human Herpes Virus 8 (HHV8). It forms lesions on the skin. If untreated, it affects all organs except for the brain.
- Cytomegalovirus (CMV/HHV5): Many adults are seropositive for CMV (a.k.a. HHV5), but it is only reactivated when CD4 cell counts fall below 100. CMV may manifest as CMV retinitis in HIV/AIDS patients.
- Cryptosporidium parvum: Cryptosporidium is a protozoan that can cause acute and chronic diarrhoea. It can be life-threatening in HIV patients.
- Pneumocystis jirovecii pneumonia: Caused by a fungus. Common opportunistic infection in HIV/AIDS patients.
- Tuberculosis: Many HIV patients are co-infected with TB. TB is becoming more drug-resistant, which is a problem.
- Cryptococcus neoformans meningitis: Cryptococcus is a yeast-like fungus that can cause meningitis when CD4 counts fall below 100.
- Toxoplasma gondii cerebral toxoplasmosis: Toxoplasma is a protozoan spread in cat faeces and meat. It can cause cysts that are dormant in the brain, but are activated when CD4 counts fall below 100.
Emerging Infectious Diseases
The slides for this lecture aren't actually online yet, so I'm just going to make do with my own shitty notes. Hold on tight...
Influenza
Influenza is often seen as a common, relatively mild illness, but it has also been responsible for some major pandemics around the world. The only type of influenza responsible for pandemics is influenza A, as it can skip species and mutate in order to evade immunity from the population as a whole. Migrating birds are common vectors for influenza, as are pigs, as they have receptors for both bird and human influenza. The main types of mutations include reassortment, where packets of nucleic acid are swapped between human and animal viruses, and adaptation, in which enough mutations occur within the virus for it to be unrecognisable by human immune systems. The 1918 Spanish Flu was a result of viral adaptation.
Most influenza deaths are in the young and in the elderly. However, there are some exceptions. In 1918, there were also a lot of deaths in young adults, possibly because people of this age group were exposed to poor conditions when fighting in the war. In the 2009 swine flu pandemic, there was a peak in the 40-60-year-old age group, but the elderly were not too badly affected. It has been speculated that this is because the swine flu was similar enough to some earlier strains circulating between 1918 and 1957 for the elderly to have immunity.
The most likely candidates for future pandemics are H5N1, H7N9 and H3N2. H5N1 is mainly spread by bird migration and transport, whereas H7N9 is associated with contact with songbirds. H7N9 is mostly confined to China. It has evolved in southern China to produce a strain that is more pathogenic in birds, but it remains to be seen exactly how pathogenic this strain is in humans.
Coronaviruses
SARS
SARS is a bat virus that obtained a mutation that increased its human transmission. Bats can pass on SARS to civic cats, which in turn can pass it on to us. Thankfully, SARS is not that great at human transmission, requiring close contact for its spread. Furthermore, it is most infectious during the second week when patients are showing symptoms. As such, SARS can be readily controlled by quarantine and hygiene.
MERS
MERS, or Middle Eastern Respiratory Syndrome, may also originate from bats. Camels are the intermediate host for this virus. As you might expect from the name, most cases of MERS are in the Middle East, particularly in people with close contact with camels.
Exotic arboviruses
The good thing about arboviruses, or insect-borne viruses, is that you need that particular insect in order for the disease to spread. As such, controlling mosquito breeding can be very helpful in stopping the spread of the disease. The bad thing is that we have some of these mosquitoes in Australia: Aedes aegypti is found in northern Queensland, and Aedes albopictus is found in the Torres Strait Islands. Outbreaks of Dengue fever have occurred in northern Queensland due to the presence of Aedes aegypti.
Zika virus
Zika virus is primarily transmitted by Aedes aegypti and Aedes albopictus. Zika used to cycle between apes and these mosquitoes, but now it cycles between humans and mosquitoes. Zika can also be transmitted from human-to-human, either transplacentally, sexually or via blood transfusion. The incubation period of Zika virus is around 2 weeks, after which mild symptoms such as headache, fever and rash usually occur. Even though Zika is normally mild, it can have disastrous neurological and fetal complications. In congenital Zika syndrome, there is microcephaly (small brain due to tissue destruction), retinal damage and musculoskeletal problems.
Chikungunya
Chikungunya fever has symptoms similar to Ross River Virus, such as aches, headaches and fever. It is mainly transmitted by Aedes aegypti, but there have been some mutations that allow it to be transmitted by Aedes albopictus as well.
Nipah virus
Nipah virus originates from bats, but can be passed on to pigs and then to humans. It was originally found in Singapore and Malaysia, but there have also been outbreaks in Bangladesh and India. Symptoms include encephalitis and pneumonia, which are obviously pretty nasty, but as Nipah virus has limited person-to-person transmission, you're likely to be safe if you avoid bats.
Other viruses
Other viruses that were mentioned during the lecture include Lassa virus (from rodents), Ebola and Marburg viruses (from bats) and Congo-Crimean haemorrhagic fever (CCHF). However, the lecturer didn't go into much more detail, so that's pretty much all I'm going to say about them.
Influenza
Influenza is often seen as a common, relatively mild illness, but it has also been responsible for some major pandemics around the world. The only type of influenza responsible for pandemics is influenza A, as it can skip species and mutate in order to evade immunity from the population as a whole. Migrating birds are common vectors for influenza, as are pigs, as they have receptors for both bird and human influenza. The main types of mutations include reassortment, where packets of nucleic acid are swapped between human and animal viruses, and adaptation, in which enough mutations occur within the virus for it to be unrecognisable by human immune systems. The 1918 Spanish Flu was a result of viral adaptation.
Most influenza deaths are in the young and in the elderly. However, there are some exceptions. In 1918, there were also a lot of deaths in young adults, possibly because people of this age group were exposed to poor conditions when fighting in the war. In the 2009 swine flu pandemic, there was a peak in the 40-60-year-old age group, but the elderly were not too badly affected. It has been speculated that this is because the swine flu was similar enough to some earlier strains circulating between 1918 and 1957 for the elderly to have immunity.
The most likely candidates for future pandemics are H5N1, H7N9 and H3N2. H5N1 is mainly spread by bird migration and transport, whereas H7N9 is associated with contact with songbirds. H7N9 is mostly confined to China. It has evolved in southern China to produce a strain that is more pathogenic in birds, but it remains to be seen exactly how pathogenic this strain is in humans.
Coronaviruses
SARS
SARS is a bat virus that obtained a mutation that increased its human transmission. Bats can pass on SARS to civic cats, which in turn can pass it on to us. Thankfully, SARS is not that great at human transmission, requiring close contact for its spread. Furthermore, it is most infectious during the second week when patients are showing symptoms. As such, SARS can be readily controlled by quarantine and hygiene.
MERS
MERS, or Middle Eastern Respiratory Syndrome, may also originate from bats. Camels are the intermediate host for this virus. As you might expect from the name, most cases of MERS are in the Middle East, particularly in people with close contact with camels.
Exotic arboviruses
The good thing about arboviruses, or insect-borne viruses, is that you need that particular insect in order for the disease to spread. As such, controlling mosquito breeding can be very helpful in stopping the spread of the disease. The bad thing is that we have some of these mosquitoes in Australia: Aedes aegypti is found in northern Queensland, and Aedes albopictus is found in the Torres Strait Islands. Outbreaks of Dengue fever have occurred in northern Queensland due to the presence of Aedes aegypti.
Zika virus
Zika virus is primarily transmitted by Aedes aegypti and Aedes albopictus. Zika used to cycle between apes and these mosquitoes, but now it cycles between humans and mosquitoes. Zika can also be transmitted from human-to-human, either transplacentally, sexually or via blood transfusion. The incubation period of Zika virus is around 2 weeks, after which mild symptoms such as headache, fever and rash usually occur. Even though Zika is normally mild, it can have disastrous neurological and fetal complications. In congenital Zika syndrome, there is microcephaly (small brain due to tissue destruction), retinal damage and musculoskeletal problems.
Chikungunya
Chikungunya fever has symptoms similar to Ross River Virus, such as aches, headaches and fever. It is mainly transmitted by Aedes aegypti, but there have been some mutations that allow it to be transmitted by Aedes albopictus as well.
Nipah virus
Nipah virus originates from bats, but can be passed on to pigs and then to humans. It was originally found in Singapore and Malaysia, but there have also been outbreaks in Bangladesh and India. Symptoms include encephalitis and pneumonia, which are obviously pretty nasty, but as Nipah virus has limited person-to-person transmission, you're likely to be safe if you avoid bats.
Other viruses
Other viruses that were mentioned during the lecture include Lassa virus (from rodents), Ebola and Marburg viruses (from bats) and Congo-Crimean haemorrhagic fever (CCHF). However, the lecturer didn't go into much more detail, so that's pretty much all I'm going to say about them.
Thursday, October 19, 2017
Antiparasitic Agents
INFODUMP INCOMING!!!
I haven't written very much about parasites on this blog, but there's an introduction to them here if you need one. This lecture jumped all over the place, but hopefully I'll be able to provide some kind of coherent structure.
Malaria
For more information on malaria, see here.
Malaria can be treated with a range of antimalarials, such as quinine, mefloquine and artemisinin derivatives such as artemether and artesunate. Artemesia annua has been used in Chinese medicine for years, and the active ingredient was eventually isolated, earning Tu Youyou a Nobel Prize. The actual mechanism of action is not quite clear, but it is thought to involve increasing mitochondria-targeting endoperoxides in parasites and altering calcium metabolism. It is a well-tolerated drug and can clear parasites quickly. To prevent parasites from surviving and developing resistance, artemisinin is always given in combination with another antimalarial with a longer half-life.
Enteric protozoa and trichomonas
Enteric protozoa include Entamoeba histolytica, Giardia lamblia and Cryptosporidium, which can cause fun symptoms such as dysentery. Trichomonas, which can cause yellow-green discharge, has been discussed in further detail here. They are discussed together because amoebic dysentery, Giardia and Trichomonas can all be treated with metronidazole or tinidazole. Dientamoeba (another enteric protozoan) can also be treated with metronidazole, as well as doxycycline. Some other enteric protozoa, such as Isospora, Cyclospora and Blastocystis can be treated with co-trimoxazole (another name for trimethoprim/sulfamoxazole), or with pyrimethamine plus ciprofloxacin.
Metronidazole and Tinidazole
Metronidazole and tinidazole, which are classified as nitroimidazoles, are antibacterial and antiprotozoal. They are pretty cheap (metronidazole is 10c per tablet), well-tolerated and can be used in pregnancy (though not when breastfeeding). They can also be applied orally, topically or intravenously. Metronidazole treatment takes longer than tinidazole treatment.
Nitazoxanide
Nitazoxanide is a thiazolide mainly used to treat Cryptosporidia and other opportunistic protozoan infections in AIDS patients. It can also be used against Giardia and some viruses, such as HBV, HCV, rotavirus, influenza A, coronavirus, RSV, adenovirus and HSV1. It interferes with pyruvate ferredoxin oxidoreductase (PFOR).
Paromomycin
Paromomycin is an aminoglycoside that is also able to inhibit protozoan protein synthesis. It is effective against a wide range of protozoa, such as Entamoeba, Giardia, Dientamoeba and Leishmania.
Toxoplasmosis
Toxoplasmosis is caused by Toxoplasma gondii, which is spread in cat faeces and contaminated meat. It can cause cysts that can remain dormant in the brain for a while, eventually leading to cerebral disease such as fever, confusion, headache and seizures. One of the treatments for toxoplasmosis is pyrimethamine, which inhibits dihydrofolate reductase and may have synergistic activity with sulfonamides. It is also effective against malaria. Pyrimethamine can be given with sulphadiazine for effective treatment of toxoplasmosis. HIV patients may need to take pyrimethamine for life.
Leishmaniasis
Leishmaniasis is caused by leishmania, which resides in macrophages and is spread by mosquitoes. The two main forms are visceral and cutaneous leishmaniasis, but there are also mucosal, diffuse and disseminated forms.
Of the two main forms, the cutaneous form is more mild. It can self-heal within 3 months to 3 years, though it normally leaves scars. It can also be treated topically with heat, cryo or laser therapy. Other treatments include intralesional pentavalent antimonials, imiquimod, paromomycin, amphotericin B or oral miltefosine. Visceral leishmaniasis is more severe and generally requires intravenous or intramuscular pentavalent antimonials and/or amphotericin.
Pentavalent antimonials
Pentavalent antimonials, such as sodium stibogluconate and meglumine antimoniate, are first-line treatments for leishmaniasis. They have inactive SbV (Sb is antimony) which is converted to active SbIII. The possible mechanism of action is inhibition of ATP synthetase, but this is still uncertain. Pentavalent antimonials are quite toxic and are linked to cardiotoxicity, elevation of pancreatic enzymes and hepatitis. Furthermore, there is some resistance to these drugs, particularly in India, where resistance to pentavalent antimonials is around 70%.
Miltefosine
Miltefosine was originally an anti-cancer drug, but has been found to have antiprotozoal activity. It is the only oral drug for leishmaniasis. It is mostly effective at treating visceral leishmaniasis, but there is increasing evidence that it may be helpful in cutaneous leishmaniasis as well.
Trypanosomiasis
There are two main kinds of trypanosomiasis: Chagas' disease and African trypanosomiasis. I have already written about African trypanosomiasis here. Chagas' disease, which is spread by the triatomine bug, can cause the Romana's sign or Chagoma acutely (a quick Google search will give you some pictures), or cardiomegaly, megaoesophagus and/or megacolon chronically. Chagas' disease can only really be treated in the acute stage by either nifurtimox or benznidazole- there are no treatments for chronic Chagas' disease.
Nifurtimox
Nifurtimox is an anti-Chagas' drug that causes free radical production. Side-effects include anorexia and neuro-psychological reactions.
Benznidazole
Benznidazole is a nitroimidazole (like metronidazole and tinidazole) that can also be used to treat Chagas' disease. Side-effects include hypersensitivity, oedema and bone marrow suppression.
Pentamidine
Pentamidine, which can be used to treat early-stage Gambiense disease, can be given as a deep IM or slow IV injection. Side-effects include hypotension, shock and renal failure.
Eflornithine
Eflornithine, which was originally developed as an anti-cancer drug, can be used to treat late-stage Gambiense disease. It is given as an IV for 14 days, which can be a bit of a problem, given that many cases of Gambiense disease occur in countries with underdeveloped healthcare systems. Side effects include GI upset, seizures and bone marrow suppression.
Suramin
Suramin is used to treat early-stage Rhodesiense disease. Side-effects include allergic reactions, nausea, vomiting, renal impairment and fever.
Melarsoprol
Melarsoprol is used to treat late-stage Rhodesiense disease. It is an arsenical, which means it comes with some nasty side-effects, such as post-treatment reactive encephalopathy. The fatality rate from this drug is 5-10%, but unfortunately it's the best treatment available for late-stage Rhodesiense disease. (You gotta wonder what the other treatments are like...)
Onchocerciasis (and other types of filariasis)
Onchocerciasis is another name for "river blindness," which is caused by subcutaneous filariasis, as detailed here. Just like with other filarial diseases, treatments involve killing microfilaria, rather than the adult worms. Hence, repeat treatments are often required.
Ivermectin
Ivermectin blocks chloride channels, paralysing microfilaria. Unfortunately, this drug may have oncogenic (cancer-causing) and sterilising effects. Other side-effects include hypotension, headache and fever.
Doxycycline
Doxycycline is a tetracycline that is used to kill Wolbachia. As mentioned here, filarial worms require Wolbachia in order to survive.
Diethylcarbamazine (DEC)
DEC kills microfilaria, as well as some adult worms. It is thought to sensitise microfilaria to phagocytosis by the immune system. DEC also inhibits arachidonic acid metabolism.
Soil-transmitted helminths
Soil-transmitted helminths, such as Ascaris and Enterobius, are mainly treated with benzimidazoles such as albendazole and mebendazole. Benzimidazoles are broad-spectrum anti-helminthics that are usually single-dose as they generally reach their target pretty well with few side-effects.
Albendazole
Albendazole binds to intracellular tubulin, impairing absorptive functions of the worm. Side-effects include transient GI upset, dizziness, itch and dry mouth. Albendazole is also potentially teratogenic, but the benefits usually outweigh the risks.
Mebendazole
Mebendazole also binds to tubulin, preventing microtubule formation, cell division and glucose uptake, which in turn leads to increased glycogen utilisation by the worm. Side-effects of mebendazole are similar to those of albendazole.
Trematodes (flukes)
I've written more about trematodes here. The main treatment is praziquantel, but albendazole, triclabendazole, nitazoxanide and chloroquine may also be used. Praziquantel increases calcium permeability of parasite membranes, but may also result in neuromuscular paralysis (presumably via the same mechanism).
Hydatid disease
Hydatid disease, as described here, is caused by tissue tapeworms. It is characterised by the formation of cysts that may rupture, so surgical intervention may be required. Before surgery, the cyst is sterilised with scolicidal agents, such as benzimidazoles and ethanol.
Taenia species
I've already written about Taenia here. T. saginata and T. solium can both be treated with praziquantel. If cysticercosis occurs (formation of cysts due to ingesting the eggs of Taenia spp.), prolonged courses may be required, as well as high-dose steroids to suppress the "immune storm" that results when worms are killed.
I haven't written very much about parasites on this blog, but there's an introduction to them here if you need one. This lecture jumped all over the place, but hopefully I'll be able to provide some kind of coherent structure.
Malaria
For more information on malaria, see here.
Malaria can be treated with a range of antimalarials, such as quinine, mefloquine and artemisinin derivatives such as artemether and artesunate. Artemesia annua has been used in Chinese medicine for years, and the active ingredient was eventually isolated, earning Tu Youyou a Nobel Prize. The actual mechanism of action is not quite clear, but it is thought to involve increasing mitochondria-targeting endoperoxides in parasites and altering calcium metabolism. It is a well-tolerated drug and can clear parasites quickly. To prevent parasites from surviving and developing resistance, artemisinin is always given in combination with another antimalarial with a longer half-life.
Enteric protozoa and trichomonas
Enteric protozoa include Entamoeba histolytica, Giardia lamblia and Cryptosporidium, which can cause fun symptoms such as dysentery. Trichomonas, which can cause yellow-green discharge, has been discussed in further detail here. They are discussed together because amoebic dysentery, Giardia and Trichomonas can all be treated with metronidazole or tinidazole. Dientamoeba (another enteric protozoan) can also be treated with metronidazole, as well as doxycycline. Some other enteric protozoa, such as Isospora, Cyclospora and Blastocystis can be treated with co-trimoxazole (another name for trimethoprim/sulfamoxazole), or with pyrimethamine plus ciprofloxacin.
Metronidazole and Tinidazole
Metronidazole and tinidazole, which are classified as nitroimidazoles, are antibacterial and antiprotozoal. They are pretty cheap (metronidazole is 10c per tablet), well-tolerated and can be used in pregnancy (though not when breastfeeding). They can also be applied orally, topically or intravenously. Metronidazole treatment takes longer than tinidazole treatment.
Nitazoxanide
Nitazoxanide is a thiazolide mainly used to treat Cryptosporidia and other opportunistic protozoan infections in AIDS patients. It can also be used against Giardia and some viruses, such as HBV, HCV, rotavirus, influenza A, coronavirus, RSV, adenovirus and HSV1. It interferes with pyruvate ferredoxin oxidoreductase (PFOR).
Paromomycin
Paromomycin is an aminoglycoside that is also able to inhibit protozoan protein synthesis. It is effective against a wide range of protozoa, such as Entamoeba, Giardia, Dientamoeba and Leishmania.
Toxoplasmosis
Toxoplasmosis is caused by Toxoplasma gondii, which is spread in cat faeces and contaminated meat. It can cause cysts that can remain dormant in the brain for a while, eventually leading to cerebral disease such as fever, confusion, headache and seizures. One of the treatments for toxoplasmosis is pyrimethamine, which inhibits dihydrofolate reductase and may have synergistic activity with sulfonamides. It is also effective against malaria. Pyrimethamine can be given with sulphadiazine for effective treatment of toxoplasmosis. HIV patients may need to take pyrimethamine for life.
Leishmaniasis
Leishmaniasis is caused by leishmania, which resides in macrophages and is spread by mosquitoes. The two main forms are visceral and cutaneous leishmaniasis, but there are also mucosal, diffuse and disseminated forms.
Of the two main forms, the cutaneous form is more mild. It can self-heal within 3 months to 3 years, though it normally leaves scars. It can also be treated topically with heat, cryo or laser therapy. Other treatments include intralesional pentavalent antimonials, imiquimod, paromomycin, amphotericin B or oral miltefosine. Visceral leishmaniasis is more severe and generally requires intravenous or intramuscular pentavalent antimonials and/or amphotericin.
Pentavalent antimonials
Pentavalent antimonials, such as sodium stibogluconate and meglumine antimoniate, are first-line treatments for leishmaniasis. They have inactive SbV (Sb is antimony) which is converted to active SbIII. The possible mechanism of action is inhibition of ATP synthetase, but this is still uncertain. Pentavalent antimonials are quite toxic and are linked to cardiotoxicity, elevation of pancreatic enzymes and hepatitis. Furthermore, there is some resistance to these drugs, particularly in India, where resistance to pentavalent antimonials is around 70%.
Miltefosine
Miltefosine was originally an anti-cancer drug, but has been found to have antiprotozoal activity. It is the only oral drug for leishmaniasis. It is mostly effective at treating visceral leishmaniasis, but there is increasing evidence that it may be helpful in cutaneous leishmaniasis as well.
Trypanosomiasis
There are two main kinds of trypanosomiasis: Chagas' disease and African trypanosomiasis. I have already written about African trypanosomiasis here. Chagas' disease, which is spread by the triatomine bug, can cause the Romana's sign or Chagoma acutely (a quick Google search will give you some pictures), or cardiomegaly, megaoesophagus and/or megacolon chronically. Chagas' disease can only really be treated in the acute stage by either nifurtimox or benznidazole- there are no treatments for chronic Chagas' disease.
Nifurtimox
Nifurtimox is an anti-Chagas' drug that causes free radical production. Side-effects include anorexia and neuro-psychological reactions.
Benznidazole
Benznidazole is a nitroimidazole (like metronidazole and tinidazole) that can also be used to treat Chagas' disease. Side-effects include hypersensitivity, oedema and bone marrow suppression.
Pentamidine
Pentamidine, which can be used to treat early-stage Gambiense disease, can be given as a deep IM or slow IV injection. Side-effects include hypotension, shock and renal failure.
Eflornithine
Eflornithine, which was originally developed as an anti-cancer drug, can be used to treat late-stage Gambiense disease. It is given as an IV for 14 days, which can be a bit of a problem, given that many cases of Gambiense disease occur in countries with underdeveloped healthcare systems. Side effects include GI upset, seizures and bone marrow suppression.
Suramin
Suramin is used to treat early-stage Rhodesiense disease. Side-effects include allergic reactions, nausea, vomiting, renal impairment and fever.
Melarsoprol
Melarsoprol is used to treat late-stage Rhodesiense disease. It is an arsenical, which means it comes with some nasty side-effects, such as post-treatment reactive encephalopathy. The fatality rate from this drug is 5-10%, but unfortunately it's the best treatment available for late-stage Rhodesiense disease. (You gotta wonder what the other treatments are like...)
Onchocerciasis (and other types of filariasis)
Onchocerciasis is another name for "river blindness," which is caused by subcutaneous filariasis, as detailed here. Just like with other filarial diseases, treatments involve killing microfilaria, rather than the adult worms. Hence, repeat treatments are often required.
Ivermectin
Ivermectin blocks chloride channels, paralysing microfilaria. Unfortunately, this drug may have oncogenic (cancer-causing) and sterilising effects. Other side-effects include hypotension, headache and fever.
Doxycycline
Doxycycline is a tetracycline that is used to kill Wolbachia. As mentioned here, filarial worms require Wolbachia in order to survive.
Diethylcarbamazine (DEC)
DEC kills microfilaria, as well as some adult worms. It is thought to sensitise microfilaria to phagocytosis by the immune system. DEC also inhibits arachidonic acid metabolism.
Soil-transmitted helminths
Soil-transmitted helminths, such as Ascaris and Enterobius, are mainly treated with benzimidazoles such as albendazole and mebendazole. Benzimidazoles are broad-spectrum anti-helminthics that are usually single-dose as they generally reach their target pretty well with few side-effects.
Albendazole
Albendazole binds to intracellular tubulin, impairing absorptive functions of the worm. Side-effects include transient GI upset, dizziness, itch and dry mouth. Albendazole is also potentially teratogenic, but the benefits usually outweigh the risks.
Mebendazole
Mebendazole also binds to tubulin, preventing microtubule formation, cell division and glucose uptake, which in turn leads to increased glycogen utilisation by the worm. Side-effects of mebendazole are similar to those of albendazole.
Trematodes (flukes)
I've written more about trematodes here. The main treatment is praziquantel, but albendazole, triclabendazole, nitazoxanide and chloroquine may also be used. Praziquantel increases calcium permeability of parasite membranes, but may also result in neuromuscular paralysis (presumably via the same mechanism).
Hydatid disease
Hydatid disease, as described here, is caused by tissue tapeworms. It is characterised by the formation of cysts that may rupture, so surgical intervention may be required. Before surgery, the cyst is sterilised with scolicidal agents, such as benzimidazoles and ethanol.
Taenia species
I've already written about Taenia here. T. saginata and T. solium can both be treated with praziquantel. If cysticercosis occurs (formation of cysts due to ingesting the eggs of Taenia spp.), prolonged courses may be required, as well as high-dose steroids to suppress the "immune storm" that results when worms are killed.
Wednesday, October 18, 2017
Infections in Returned Travellers
Fasten your seatbelts, because we're back to blogging about MICR3350!
Most travel infections result from visiting countries in tropical areas. The most common identified GI pathogens in returned travellers are Giardia, Salmonella, Shigella and Campylobacter, the most common febrile conditions include malaria, dengue, typhoid, chikungunya and so on, and the most common respiratory illnesses include flu and legionellosis. Very few people who catch a disease abroad actually die from it, and a large proportion of those deaths are from falciparum malaria (the worst form of malaria).
Diagnosis
Diagnosis can be very difficult, as most of these illnesses have non-specific and overlapping symptoms (like fever, fatigue, nausea, etc.). Therefore, it is important to obtain a thorough history that includes travel history and measures taken to avoid risk (e.g. anti-mosquito measures, pre-travel vaccinations).
Sometimes, you get lucky and find a symptom that is specific to an infection. For example, an eschar, which has a dark area in the middle surrounded by a ring of inflammation, is associated with Rickettsia infections. Chancres are associated with primary syphilis. A "migrating track" under the skin" is associated with cutaneous larva migrans (animal hookworms under the skin). A similar-shaped lesion in the eye might indicate loa loa microfilarial worms that cause loiasis. Myiasis- infection by human botflies- can be quite distinctive too.
Incubation periods can also be used to help narrow down a diagnosis. For example, the incubation period for yellow fever is only 3-6 days. If a patient visited a yellow fever-endemic country more than 6 days ago, it is unlikely that their current infection is due to yellow fever. Other incubation periods that might be helpful to know are malaria (9 days - several months), typhoid (3-60 days), dengue (3-14 days) and typhus (Rickettsia infection not to be confused with typhoid. Incubation period for this one is 6-21 days).
Malaria
Since a large proportion of deaths from travel illnesses are due to malaria, it should always be considered in a returned traveller with fever. Malaria is transmitted by Anopheles mosquitoes, and comes in five flavours: Plasmodium vivax, P. ovale, P. malariae, P. falciparum and P. knowlesi. P. falciparum is the most severe and life-threatening. P. knowlesi is not very common (originated in monkeys), but it can also be severe.
The life cycle of malaria is quite complex. The infective form, called a sporozoite, lives in the salivary gland of mosquitoes. Sporozoites are transmitted to us when mosquitoes bite. Once inside us, sporozoites move through the blood and lymphatics to the liver, where they multiply into haploid forms called merozoites, which gather in lesions called schizonts. At this stage, P. vivax and P. ovale can also produce hypnozoites, which are latent forms. Hence, P. vivax and P. ovale malaria can recur.
Eventually, liver schizonts rupture, releasing merozoites into the bloodstream. Merozoites infect red blood cells and develop into the trophozoite form. Trophozoites sometimes appear as rings under the microscope due to the presence of a large vacuole in the middle. As the trophozoites develop, a mature erythrocytic schizont that contains thousands of merozoites is formed. These erythrocytic schizonts can eventually rupture, releasing merozoites to infect fresh red cells. In untreated malaria, this cycle can cause regular waxing and waning of fever.
Some trophozoites can exit the cycle and develop into male and female gametocytes. Gametocytes can be taken up by mosquitoes. Once in the mosquito's gut, the gametocytes undergo sexual reproduction, producing new sporozoites that live in the mosquito's salivary glands. And so the process continues!
Malaria can present in many different ways. Fever is usually the main symptom, but there are many other nonspecific symptoms, from headache, to abdominal pain, to seizures. In severe cases, it may proceed to pulmonary oedema, acute renal failure, severe anaemia, and so on. Therefore, diagnosis of malaria cannot be made clinically, and diagnostic tests are necessary. These diagnostic tests include a blood film to look for parasites, PCR and antigen detection.
Dengue fever
Dengue is another common cause of fever in returned travellers. It is transmitted by Aedes mosquitoes. Just like malaria, dengue fever can present with very non-specific symptoms, mainly fever. The main symptoms of "classical" dengue are fever, rash, severe muscle and joint pain, headache, retro-orbital pain, and mild haemorrhagic phenomena. If a patient is infected twice, with a different serotype each time, they can get dengue haemorrhagic fever or dengue shock syndrome, which are very serious conditions. In dengue haemorrhagic fever, platelets are diminished, leading to haemorrhagic skin rashes, leaky blood vessels and hypotension. Diagnostic tests for Dengue fever include antibody and antigen (NS1) detection and PCR.
Prevention
Some preventative measures can be taken to reduce the risk of getting a travel illness. These include getting appropriate vaccinations, prophylactic drugs for malaria, mosquito avoidance, observing food and water safety, safe sex and avoiding animals.
Tuesday, October 10, 2017
Antifungal Agents
Yup, it's another post about drugs! I've also written about antifungal drugs here.
Ergosterol Synthesis Inhibitors
Ergosterol is one of the membrane sterols in fungi (analogous to cholesterol in humans). Blocking of sterol formation by azoles or allylamines leads to the accumulation of toxic sterols in the cell membrane, disrupting packing of membrane lipids.
Azoles
Azoles work by inhibiting 14α-sterol-demethylase, which is in the ergosterol biosynthesis pathway. Inhibition of this enzyme leads to accumulation of 14α methylsterols and disrupted packing of membrane lipids.
The two main types of azoles are the imidazoles and triazoles. Imidazoles, which include ketoconazole and clotrimazole, are quite toxic and are generally only used for more superficial mycoses. Triazoles, such as fluconazole (best against Candida but has no mould activity), itraconazole (better than fluconazole at dealing with moulds, but not as good at dealing with yeasts), voriconazole and posaconazole, are less toxic and target a broader spectrum of fungi. Even though triazoles are less toxic, that's not to say that they are non-toxic: they can still cause hepatotoxicity, and around 30% of patients taking voriconazole experience visual symptoms such as blurred vision and light sensitivity. Azoles also have a huge potential for drug-drug interactions, as they are metabolised by and inhibit cytochrome P450 enzymes.
Allylamines
Allylamines, such as terbinafine, inhibit squalene epoxidase, which is an earlier step in the ergosterol synthesis pathway. Inhibition of squalene epoxidase causes accumulation of squalene and deficiency of ergosterol, which again leads to interference with membrane function and fungal death.
Glucan Synthesis Inhibitors (Echinocandins)
Echinocandins, such as caspofungin, anidulafungin and micafungin inhibit glucan formation. As humans don't have glucan, echinocandins have limited toxic effects. They are used in severe Candida infections as they inhibit the growing ends and branches of fungal hyphae (remember, Candida can form true hyphae when it enters tissue). Unfortunately, echinocandins are very expensive, limiting their use.
Polyenes
Polyenes, such as amphotericin B (AMB) and nystatin, associate with ergosterol to form pores. Large amounts of ions can move through the pore, leading to cell death. Polyenes are broad-spectrum and can be quite useful, especially against invasive fungal infections such as invasive aspergillosis, disseminated candidiasis and cryptococcal meningitis.
Unfortunately, conventional AMB (c-AMB) can be very toxic to the kidneys. Liposomal preparations (L-AMB), which have lipid carriers that ensure uptake and transport by macrophages, have similar efficacy but less toxicity than c-AMB. Thus, L-AMBs, such as AmBisome and Abelcet, are the standard of care for patients with severe fungal infections.
Susceptibility testing
Unlike with bacteria, susceptibility testing is rarely performed on fungi as the results are not very reliable. In vitro resistance usually predicts failure, but in vitro sensitivity doesn't predict success. Also, fungi take a long time to grow as compared to bacteria.
Antifungal resistance
Unfortunately, fungi have already developed some resistance mechanisms to fungi. The main resistance mechanisms are alterations or defects in the ergosterol synthesis pathway, increase of efflux pumps, decrease of drug import and alterations to drug processing.
Treatments
There were a bunch of fungi mentioned throughout this lecture with treatments, but I thought it would be more coherent if I stuck them all at the end of this post, so here they are:
Ergosterol Synthesis Inhibitors
Ergosterol is one of the membrane sterols in fungi (analogous to cholesterol in humans). Blocking of sterol formation by azoles or allylamines leads to the accumulation of toxic sterols in the cell membrane, disrupting packing of membrane lipids.
Azoles
Azoles work by inhibiting 14α-sterol-demethylase, which is in the ergosterol biosynthesis pathway. Inhibition of this enzyme leads to accumulation of 14α methylsterols and disrupted packing of membrane lipids.
The two main types of azoles are the imidazoles and triazoles. Imidazoles, which include ketoconazole and clotrimazole, are quite toxic and are generally only used for more superficial mycoses. Triazoles, such as fluconazole (best against Candida but has no mould activity), itraconazole (better than fluconazole at dealing with moulds, but not as good at dealing with yeasts), voriconazole and posaconazole, are less toxic and target a broader spectrum of fungi. Even though triazoles are less toxic, that's not to say that they are non-toxic: they can still cause hepatotoxicity, and around 30% of patients taking voriconazole experience visual symptoms such as blurred vision and light sensitivity. Azoles also have a huge potential for drug-drug interactions, as they are metabolised by and inhibit cytochrome P450 enzymes.
Allylamines
Allylamines, such as terbinafine, inhibit squalene epoxidase, which is an earlier step in the ergosterol synthesis pathway. Inhibition of squalene epoxidase causes accumulation of squalene and deficiency of ergosterol, which again leads to interference with membrane function and fungal death.
Glucan Synthesis Inhibitors (Echinocandins)
Echinocandins, such as caspofungin, anidulafungin and micafungin inhibit glucan formation. As humans don't have glucan, echinocandins have limited toxic effects. They are used in severe Candida infections as they inhibit the growing ends and branches of fungal hyphae (remember, Candida can form true hyphae when it enters tissue). Unfortunately, echinocandins are very expensive, limiting their use.
Polyenes
Polyenes, such as amphotericin B (AMB) and nystatin, associate with ergosterol to form pores. Large amounts of ions can move through the pore, leading to cell death. Polyenes are broad-spectrum and can be quite useful, especially against invasive fungal infections such as invasive aspergillosis, disseminated candidiasis and cryptococcal meningitis.
Unfortunately, conventional AMB (c-AMB) can be very toxic to the kidneys. Liposomal preparations (L-AMB), which have lipid carriers that ensure uptake and transport by macrophages, have similar efficacy but less toxicity than c-AMB. Thus, L-AMBs, such as AmBisome and Abelcet, are the standard of care for patients with severe fungal infections.
Susceptibility testing
Unlike with bacteria, susceptibility testing is rarely performed on fungi as the results are not very reliable. In vitro resistance usually predicts failure, but in vitro sensitivity doesn't predict success. Also, fungi take a long time to grow as compared to bacteria.
Antifungal resistance
Unfortunately, fungi have already developed some resistance mechanisms to fungi. The main resistance mechanisms are alterations or defects in the ergosterol synthesis pathway, increase of efflux pumps, decrease of drug import and alterations to drug processing.
Treatments
There were a bunch of fungi mentioned throughout this lecture with treatments, but I thought it would be more coherent if I stuck them all at the end of this post, so here they are:
- Dermatophyte infections: First try topical therapy with ketoconazole, clotrimazole or terbinafine cream. Oral ketoconazole or terbinafine can be used if treatment is unsuccessful. Tinea capitis can be treated with oral terbinafine.
- Pityriasis versicolor (Malassezia): Treat with topical antifungal shampoos or with oral ketoconazole or itraconazole. Oral terbinafine is ineffective against pityriasis versicolor. (Malassezia itself isn't immune to terbinafine, but the location of the infection may play a role in its ineffectiveness in pityriasis versicolor.)
- Invasive candidasis: Intravenous azoles, such as itraconazole.
- Invasive aspergillosis: Voriconazole. Polyenes may still be used. Treatment should be started as soon as possible to reduce the risk of mortality.
- Cryptococcal meningitis: IV amphotericin plus flucytosine for two weeks, followed by high-dose fluconazole. Fluconazole may be continued to prevent reinfection.
- Penicillum marneffii: Amphotericin B, then high-dose itraconazole, then lifelong suppressive therapy.
Antiviral Agents
Now we're onto discussing antivirals! I've spoken about them a little bit for PHAR2210, but I'll be talking about even more drugs in this post. Yay drugs.
Nucleoside/Nucleotide Analogues
Nucleoside/nucleotide analogues are fairly close to the structure of a real nucleoside/nucleotide, so they get incorporated into viral DNA but cause instability or chain termination. They generally need to be triphosphorylated by viral enzymes or by host enzymes in order to become active.
Nucleoside/nucleotide analogues are mostly active against herpes viruses such as herpes simplex virus (HSV), varicella zoster virus (VZV) and cytomegalovirus (CMV), though they are also active against HIV and hepatitis viruses. HSV is usually treated with acyclovir or penciclovir cream. Oral antivirals, such as valaciclovir and famciclovir, are more effective. Treatment can reduce the risk of serious sequelae, such as HSV-1 encephalitis. VZV is not usually treated in children, but in adults (who have a more severe form of chickenpox or may have shingles), acyclovir, famciclovir and/or valacyclovir may be given. Hepatitis B can be treated and even occasionally cured with nucleoside analogues, though the main aim is usually to suppress viral replication.
Acyclovir and Valacyclovir
Acyclovir is an analogue of guanosine. It is phosphorylated once by herpes virus thymidine kinase, and then twice more by host enzymes. A prodrug of acyclovir, valacyclovir, has a different side chain that improves its oral bioavailability (3-5x that of oral acyclovir).
Penciclovir and Famciclovir
Penciclovir is pretty similar to acyclovir, but it has a longer half-life. Famciclovir is a prodrug of penciclovir with higher oral bioavailability.
Ganciclovir and Valganciclovir
Ganciclovir is a guanosine analogue with IV, oral and intravitreal formulations. The intravitreal formulation, which is an implant, can be used to treat CMV retinitis (a complication of CMV that mainly occurs in HIV patients). Unfortunately, ganciclovir is very toxic and may suppress the bone marrow. Valganciclovir, the valyl ester of ganciclovir, has higher oral bioavailability and less toxicity.
Ribavirin
Ribavirin is an analogue of both guanine and adenine. It inhibits the RNA polymerase of RNA viruses, and has multiple targets in other viruses. Daily oral ribavirin, in conjunction with weekly interferon injections, are used to treat hepatitis C. Ribavirin is also effective against respiratory syncytial virus (RSV) and Lassa virus. Unfortunately, ribavirin can cause anaemia and thrombocytopaenia, and may have interactions with HIV drugs.
Matrix protein inhibitors
The only matrix protein inhibitor I'll talk about in this post is amantadine, which is only effective against influenza A (and even then has no effect against some strains, such as H1N1 or H3N2). It blocks the ion channel protein M2, which transports hydrogen ions and lowers the pH in the virus core, which in turn results in M1 protein dissociation from the ribonucleoprotein. Treatment with amantadine can shorten the illness and reduce viral shedding. Amantadine also has neurological effects and can be used in nerve pain and Parkinson's disease.
Neuraminidase inhibitors
Neuraminidase inhibitors, such as zanamivir and oseltamivir, block neuraminidase (!), which is a viral enzyme that cleaves receptors containing sialic acid, allowing viral particles to dissociate from the host cell. They are also used to treat influenza.
HIV drugs
See earlier post: Chemotherapy III: Antiviral Drugs
Nucleoside/Nucleotide Analogues
Nucleoside/nucleotide analogues are fairly close to the structure of a real nucleoside/nucleotide, so they get incorporated into viral DNA but cause instability or chain termination. They generally need to be triphosphorylated by viral enzymes or by host enzymes in order to become active.
Nucleoside/nucleotide analogues are mostly active against herpes viruses such as herpes simplex virus (HSV), varicella zoster virus (VZV) and cytomegalovirus (CMV), though they are also active against HIV and hepatitis viruses. HSV is usually treated with acyclovir or penciclovir cream. Oral antivirals, such as valaciclovir and famciclovir, are more effective. Treatment can reduce the risk of serious sequelae, such as HSV-1 encephalitis. VZV is not usually treated in children, but in adults (who have a more severe form of chickenpox or may have shingles), acyclovir, famciclovir and/or valacyclovir may be given. Hepatitis B can be treated and even occasionally cured with nucleoside analogues, though the main aim is usually to suppress viral replication.
Acyclovir and Valacyclovir
Acyclovir is an analogue of guanosine. It is phosphorylated once by herpes virus thymidine kinase, and then twice more by host enzymes. A prodrug of acyclovir, valacyclovir, has a different side chain that improves its oral bioavailability (3-5x that of oral acyclovir).
Penciclovir and Famciclovir
Penciclovir is pretty similar to acyclovir, but it has a longer half-life. Famciclovir is a prodrug of penciclovir with higher oral bioavailability.
Ganciclovir and Valganciclovir
Ganciclovir is a guanosine analogue with IV, oral and intravitreal formulations. The intravitreal formulation, which is an implant, can be used to treat CMV retinitis (a complication of CMV that mainly occurs in HIV patients). Unfortunately, ganciclovir is very toxic and may suppress the bone marrow. Valganciclovir, the valyl ester of ganciclovir, has higher oral bioavailability and less toxicity.
Ribavirin
Ribavirin is an analogue of both guanine and adenine. It inhibits the RNA polymerase of RNA viruses, and has multiple targets in other viruses. Daily oral ribavirin, in conjunction with weekly interferon injections, are used to treat hepatitis C. Ribavirin is also effective against respiratory syncytial virus (RSV) and Lassa virus. Unfortunately, ribavirin can cause anaemia and thrombocytopaenia, and may have interactions with HIV drugs.
Matrix protein inhibitors
The only matrix protein inhibitor I'll talk about in this post is amantadine, which is only effective against influenza A (and even then has no effect against some strains, such as H1N1 or H3N2). It blocks the ion channel protein M2, which transports hydrogen ions and lowers the pH in the virus core, which in turn results in M1 protein dissociation from the ribonucleoprotein. Treatment with amantadine can shorten the illness and reduce viral shedding. Amantadine also has neurological effects and can be used in nerve pain and Parkinson's disease.
Neuraminidase inhibitors
Neuraminidase inhibitors, such as zanamivir and oseltamivir, block neuraminidase (!), which is a viral enzyme that cleaves receptors containing sialic acid, allowing viral particles to dissociate from the host cell. They are also used to treat influenza.
HIV drugs
See earlier post: Chemotherapy III: Antiviral Drugs
Tuesday, October 3, 2017
Introduction to Medically Important Helminths
Another infodumping lecture! Yay!
Classification
The two main phylums of helminths (worms) are nematodes (round worms) and platyhelminthes (flat worms). Platyhelminthes can be subdivided into two main classes: cestodes (which includes tapeworms) and trematodes (which includes flukes and Schistosoma).
Transmission
Helminths can be transmitted by direct transmission, ingestion or by vectors (e.g. mosquitoes). The most common helminths to be transmitted directly are gastrointestinal nematodes, while the most common to be transmitted by ingestion are tapeworms and some nematodes. Vector-borne helminths have a limited geographical range compared to the other two types of transmission.
Nematodes
Nematodes (round worms) come in all shapes and sizes, from under 1mm to over 1m in length. They are dioecious (have male and female forms). Nematodes can be roughly divided into gastrointestinal nematodes, or tissue/blood nematodes. Gastrointestinal nematodes are some of the commonest worm infections and are predicted to infect roughly 50% of the world's population. They can be ingested, or larvae can burrow into the skin.
Caenorhabditis elegans is one of the most important nematodes in medicine and medical research. It is known to survive space. It is also transparent and emits blue fluorescence upon death, which makes it useful for research. Other nematodes include Trichuris trichura ("whipworm"), Enterobius vermicularis ("pinworm"), Trichinella spiralis (a tissue/blood nematode that is the only intracellular helminth), Guinea Worm (a tissue/blood nematode that is predicted to be the next disease to be eradicated) and Toxocara canis (a tissue/blood nematode that often affects the eye).
In this post, we'll focus on a couple of gastrointestinal nematodes (Ascaris lumbricoides and hookworms) as well as a tissue worm (filarial worms). Let's get started!
Ascaris lumbricoides
Ascaris lumbricoides is a fairly large worm (15-50cm long) that can lay around 200 000 eggs per day. Infection by Ascaris is often asymptomatic, but can lead to abdominal pain, bowel obstruction, ulcers, bloody sputum and faeces, fever and cough. If the larvae migrate through the lungs, they can cause Loffler's syndrome, which is basically eosinophilia in the lungs. Children infected with Ascaris may also experience growth retardation.
Ascaris can be diagnosed by identifying the eggs in the faeces. In severe cases, adult worms can be seen protruding from the anus. The infection can be treated with anti-helminthic agents such as mebendazole, nitazoxanide and ivermectin.
Hookworms
The main species of hookworms are Ancylostoma duadenale and Necator americanus. Infection is usually asymptomatic, but since hookworms feed on blood, they can cause anaemia. Hookworms can also cause growth retardation in children, and if infection occurs during pregnancy, children may be born with a low birth weight. Diagnosis is by identification of eggs in faeces, and treatment is with mebendazole or albendazole.
Filarial Worms
Filarial worms can cause three types of filariasis, all of which are spread via vectors such as mosquitoes. In lymphatic filariasis, the worms live in the lymphatic system and cause chronic disease, which may lead to elephantiasis (swelling of the legs). Subcutaneous filariasis, where worms stay in the subcutaneous layer of the skin, may lead to "river blindness." Finally, serous cavity filariasis has relatively mild symptoms.
Filarial worms can enter our blood when mosquitoes bite us. Once in the human body, adults can produce sheathed microfilariae that can enter lymph and blood. At night, these microfilariae enter the peripheral blood, where they can be taken up by the next mosquito to bite us. Once in the mosquito, microfilariae shed their sheaths and move to the mosquito's thoracic cavity to form larvae, which grow and move back to the head. The worms can then enter the blood of the mosquito's next victim.
Lymphatic filariasis can be diagnosed by looking for microfilaria in a finger prick test. As microfilaria move towards the peripheral blood during the night, the timing of the test is important. Subcutaneous filariasis can be diagnosed via a skin snip. All kinds of filariasis can be diagnosed with antigen tests.
Treatment for filarial worms mainly targets microfilaria. Albendazole combined with ivermectin kills microfilaria, as does diethylcarbamazine. A new treatment for filariasis uses the antibiotic doxycycline to kill off Wolbachia, which is a bacteria that is needed for the worms to survive.
Platyhelminthes- Cestodes
Cestodes (tapeworms) are haemaphrodites (each worm has male and female reproductive organs). Their bodies are composed of segments and their heads have a structure called a scolex (suckers and hooks), which helps them to attach to the gut. They can cause intestinal infections (which are mainly asymptomatic, with some exceptions) or tissue infections (which tend to be pathological).
Intestinal tapeworms
Intestinal tapeworms, which include Taenia solium and Taenia saginata, can be acquired by eating undercooked meat. Cattle and pigs acquire intestinal tapeworms by ingesting contaminated vegetation and then form cysts in their muscles. Humans can then acquire the infection by eating undercooked meat of infected animals. Once inside humans, these worms can attach to the gut via the scolex and grow up to around 3m long. If humans directly ingest the eggs of Taenia solium (not Taenia saginata), they can develop cysticercosis (cysts). These cysts usually form in subcutaneous tissue, muscle or brain.
Intestinal tapeworms can be diagnosed by identifying eggs in stool sample. The eggs are identical between T. solium and T. saginata, so proglottids (one of the segments or joints) need to be visualised with India ink, or adult worms need to be found, in order to differentiate the species. PCR of ribosomal genes can also be done. Cysticercosis can also be diagnosed with X-rays, CT or biopsy.
Treatment of intestinal tapeworms can be done with praziquantel, niclosamide or albendazole. Some care needs to be taken as dying larvae can elicit a major immune response. Corticosteroids may be given at the same time as anti-helminthic therapy in order to prevent this major immune response.
Tissue tapeworms
Tissue tapeworms include Echinococcus species, such as Echinococcus granulosus and Echinococcus multilocularis. They are acquired by ingesting ova in contaminated food or by direct contact with certain animals, such as dogs. In contrast to intestinal tapeworms, these worms tend to be quite small (~2-7mm). They can form "bladder-like" cysts in Hydatid disease, or they can form new "daughter cysts." The "daughter cysts" of E. granulosus are membrane-bound and are a bit more like a benign tumour, whereas the daughter cysts of E. multilocularis tend to spread more like a metastatic tumour.
Cysts caused by tissue tapeworms can be detected via X-ray or MRI, and diagnosis can be confirmed with biopsy or PCR. Treatment begins with surgery to remove the cysts (if possible), followed by long-term (>2 years) benzimidazoles (class of drugs including albendazole and mebendazole). In non-responsive cases, intravenous amphotericin B may also be used.
Platyhelminthes- Trematodes
The trematodes include tissue flukes and schistosomes. Tissue flukes are haemaphrodites, whereas schistosomes are dioecious. They are obligate parasites (i.e. need to live in a host cell in order to reproduce) and can live for a long time.
Schistosomes
Schistosomes cause schistosomiasis, also known as Bilharzia. Adult schistosomes are asymptomatic, but eggs can cause tissue damage, which over time can lead to chronic organ damage and cancer. Schistosomes are spread by snails: miracidia (trematode larvae) can penetrate snail tissue. Once inside snails, schistosomes form sporocysts, and eventually are released into the water as cercariae (which I *think* is the larval form? Too many unexplained words). Cercariae can penetrate human skin. Once inside humans, they lose their tails and migrate to the liver, where they mature.
Diagnosis of schistosomes is done through identification of eggs, serology tests and biopsy. Treatment is done via praziquantel. As with intestinal tapeworms, dying worms can elicit an immune response, so steroids are also necessary.
Flukes
Flukes include Fasciola, Clonorchis and Opisthorchis, which live in the liver, and Paragonimus, which lives in the lung. The life cycle of flukes is similar to that of schistosomes, aside from the fact that they don't mate (they are haemaphrodites). Flukes are often acquired by eating seafood.
Diagnosis of flukes is done via identifying eggs in the faeces or bile. There is a risk of false positives, however, as no eggs are laid in the first few months. A serology test for antibodies (ELISA) can also be done, as can PCR on faecal material. Treatments include triclabendazole (a benzimidazole) and praziquantel.
Anti-Helminthic Drugs
I'm just going to cross my fingers and hope that we don't need to know everything on this slide. Anyway, apparently we will be getting a lecture on antiparasitic agents in two weeks' time, so stay tuned, I guess!
Classification
The two main phylums of helminths (worms) are nematodes (round worms) and platyhelminthes (flat worms). Platyhelminthes can be subdivided into two main classes: cestodes (which includes tapeworms) and trematodes (which includes flukes and Schistosoma).
Transmission
Helminths can be transmitted by direct transmission, ingestion or by vectors (e.g. mosquitoes). The most common helminths to be transmitted directly are gastrointestinal nematodes, while the most common to be transmitted by ingestion are tapeworms and some nematodes. Vector-borne helminths have a limited geographical range compared to the other two types of transmission.
Nematodes
Nematodes (round worms) come in all shapes and sizes, from under 1mm to over 1m in length. They are dioecious (have male and female forms). Nematodes can be roughly divided into gastrointestinal nematodes, or tissue/blood nematodes. Gastrointestinal nematodes are some of the commonest worm infections and are predicted to infect roughly 50% of the world's population. They can be ingested, or larvae can burrow into the skin.
Caenorhabditis elegans is one of the most important nematodes in medicine and medical research. It is known to survive space. It is also transparent and emits blue fluorescence upon death, which makes it useful for research. Other nematodes include Trichuris trichura ("whipworm"), Enterobius vermicularis ("pinworm"), Trichinella spiralis (a tissue/blood nematode that is the only intracellular helminth), Guinea Worm (a tissue/blood nematode that is predicted to be the next disease to be eradicated) and Toxocara canis (a tissue/blood nematode that often affects the eye).
In this post, we'll focus on a couple of gastrointestinal nematodes (Ascaris lumbricoides and hookworms) as well as a tissue worm (filarial worms). Let's get started!
Ascaris lumbricoides
Ascaris lumbricoides is a fairly large worm (15-50cm long) that can lay around 200 000 eggs per day. Infection by Ascaris is often asymptomatic, but can lead to abdominal pain, bowel obstruction, ulcers, bloody sputum and faeces, fever and cough. If the larvae migrate through the lungs, they can cause Loffler's syndrome, which is basically eosinophilia in the lungs. Children infected with Ascaris may also experience growth retardation.
Ascaris can be diagnosed by identifying the eggs in the faeces. In severe cases, adult worms can be seen protruding from the anus. The infection can be treated with anti-helminthic agents such as mebendazole, nitazoxanide and ivermectin.
Hookworms
The main species of hookworms are Ancylostoma duadenale and Necator americanus. Infection is usually asymptomatic, but since hookworms feed on blood, they can cause anaemia. Hookworms can also cause growth retardation in children, and if infection occurs during pregnancy, children may be born with a low birth weight. Diagnosis is by identification of eggs in faeces, and treatment is with mebendazole or albendazole.
Filarial Worms
Filarial worms can cause three types of filariasis, all of which are spread via vectors such as mosquitoes. In lymphatic filariasis, the worms live in the lymphatic system and cause chronic disease, which may lead to elephantiasis (swelling of the legs). Subcutaneous filariasis, where worms stay in the subcutaneous layer of the skin, may lead to "river blindness." Finally, serous cavity filariasis has relatively mild symptoms.
Filarial worms can enter our blood when mosquitoes bite us. Once in the human body, adults can produce sheathed microfilariae that can enter lymph and blood. At night, these microfilariae enter the peripheral blood, where they can be taken up by the next mosquito to bite us. Once in the mosquito, microfilariae shed their sheaths and move to the mosquito's thoracic cavity to form larvae, which grow and move back to the head. The worms can then enter the blood of the mosquito's next victim.
Lymphatic filariasis can be diagnosed by looking for microfilaria in a finger prick test. As microfilaria move towards the peripheral blood during the night, the timing of the test is important. Subcutaneous filariasis can be diagnosed via a skin snip. All kinds of filariasis can be diagnosed with antigen tests.
Treatment for filarial worms mainly targets microfilaria. Albendazole combined with ivermectin kills microfilaria, as does diethylcarbamazine. A new treatment for filariasis uses the antibiotic doxycycline to kill off Wolbachia, which is a bacteria that is needed for the worms to survive.
Platyhelminthes- Cestodes
Cestodes (tapeworms) are haemaphrodites (each worm has male and female reproductive organs). Their bodies are composed of segments and their heads have a structure called a scolex (suckers and hooks), which helps them to attach to the gut. They can cause intestinal infections (which are mainly asymptomatic, with some exceptions) or tissue infections (which tend to be pathological).
Intestinal tapeworms
Intestinal tapeworms, which include Taenia solium and Taenia saginata, can be acquired by eating undercooked meat. Cattle and pigs acquire intestinal tapeworms by ingesting contaminated vegetation and then form cysts in their muscles. Humans can then acquire the infection by eating undercooked meat of infected animals. Once inside humans, these worms can attach to the gut via the scolex and grow up to around 3m long. If humans directly ingest the eggs of Taenia solium (not Taenia saginata), they can develop cysticercosis (cysts). These cysts usually form in subcutaneous tissue, muscle or brain.
Intestinal tapeworms can be diagnosed by identifying eggs in stool sample. The eggs are identical between T. solium and T. saginata, so proglottids (one of the segments or joints) need to be visualised with India ink, or adult worms need to be found, in order to differentiate the species. PCR of ribosomal genes can also be done. Cysticercosis can also be diagnosed with X-rays, CT or biopsy.
Treatment of intestinal tapeworms can be done with praziquantel, niclosamide or albendazole. Some care needs to be taken as dying larvae can elicit a major immune response. Corticosteroids may be given at the same time as anti-helminthic therapy in order to prevent this major immune response.
Tissue tapeworms
Tissue tapeworms include Echinococcus species, such as Echinococcus granulosus and Echinococcus multilocularis. They are acquired by ingesting ova in contaminated food or by direct contact with certain animals, such as dogs. In contrast to intestinal tapeworms, these worms tend to be quite small (~2-7mm). They can form "bladder-like" cysts in Hydatid disease, or they can form new "daughter cysts." The "daughter cysts" of E. granulosus are membrane-bound and are a bit more like a benign tumour, whereas the daughter cysts of E. multilocularis tend to spread more like a metastatic tumour.
Cysts caused by tissue tapeworms can be detected via X-ray or MRI, and diagnosis can be confirmed with biopsy or PCR. Treatment begins with surgery to remove the cysts (if possible), followed by long-term (>2 years) benzimidazoles (class of drugs including albendazole and mebendazole). In non-responsive cases, intravenous amphotericin B may also be used.
Platyhelminthes- Trematodes
The trematodes include tissue flukes and schistosomes. Tissue flukes are haemaphrodites, whereas schistosomes are dioecious. They are obligate parasites (i.e. need to live in a host cell in order to reproduce) and can live for a long time.
Schistosomes
Schistosomes cause schistosomiasis, also known as Bilharzia. Adult schistosomes are asymptomatic, but eggs can cause tissue damage, which over time can lead to chronic organ damage and cancer. Schistosomes are spread by snails: miracidia (trematode larvae) can penetrate snail tissue. Once inside snails, schistosomes form sporocysts, and eventually are released into the water as cercariae (which I *think* is the larval form? Too many unexplained words). Cercariae can penetrate human skin. Once inside humans, they lose their tails and migrate to the liver, where they mature.
Diagnosis of schistosomes is done through identification of eggs, serology tests and biopsy. Treatment is done via praziquantel. As with intestinal tapeworms, dying worms can elicit an immune response, so steroids are also necessary.
Flukes
Flukes include Fasciola, Clonorchis and Opisthorchis, which live in the liver, and Paragonimus, which lives in the lung. The life cycle of flukes is similar to that of schistosomes, aside from the fact that they don't mate (they are haemaphrodites). Flukes are often acquired by eating seafood.
Diagnosis of flukes is done via identifying eggs in the faeces or bile. There is a risk of false positives, however, as no eggs are laid in the first few months. A serology test for antibodies (ELISA) can also be done, as can PCR on faecal material. Treatments include triclabendazole (a benzimidazole) and praziquantel.
Anti-Helminthic Drugs
I'm just going to cross my fingers and hope that we don't need to know everything on this slide. Anyway, apparently we will be getting a lecture on antiparasitic agents in two weeks' time, so stay tuned, I guess!
Fungal Infections
Just when I thought that we were done with infodumping lectures, we get another one! Maybe this is our "welcome back from study break" treat >_> Just like in the first few posts, anything in red can be found on Microbe Invader (http://microbeinvader.com/).
Descriptions of Fungal Infection
Location
Fungal infections can be described as superficial, cutaneous, subcutaneous or systemic. Superficial infections, such as dandruff and oral candidiasis, generally don't cause an immune response, but they are also very rarely fatal. Cutaneous infections, such as tinea, infect keratinised tissue. Subcutaneous infections, such as sporotrichosis, can lead to granuloma formation and cysts. Finally, systemic infections can cause severe illnesses that are often fatal, and are often caused by the more virulent fungi, including the primary pathogens Coccidioides, Histoplasma, Blastomyces and Paracoccidioides. Even opportunistic pathogens, such as Candida, may be able to cause systemic infections in the immunocompromised.
Endemic vs. Opportunistic
Fungi can also be classified as endemic or opportunistic. Endemic mycoses are caused by fungi which are not part of the normal human flora, but are acquired from environmental sources. They are often associated with outdoor activities and other specific exposures: for example, Histoplasma is associated with old buildings with bird or bat droppings, Coccidioides is associated with deserts, and Blastomyces is associated with wetlands and wooded areas. (All of those three are mainly found in North America.) Other risk factors for endemic mycoses include increasing age and decreasing cell-mediated immunity. Opportunistic mycoses, on the other hand, are caused by organisms that are often part of the normal flora, and mainly cause disease in the immunocompromised.
Morphology
Yeasts are fungi that are round and multiply by budding. Filamentous fungi grow as filaments and hyphae (chain of cells that line up nicely to form smooth "branches"), which can intertwine to form a network called a mycelium. Filamentous fungi often produce asexual spores called conidia, which are located on long chains on a conidiophore.
There are also dimorphic (or more accurately polyphenic) "yeast-like fungi" which can grow as yeasts or form chains. Some of these chains might be hyphae, and some might be pseudohyphae (kind of like hyphae, but with "constrictions" between the cells in the chain). The ability of dimorphic fungi to change shape can help with overcoming defences. Dimorphic fungi tend to be in the yeast form at body temperature, aiding in dissemination. Their filamentous form, which is more common at room temperature, helps with tissue damage and invasion.
Cell Walls
Just a quick note on cell walls: fungal cell walls are mainly made up of polysaccharides, particularly glucan, chitin and mannan. The cell wall composition changes between different species.
Virulence Factors
Just like bacteria, fungi have several virulence factors that help them to fight against their host.
Phenotypic Switching and Dimorphism
As I mentioned above, some fungi can change their morphology. Aside from dimorphic fungi, which can change between yeasts and hyphae, several different fungi can change their cell surface properties, colony appearance, and so on. For example, Candida albicans can switch from white cells to opaque cells for sexual mating.
Adhesion
Like bacteria, some fungi have adhesins, which help them to bind to host cells.
Secreted hydrolytic enzymes
Some fungi can secrete enzymes, such as proteases, lipases and phospholipases. These have a wide range of roles, from acquiring nutrition (e.g. pinching iron from RBCs), damaging tissue and overcoming the host immune system.
Biofilm
Fungi, like bacteria, can form biofilm or "slime" which provides a physical barrier from phagocytosis, lysozyme and antimicrobials.
Candida
Since Candida (especially Candida albicans) is a relatively common fungus, let's focus on it for a little while! Candida mainly exists in yeast form, and is thus oval or round and can reproduce by budding. It is, however, polyphenic, and can form pseudohyphae when exposed to certain physiological conditions. When Candida enters tissue, it can form true hyphae, which aid in invasion and infection.
Candida is found in a lot of places, including soil, food, animals and humans (yep, it is part of our commensal flora). There are over 150 species, but only a handful cause infection, and even then they mainly only cause infections in the immunocompromised. Conditions caused by Candida include intertrigo (skin fold infection), paronychia (nail bed infection), onychomycosis (nail infection), nappy rash, oral candidiasis, vaginitis and oesophagitis.
Candida can even enter the blood, causing Candidaemia. The usual causes of candidaemia are colonisation of intravenous cannulae and contaminated injections by drug users. Invasive candidiasis can cause some pretty nasty conditions, such as endophthalmitis (inflammation of the interior of the eye), meningitis, endocarditis, pyelonephritis and osteomyelitis.
Diagnosis of Candida can be done via microscopy and culture. On agar, Candida forms smoth, creamy white colonies. In serum, Candida albicans forms germ tubes (which are basically precursors of hyphae), which differentiates it from other Candida species.
Dermatophytes
Dermatophytes are fungi that usually cause milder skin diseases, such as tinea. The main genera are Microsporum, Epidermophyton and Trichophyton. They invade the epidermis and release proteinases and keratinases, causing inflammation of the skin and keratinised tissues. Dermatophyte infections include tinea pedis ("athlete's foot"), tinea corporis ("body ringworm"), tinea cruris (infection of warm, moist creases), tinea capitis (scalp infection) and tinea unguinum (thick, crumbly and discoloured nails).
For some reason, there was a slide on pityriasis versicolor here, even though it's caused by Malassezia, which is not a dermatophyte. (I've also covered it here.)
To diagnose a dermatophyte infection, skin scrapings or nail clippings are often used. Since the fungus is most active in the advancing margin of the lesion, skin scrapings are usually done in this area.
Aspergillus
The main type of Aspergillus is Aspergillus fumigatus, which is fairly ubiquitous. Most people will inhale many A. fumigatus conidia (spores) daily without any ill effect, though the immunocompromised are a different story.
There are three main manifestations of aspergillosis: allergic bronchopulmonary aspergillosis (ABPE), aspergilloma and invasive aspergillosis. ABPE is caused by an allergic or hypersensitivity response to Aspergillus spores, and causes breathlessness and fevers. Aspergilloma is a fungal ball which grows inside a pre-existing lung cavity. Invasive aspergillosis, which is an acute or subacute infection, mainly affects the lungs and causes fever, cough and haemoptysis (coughing up blood). Invasive aspergillosis occurs mainly in transplant patients. Mortality rates are fairly high, though they are not so bad if treatment is started within 10 days of symptom commencement.
Aspergillus can be diagnosed via culture, the galactomannan assay and/or the β-D-glucan assay, though these diagnostic techniques may also need to be backed up by histological proof (i.e. looking at the tissue). Culture of Aspergillus has a high likelihood of false positives, and has only around 30% sensitivity. The galactomannan assay, which is an ELISA for a particular cell wall component of Aspergillus, has some inter-lab variability, problems with false positives and negatives, and requires serial testing (i.e. test the same patient several times over several days). The β-D-glucan assay also looks for part of the cell wall of Aspergillus, but amoxicillin-clavulanate, piperacillin-tazobactam and Streptococcus pneumoniae (which also has some β-D-glucan) can all cause false positives.
Mucormycosis
Mucormycosis is an invasive lethal mycosis caused by moulds such as Rhizopus, Rhizomucor, Mucorales and Absidia. It is a severe infection of the facial sinuses that mainly affects diabetics and the immunocompromised.
Cryptococcus
I've spoken about Cryptococcus a bit here, so now I'll go into a bit more detail! Cryptococcus neoformans is mainly associated with soil and bird poo, whereas Cryptococcus gattii is mainly associated with eucalyptus trees. It mainly causes disease in the immunocompromised, but it may also cause disease in healthy people. Cryptococcus can be diagnosed via microscopy (particularly with the India ink test), testing for cryptococcal antigen in the serum or CSF, as well as by looking at the manifestations (symptoms include meningoencephalitis, pneumonia and skin lesions).
Descriptions of Fungal Infection
Location
Fungal infections can be described as superficial, cutaneous, subcutaneous or systemic. Superficial infections, such as dandruff and oral candidiasis, generally don't cause an immune response, but they are also very rarely fatal. Cutaneous infections, such as tinea, infect keratinised tissue. Subcutaneous infections, such as sporotrichosis, can lead to granuloma formation and cysts. Finally, systemic infections can cause severe illnesses that are often fatal, and are often caused by the more virulent fungi, including the primary pathogens Coccidioides, Histoplasma, Blastomyces and Paracoccidioides. Even opportunistic pathogens, such as Candida, may be able to cause systemic infections in the immunocompromised.
Endemic vs. Opportunistic
Fungi can also be classified as endemic or opportunistic. Endemic mycoses are caused by fungi which are not part of the normal human flora, but are acquired from environmental sources. They are often associated with outdoor activities and other specific exposures: for example, Histoplasma is associated with old buildings with bird or bat droppings, Coccidioides is associated with deserts, and Blastomyces is associated with wetlands and wooded areas. (All of those three are mainly found in North America.) Other risk factors for endemic mycoses include increasing age and decreasing cell-mediated immunity. Opportunistic mycoses, on the other hand, are caused by organisms that are often part of the normal flora, and mainly cause disease in the immunocompromised.
Morphology
Yeasts are fungi that are round and multiply by budding. Filamentous fungi grow as filaments and hyphae (chain of cells that line up nicely to form smooth "branches"), which can intertwine to form a network called a mycelium. Filamentous fungi often produce asexual spores called conidia, which are located on long chains on a conidiophore.
There are also dimorphic (or more accurately polyphenic) "yeast-like fungi" which can grow as yeasts or form chains. Some of these chains might be hyphae, and some might be pseudohyphae (kind of like hyphae, but with "constrictions" between the cells in the chain). The ability of dimorphic fungi to change shape can help with overcoming defences. Dimorphic fungi tend to be in the yeast form at body temperature, aiding in dissemination. Their filamentous form, which is more common at room temperature, helps with tissue damage and invasion.
Cell Walls
Just a quick note on cell walls: fungal cell walls are mainly made up of polysaccharides, particularly glucan, chitin and mannan. The cell wall composition changes between different species.
Virulence Factors
Just like bacteria, fungi have several virulence factors that help them to fight against their host.
Phenotypic Switching and Dimorphism
As I mentioned above, some fungi can change their morphology. Aside from dimorphic fungi, which can change between yeasts and hyphae, several different fungi can change their cell surface properties, colony appearance, and so on. For example, Candida albicans can switch from white cells to opaque cells for sexual mating.
Adhesion
Like bacteria, some fungi have adhesins, which help them to bind to host cells.
Secreted hydrolytic enzymes
Some fungi can secrete enzymes, such as proteases, lipases and phospholipases. These have a wide range of roles, from acquiring nutrition (e.g. pinching iron from RBCs), damaging tissue and overcoming the host immune system.
Biofilm
Fungi, like bacteria, can form biofilm or "slime" which provides a physical barrier from phagocytosis, lysozyme and antimicrobials.
Candida
Since Candida (especially Candida albicans) is a relatively common fungus, let's focus on it for a little while! Candida mainly exists in yeast form, and is thus oval or round and can reproduce by budding. It is, however, polyphenic, and can form pseudohyphae when exposed to certain physiological conditions. When Candida enters tissue, it can form true hyphae, which aid in invasion and infection.
Candida is found in a lot of places, including soil, food, animals and humans (yep, it is part of our commensal flora). There are over 150 species, but only a handful cause infection, and even then they mainly only cause infections in the immunocompromised. Conditions caused by Candida include intertrigo (skin fold infection), paronychia (nail bed infection), onychomycosis (nail infection), nappy rash, oral candidiasis, vaginitis and oesophagitis.
Candida can even enter the blood, causing Candidaemia. The usual causes of candidaemia are colonisation of intravenous cannulae and contaminated injections by drug users. Invasive candidiasis can cause some pretty nasty conditions, such as endophthalmitis (inflammation of the interior of the eye), meningitis, endocarditis, pyelonephritis and osteomyelitis.
Diagnosis of Candida can be done via microscopy and culture. On agar, Candida forms smoth, creamy white colonies. In serum, Candida albicans forms germ tubes (which are basically precursors of hyphae), which differentiates it from other Candida species.
Dermatophytes
Dermatophytes are fungi that usually cause milder skin diseases, such as tinea. The main genera are Microsporum, Epidermophyton and Trichophyton. They invade the epidermis and release proteinases and keratinases, causing inflammation of the skin and keratinised tissues. Dermatophyte infections include tinea pedis ("athlete's foot"), tinea corporis ("body ringworm"), tinea cruris (infection of warm, moist creases), tinea capitis (scalp infection) and tinea unguinum (thick, crumbly and discoloured nails).
For some reason, there was a slide on pityriasis versicolor here, even though it's caused by Malassezia, which is not a dermatophyte. (I've also covered it here.)
To diagnose a dermatophyte infection, skin scrapings or nail clippings are often used. Since the fungus is most active in the advancing margin of the lesion, skin scrapings are usually done in this area.
Aspergillus
The main type of Aspergillus is Aspergillus fumigatus, which is fairly ubiquitous. Most people will inhale many A. fumigatus conidia (spores) daily without any ill effect, though the immunocompromised are a different story.
There are three main manifestations of aspergillosis: allergic bronchopulmonary aspergillosis (ABPE), aspergilloma and invasive aspergillosis. ABPE is caused by an allergic or hypersensitivity response to Aspergillus spores, and causes breathlessness and fevers. Aspergilloma is a fungal ball which grows inside a pre-existing lung cavity. Invasive aspergillosis, which is an acute or subacute infection, mainly affects the lungs and causes fever, cough and haemoptysis (coughing up blood). Invasive aspergillosis occurs mainly in transplant patients. Mortality rates are fairly high, though they are not so bad if treatment is started within 10 days of symptom commencement.
Aspergillus can be diagnosed via culture, the galactomannan assay and/or the β-D-glucan assay, though these diagnostic techniques may also need to be backed up by histological proof (i.e. looking at the tissue). Culture of Aspergillus has a high likelihood of false positives, and has only around 30% sensitivity. The galactomannan assay, which is an ELISA for a particular cell wall component of Aspergillus, has some inter-lab variability, problems with false positives and negatives, and requires serial testing (i.e. test the same patient several times over several days). The β-D-glucan assay also looks for part of the cell wall of Aspergillus, but amoxicillin-clavulanate, piperacillin-tazobactam and Streptococcus pneumoniae (which also has some β-D-glucan) can all cause false positives.
Mucormycosis
Mucormycosis is an invasive lethal mycosis caused by moulds such as Rhizopus, Rhizomucor, Mucorales and Absidia. It is a severe infection of the facial sinuses that mainly affects diabetics and the immunocompromised.
Cryptococcus
I've spoken about Cryptococcus a bit here, so now I'll go into a bit more detail! Cryptococcus neoformans is mainly associated with soil and bird poo, whereas Cryptococcus gattii is mainly associated with eucalyptus trees. It mainly causes disease in the immunocompromised, but it may also cause disease in healthy people. Cryptococcus can be diagnosed via microscopy (particularly with the India ink test), testing for cryptococcal antigen in the serum or CSF, as well as by looking at the manifestations (symptoms include meningoencephalitis, pneumonia and skin lesions).
Tuesday, September 19, 2017
Antibiotic Resistance
In my last post, I discussed antibiotics. In this post, I'm going to discuss what happens when bacteria are resistant to antibiotics! Hooray! (not...)
Understand how antimicrobial resistance has arisen and spread in bacterial populations
Antimicrobial resistance in populations comes about due to selective pressures. Without selective pressures, only a small number of microbes will have resistance, but when exposed to an antimicrobial agent, only those microbes with resistance will be able to survive and reproduce, such that resistant microbes eventually become dominant in the population.
Antimicrobial resistance has probably been around for as long as there have been microbes. For example, some microbes living in soil secrete their own antimicrobial agents, so neighbouring microbes may have gained resistance. Of course, nowadays we are mainly interested in looking at resistance to conventional antibiotics. There are many reasons why microbes may be developing resistance: some doctors may be prescribing antibiotics unnecessarily, some patients may not be complying with treatment (allowing some microbes to survive and mutate), and the use of antibiotics in agriculture may be problematic. One of the more problematic agricultural antibiotics is avoparcin, which is a glycopeptide (like vancomycin), and gives cross-resistance to vancomycin. It has been hypothesised that vancomycin resistance might be linked to antibiotic use in animals.
Understand the genetics of antibiotic resistance
Sometimes, resistance is already coded within bacterial genes. Other times, mutations might be acquired from mutations, vertical transfer via chromosomes, or horizontal transfer via transfer of chromosomal genes or transfer of genes on mobile genetic elements. These mobile genetic elements include plasmids, transposons, and integrons. Plasmids are self-replicating circular dsDNA that come in a few varieties. Conjugative plasmids can transfer information between bacteria, while R plasmids (which may also be conjugative) encode antibiotic resistance. Transposons and integrons are linear dsDNA that cannot self-replicate. Integrons can integrate into transposons, and transposons can integrate into chromosomes or plasmids.
In the process of conjugation, two bacterial cells must first come together. Sometimes this occurs through retraction of a sex pilus, which is a structure that is present on some bacterial cells. A "conjugation tube" forms between the two bacteria, and plasmid DNA replication begins. While DNA replication occurs, the free DNA strand begins moving through the conjugation tube. In the recipient cell, the free strand is replicated. The end result is that each cell ends up with a full copy of the plasmid.
Know the five mechanisms of antibiotic resistance and some examples
The main mechanisms are as follows:
Streptococcus pneumoniae
S. pneumoniae is a common cause of ear infections, but since antibiotics don't penetrate the middle ear very well, there's a high chance that treatment isn't 100% effective. As such, a small number of bacteria are left hanging around and are able to mutate. Some S. pneumoniae has developed penicillin resistance via mutation of penicillin-binding proteins (i.e. antibiotic target site alteration). Since very high level resistance is unusual, it can usually be overcome by increasing the dose of penicillin.
Staphylococcus aureus
S. aureus has a particularly nasty form called MRSA (methicillin-resistant S. aureus), which is also known as a "superbug." MRSA contains the Staphylococcal Cassette Chromosome mec (SCCmec), which is a mobile genetic element that integrates into the S. aureus chromosome. SCCmec contains the antibiotic resistance gene mecA, which codes for PBP2a, which does not bind beta-lactam antibiotics. Usually alternative antibiotics, such as vancomycin, are required to treat this bad boy.
β-lactamase resistance in Gram negatives
Most beta-lactamase resistance is due to beta-lactamases in Gram-negative bacteria. Extended-spectrum beta-lactamases (ESBLs) are resistant to penicillins, cephalosporins, and monobactams (but not to carbapenems). ESBLs can be inhibited by clavulanic acid, which is why clavulanic acid is often packaged together with some antibiotics (e.g. amoxicillin-clavulanate). Some bacteria have carbapenemases which, as their name suggests, are also capable of hydrolysing carbapenems. Recently, a new carbapenemase, called New Delhi metallo-beta-lactamase (NDM-1), has been discovered. It is found on a plasmid encoding the blaNDM gene, and has spread rapidly. If you have a superbug with this carbapenemase, you're pretty much stuck with the polymixins and tigecycline.
Other resistances
Aside from beta-lactams, bacteria have also developed resistance to some other antibiotics (sneaky buggers!). To defend against aminoglycosides, some bacteria have inactivating enzymes (e.g. streptomycin acetyltransferase) and/or have a decreased expression of porins, decreasing membrane permeability to aminoglycosides. To defend against the MLSB group (macrolides, lincosamide, streptogramin B), some bacteria methylate 16S rRNA to alter the binding site and/or have increased efflux through multidrug resistance (MDR) efflux pumps. Bacteria can also defend against tetracyclines by increasing efflux through a specific Tet pump.
Understand the implications of antibiotic resistance and strategies to address the problem
Obviously, antibiotic resistance is kind of problematic. When bacteria are resistant, it narrows down our treatment choices and we might be stuck with only very toxic drugs (or, worse still, no options at all). Some strategies that have been suggested to address this problem include:
Understand how antimicrobial resistance has arisen and spread in bacterial populations
Antimicrobial resistance in populations comes about due to selective pressures. Without selective pressures, only a small number of microbes will have resistance, but when exposed to an antimicrobial agent, only those microbes with resistance will be able to survive and reproduce, such that resistant microbes eventually become dominant in the population.
Antimicrobial resistance has probably been around for as long as there have been microbes. For example, some microbes living in soil secrete their own antimicrobial agents, so neighbouring microbes may have gained resistance. Of course, nowadays we are mainly interested in looking at resistance to conventional antibiotics. There are many reasons why microbes may be developing resistance: some doctors may be prescribing antibiotics unnecessarily, some patients may not be complying with treatment (allowing some microbes to survive and mutate), and the use of antibiotics in agriculture may be problematic. One of the more problematic agricultural antibiotics is avoparcin, which is a glycopeptide (like vancomycin), and gives cross-resistance to vancomycin. It has been hypothesised that vancomycin resistance might be linked to antibiotic use in animals.
Understand the genetics of antibiotic resistance
Sometimes, resistance is already coded within bacterial genes. Other times, mutations might be acquired from mutations, vertical transfer via chromosomes, or horizontal transfer via transfer of chromosomal genes or transfer of genes on mobile genetic elements. These mobile genetic elements include plasmids, transposons, and integrons. Plasmids are self-replicating circular dsDNA that come in a few varieties. Conjugative plasmids can transfer information between bacteria, while R plasmids (which may also be conjugative) encode antibiotic resistance. Transposons and integrons are linear dsDNA that cannot self-replicate. Integrons can integrate into transposons, and transposons can integrate into chromosomes or plasmids.
In the process of conjugation, two bacterial cells must first come together. Sometimes this occurs through retraction of a sex pilus, which is a structure that is present on some bacterial cells. A "conjugation tube" forms between the two bacteria, and plasmid DNA replication begins. While DNA replication occurs, the free DNA strand begins moving through the conjugation tube. In the recipient cell, the free strand is replicated. The end result is that each cell ends up with a full copy of the plasmid.
Know the five mechanisms of antibiotic resistance and some examples
The main mechanisms are as follows:
- Decreased influx of antibiotic (e.g. permeability barriers)
- Increased efflux of antibiotic (e.g. efflux pumps)
- Antibiotic inactivation (e.g. beta-lactamases, aminoglycoside-modifying enzymes)
- Antibiotic target site alteration (e.g. altered pencillin-binding-proteins / transpeptidases, altered DNA gyrase)
- Antibiotic target amplification or alternate pathway (e.g. producing more folate to overcome folate synthesis inhibitors)
Streptococcus pneumoniae
S. pneumoniae is a common cause of ear infections, but since antibiotics don't penetrate the middle ear very well, there's a high chance that treatment isn't 100% effective. As such, a small number of bacteria are left hanging around and are able to mutate. Some S. pneumoniae has developed penicillin resistance via mutation of penicillin-binding proteins (i.e. antibiotic target site alteration). Since very high level resistance is unusual, it can usually be overcome by increasing the dose of penicillin.
Staphylococcus aureus
S. aureus has a particularly nasty form called MRSA (methicillin-resistant S. aureus), which is also known as a "superbug." MRSA contains the Staphylococcal Cassette Chromosome mec (SCCmec), which is a mobile genetic element that integrates into the S. aureus chromosome. SCCmec contains the antibiotic resistance gene mecA, which codes for PBP2a, which does not bind beta-lactam antibiotics. Usually alternative antibiotics, such as vancomycin, are required to treat this bad boy.
β-lactamase resistance in Gram negatives
Most beta-lactamase resistance is due to beta-lactamases in Gram-negative bacteria. Extended-spectrum beta-lactamases (ESBLs) are resistant to penicillins, cephalosporins, and monobactams (but not to carbapenems). ESBLs can be inhibited by clavulanic acid, which is why clavulanic acid is often packaged together with some antibiotics (e.g. amoxicillin-clavulanate). Some bacteria have carbapenemases which, as their name suggests, are also capable of hydrolysing carbapenems. Recently, a new carbapenemase, called New Delhi metallo-beta-lactamase (NDM-1), has been discovered. It is found on a plasmid encoding the blaNDM gene, and has spread rapidly. If you have a superbug with this carbapenemase, you're pretty much stuck with the polymixins and tigecycline.
Other resistances
Aside from beta-lactams, bacteria have also developed resistance to some other antibiotics (sneaky buggers!). To defend against aminoglycosides, some bacteria have inactivating enzymes (e.g. streptomycin acetyltransferase) and/or have a decreased expression of porins, decreasing membrane permeability to aminoglycosides. To defend against the MLSB group (macrolides, lincosamide, streptogramin B), some bacteria methylate 16S rRNA to alter the binding site and/or have increased efflux through multidrug resistance (MDR) efflux pumps. Bacteria can also defend against tetracyclines by increasing efflux through a specific Tet pump.
Understand the implications of antibiotic resistance and strategies to address the problem
Obviously, antibiotic resistance is kind of problematic. When bacteria are resistant, it narrows down our treatment choices and we might be stuck with only very toxic drugs (or, worse still, no options at all). Some strategies that have been suggested to address this problem include:
- Use more specific agents if possible (rather than broad-spectrum)
- Avoid vancomycin unless necessary
- Only prescribe if required, and prescribe for the optimal duration
- Shorten hospital stays (less likelihood of a patient getting a nosocomial infection and needing antibiotics)
- Prevent infections (via immunisation, sanitation etc.)
Antibacterial agents and susceptibility testing
This post was mainly a recap of this lecture from PHAR2210, but with some more details. Enjoy!
Know the terminology describing the general
characteristics of antimicrobial agents and drugs
- Broad spectrum- Antibiotic inhibits or kills lots of things (e.g. tetracycline inhibits Gram positives and negatives, as well as Chlamydia and Rickettsia)
- Narrow spectrum- Inhibits or kills only a few things (e.g. pencillin G only kills Gram positives)
- Bacteriostatic- Inhibits growth, but doesn't kill (e.g. chloramphenicol)
- Bactericidal- Kills microbes (e.g. penicillins)
- Toxic dose- Dose at which the drug becomes too toxic for the host
- Therapeutic dose- Dose needed to treat the infection
- Therapeutic index- Toxic dose divided by therapeutic dose. The larger the therapeutic index, the better
Know the five main mechanisms of antibacterial action
The five main mechanisms are as follows:
- Inhibiting protein synthesis
- Inhibiting cell wall synthesis
- Metabolic antagonists/antimetabolites (i.e. blockers of enzymatic activity etc.)
- Inhibition of nucleic acid synthesis
- Cell membrane disruption
I'll expand on these in the next section...
Be able to describe the effect, mechanism of action,
group members and spectrum of activity for the
antibiotics given as examples in each case
A lot of the drugs that I'm about to mention have already been mentioned here, but time to go into more detail! Yay!
Protein synthesis inhibitors
The main classes of drugs here are aminoglycosides, tetracyclines, macrolides, and chloramphenicol. With the exception of aminoglycosides, which are bactericidal, most protein synthesis inhibitors are bacteriostatic.
Aminoglycosides, such as streptomycin, gentamicin and kanamycin, bind to the 16S rRNA of the 30S ribosomal subunit at the A site. They inhibit translation elongation and make ribosomes error-prone. Aminoglycosides are effective against Gram-negative bacteria, particularly enteric bacteria and Pseudomonas aeruginosa. However, they are quite toxic.
Tetracyclines, such as tetracycline, chlortetracycline (tetracycline with an extra -Cl), doxycycline (extra -OH) and minocycline (extra N(CH3)2) also bind to the 16S rRNA in the 30S subunit. They block the binding of incoming aminoacyl-tRNAs to the A site. Tetracyclines are broad-spectrum drugs that work against Gram-positives and Gram-negatives, as well as Chlamydia, Mycoplasma, and Rickettsia.
Macrolides, such as erythromycin and clindamycin, bind to the 23S rRNA in the 50S ribosomal subunit. They are quite bulky drugs that "plug" the ribosomal tunnel. They are also quite broad-spectrum and are able to act against Gram-positives, mycoplasmas, and some Gram-negatives. A related drug called clindamycin inhibits peptidyl transferase and is good against anaerobes.
Chloramphenicol, like macrolides, bind to the 23S rRNA in the 50S subunit. They affect the binding of aminoacyl-tRNA to the A-site. Chloramphnicol is broad-spectrum but is very toxic, so it is only used in life-threatening situations or in topical treatment of conjunctivitis.
Cell wall synthesis inhibitors
Cell wall synthesis inhibitors have very good selective toxicity as they only target components of cell walls, which humans don't have. Many cell wall inhibitors block transpeptidation, which is the last step in bacterial cell wall synthesis, and are usually bactericidal. During this step, an amino group in one chain attacks the second last D-alanine of the other chain, forming a peptide link. This may form a direct crossbridge (as in E. coli) or a different kind of interbridge (e.g. S. aureus has a pentaglycine crossbridge). Either way, transpeptidation is mediated by transpeptidases, which are also known as penicillin-binding proteins (PBPs).
The most well-known cell wall synthesis inhibitor is probably penicillin. Penicillins contain a beta-lactam ring, which resembles the terminal D-alanyl-D-alanine in peptidoglycans, thus blocking cell wall formation. Cell walls are actually kind of important for bacteria- without it, they can't resist osmotic pressure, so they are easily lysed. Cephalosporins (e.g. cefoxitin) also function in the same way- they are somewhat structurally different to penicillin, but they still have the beta-lactam ring.
Another class of cell wall synthesis inhibitor is the glycopeptides. Glycopeptides bind to the D-alanyl-D-alanine in peptidoglycans. They are relatively narrow-spectrum, limited to Gram-positives, but they can be useful as last-resort drugs in some cases (e.g. MRSA). An example of a glycopeptide is vancomycin (which also happens to be my favourite "nuke" in Microbe Invader).
Metabolic antagonists/antimetabolites
Sulfonamides and trimethoprim are both metabolic antagonists, and are often combined into one drug (trimethoprim-sulfamethoxazole). Sulfonamides block the first step in the folic acid pathway by competing with PABA. Trimethoprim blocks a later step by inhibiting the dihydrofolate reductase enzyme. The end result is that folic acid is not produced, and since bacteria require folic acid to form DNA bases, they're kind of screwed when this pathway is blocked. Both sulfonamides and trimethoprim are bacteriostatic.
Inhibition of nucleic acid synthesis
Nucleic acid synthesis inhibitors, which are mostly bactericidal, have poor selective toxicity because synthesis pathways are pretty similar between eukaryotes and prokaryotes. Quinolones and fluoroquinolones inhibit DNA gyrase (the enzyme that uncoils parent DNA), while rifampin inhibits RNA polymerase. Quinolones and fluoroquinolones vary in specificity, whereas rifampin is a narrow spectrum drug used for tuberculosis and some Gram-negatives.
Cell membrane disruption
Cell membrane disruptors are also bactericidal and have poor selective toxicity. The main class here are polymixins, such as Polymixin B and colistin (a.k.a. Polymixin E). They are narrow-spectrum and are mainly used topically for Gram-negative infections.
Macrolides, such as erythromycin and clindamycin, bind to the 23S rRNA in the 50S ribosomal subunit. They are quite bulky drugs that "plug" the ribosomal tunnel. They are also quite broad-spectrum and are able to act against Gram-positives, mycoplasmas, and some Gram-negatives. A related drug called clindamycin inhibits peptidyl transferase and is good against anaerobes.
Chloramphenicol, like macrolides, bind to the 23S rRNA in the 50S subunit. They affect the binding of aminoacyl-tRNA to the A-site. Chloramphnicol is broad-spectrum but is very toxic, so it is only used in life-threatening situations or in topical treatment of conjunctivitis.
Cell wall synthesis inhibitors
Cell wall synthesis inhibitors have very good selective toxicity as they only target components of cell walls, which humans don't have. Many cell wall inhibitors block transpeptidation, which is the last step in bacterial cell wall synthesis, and are usually bactericidal. During this step, an amino group in one chain attacks the second last D-alanine of the other chain, forming a peptide link. This may form a direct crossbridge (as in E. coli) or a different kind of interbridge (e.g. S. aureus has a pentaglycine crossbridge). Either way, transpeptidation is mediated by transpeptidases, which are also known as penicillin-binding proteins (PBPs).
The most well-known cell wall synthesis inhibitor is probably penicillin. Penicillins contain a beta-lactam ring, which resembles the terminal D-alanyl-D-alanine in peptidoglycans, thus blocking cell wall formation. Cell walls are actually kind of important for bacteria- without it, they can't resist osmotic pressure, so they are easily lysed. Cephalosporins (e.g. cefoxitin) also function in the same way- they are somewhat structurally different to penicillin, but they still have the beta-lactam ring.
Another class of cell wall synthesis inhibitor is the glycopeptides. Glycopeptides bind to the D-alanyl-D-alanine in peptidoglycans. They are relatively narrow-spectrum, limited to Gram-positives, but they can be useful as last-resort drugs in some cases (e.g. MRSA). An example of a glycopeptide is vancomycin (which also happens to be my favourite "nuke" in Microbe Invader).
Metabolic antagonists/antimetabolites
Sulfonamides and trimethoprim are both metabolic antagonists, and are often combined into one drug (trimethoprim-sulfamethoxazole). Sulfonamides block the first step in the folic acid pathway by competing with PABA. Trimethoprim blocks a later step by inhibiting the dihydrofolate reductase enzyme. The end result is that folic acid is not produced, and since bacteria require folic acid to form DNA bases, they're kind of screwed when this pathway is blocked. Both sulfonamides and trimethoprim are bacteriostatic.
Inhibition of nucleic acid synthesis
Nucleic acid synthesis inhibitors, which are mostly bactericidal, have poor selective toxicity because synthesis pathways are pretty similar between eukaryotes and prokaryotes. Quinolones and fluoroquinolones inhibit DNA gyrase (the enzyme that uncoils parent DNA), while rifampin inhibits RNA polymerase. Quinolones and fluoroquinolones vary in specificity, whereas rifampin is a narrow spectrum drug used for tuberculosis and some Gram-negatives.
Cell membrane disruption
Cell membrane disruptors are also bactericidal and have poor selective toxicity. The main class here are polymixins, such as Polymixin B and colistin (a.k.a. Polymixin E). They are narrow-spectrum and are mainly used topically for Gram-negative infections.
Understand the three methods of antibacterial
susceptibility testing described
Antibacterial susceptibility tests are often used to determine MIC (minimal inhibitory concentration), which is the lowest concentration of a drug required to prevent bacterial growth. There are three main methods used: disk diffusion tests, Etests, and broth and agar dilution tests.
Disk diffusion tests (a.k.a. Kirby-Bauer tests)
In disk diffusion tests, the microbe is spread onto an agar plate. Sterile paper disks impregnated with an antibiotic are placed onto the surface of the plate. If the antibiotic kills off the microbe, there will be a clear zone around that disk, also known as an inhibition zone. The size of the inhibition zone can be used to determine MIC.
Etests
Etests are kind of like disk diffusion tests in that the microbe is spread onto an agar plate. Instead of disks, Etests use plastic strips that have a concentration gradient of an antibiotic, which is labelled with a scale. The strips are placed on the agar plate so that the lowest concentration of antibiotic is at the centre of the disk. The MIC can be determined by finding the place where the inhibition zone intersects with the strip.
Broth and agar dilution tests
In a broth dilution test, the microbe is added to a bunch of different broths, each containing a different concentration of the antibiotic. The MIC is the tube with the lowest concentration of antibiotic without any bacterial growth. Agar dilution tests are similar, but they use a concentration gradient of antibiotic across the agar (I *think*).
Tuesday, September 12, 2017
The Patient with an Infection
Last post covering content for the next test! Whoop-de-doop...
This lecture jumped around a bit, so I'm going to make my own headings and try and summarise the main points. Not sure how well I'll do, but I'll try.
What is a pathogen?
Pathogens are microbes that can cause disease. Traditionally, pathogens were distinguished from non-pathogens by virulence, which was defined as an ability to deliver "poison" and cause disease. The story is a bit more complicated than this, however, as host factors (e.g. immunosuppression, nutritional state, and previous exposure) and environmental factors may also affect the virulence of a pathogen. (I have described pathogen virulence factors here. If virulence factors are removed by gene technology or otherwise, pathogenicity is affected, but not viability.)
Pathogens, as I'm sure you should know by now, can cause an array of different diseases. It is important to figure out which pathogen type (and preferably which pathogen) a patient is infected with so that an appropriate treatment can be chosen. For example, antibiotics are ineffective on viruses and fungi. Usually, localised infections are due to bacteria or fungi and systemic infections are usually due to viruses, but this isn't always true. Meningococcal disease is pretty damn systemic, and that's caused by bacteria.
Of course, to get a disease, you must first be infected. I've touched on different routes of transmission here.
Meningococcal disease
This lecture focused quite a bit on meningococcal, so I guess I'll talk about it here. Outbreaks are usually in places where there is a lot of close contact with others, such as in university or military dormitories. There is also a region in Africa called the "meningitis belt," as the rate of incidence there is very high. Meningococcal is mainly transmitted by contact with respiratory secretions and saliva, which can result in colonisation (which lasts for months), or invasive disease. Infections mainly occur in the winter and early spring.
N. meningitidis, which causes meningococcal, has a few virulence factors that allow it to wreak havoc on the body. It has fimbriae, allowing it to adhere to the nasopharynx and hang around in there for a while. It also has a polysaccharide capsule, which prevents phagocytosis. Finally, it can release a potent endotoxin called outer membrane lipooligosaccharide (LOS), which binds to receptors on macrophages and neutrophils, triggering inflammatory and coagulation cascades. The characteristic "rash" sometimes seen in meningococcal patients is actually a result of coagulated blood under the skin.
The importance of taking a history
When dealing with a patient with an infection, it is important to take their history into account. Travel may increase the likelihood that a patient has come into contact with a certain disease (e.g. malaria and Dengue fever are more common in tropical areas). Mosquitoes may spread diseases such as Ross River virus and malaria. Other important exposures include contact with certain animals, consumption of certain foods and drinks, exposure to contaminated water, soil or dust (potting mix increases your risk of Legionella infection), sexual contact, and drug use.
Medical examination
In a medical exam, a doctor might look for signs of a "systemic inflammatory response" (i.e. high temperature, rapid pulse, rapid respiratory rate, and high blood pressure), as well as some more localised signs, such as rashes, heart murmurs, lung crepitations (crackling sounds made by inflamed lungs), abdominal tenderness, neck stiffness, and so on. There were a couple of slides on fever, but I've already touched on it here, so all I will say is that patients often shiver when they have a fever, and if they have extreme shivering ("rigors"), this is usually indicative of a serious infection. A blood test may also be ordered, with a full blood count to detect white blood cell levels, erythrocyte sedimentation rate (a non-specific indicator of inflammation), C-reactive protein (which I'm pretty sure is also a non-specific indicator of inflammation), and so on. Specimens may also be collected and sent off to the lab for further testing.
This lecture jumped around a bit, so I'm going to make my own headings and try and summarise the main points. Not sure how well I'll do, but I'll try.
What is a pathogen?
Pathogens are microbes that can cause disease. Traditionally, pathogens were distinguished from non-pathogens by virulence, which was defined as an ability to deliver "poison" and cause disease. The story is a bit more complicated than this, however, as host factors (e.g. immunosuppression, nutritional state, and previous exposure) and environmental factors may also affect the virulence of a pathogen. (I have described pathogen virulence factors here. If virulence factors are removed by gene technology or otherwise, pathogenicity is affected, but not viability.)
Pathogens, as I'm sure you should know by now, can cause an array of different diseases. It is important to figure out which pathogen type (and preferably which pathogen) a patient is infected with so that an appropriate treatment can be chosen. For example, antibiotics are ineffective on viruses and fungi. Usually, localised infections are due to bacteria or fungi and systemic infections are usually due to viruses, but this isn't always true. Meningococcal disease is pretty damn systemic, and that's caused by bacteria.
Of course, to get a disease, you must first be infected. I've touched on different routes of transmission here.
Meningococcal disease
This lecture focused quite a bit on meningococcal, so I guess I'll talk about it here. Outbreaks are usually in places where there is a lot of close contact with others, such as in university or military dormitories. There is also a region in Africa called the "meningitis belt," as the rate of incidence there is very high. Meningococcal is mainly transmitted by contact with respiratory secretions and saliva, which can result in colonisation (which lasts for months), or invasive disease. Infections mainly occur in the winter and early spring.
N. meningitidis, which causes meningococcal, has a few virulence factors that allow it to wreak havoc on the body. It has fimbriae, allowing it to adhere to the nasopharynx and hang around in there for a while. It also has a polysaccharide capsule, which prevents phagocytosis. Finally, it can release a potent endotoxin called outer membrane lipooligosaccharide (LOS), which binds to receptors on macrophages and neutrophils, triggering inflammatory and coagulation cascades. The characteristic "rash" sometimes seen in meningococcal patients is actually a result of coagulated blood under the skin.
The importance of taking a history
When dealing with a patient with an infection, it is important to take their history into account. Travel may increase the likelihood that a patient has come into contact with a certain disease (e.g. malaria and Dengue fever are more common in tropical areas). Mosquitoes may spread diseases such as Ross River virus and malaria. Other important exposures include contact with certain animals, consumption of certain foods and drinks, exposure to contaminated water, soil or dust (potting mix increases your risk of Legionella infection), sexual contact, and drug use.
Medical examination
In a medical exam, a doctor might look for signs of a "systemic inflammatory response" (i.e. high temperature, rapid pulse, rapid respiratory rate, and high blood pressure), as well as some more localised signs, such as rashes, heart murmurs, lung crepitations (crackling sounds made by inflamed lungs), abdominal tenderness, neck stiffness, and so on. There were a couple of slides on fever, but I've already touched on it here, so all I will say is that patients often shiver when they have a fever, and if they have extreme shivering ("rigors"), this is usually indicative of a serious infection. A blood test may also be ordered, with a full blood count to detect white blood cell levels, erythrocyte sedimentation rate (a non-specific indicator of inflammation), C-reactive protein (which I'm pretty sure is also a non-specific indicator of inflammation), and so on. Specimens may also be collected and sent off to the lab for further testing.
Viral Pathogenesis
Describe the various patterns of viral infection
The main patterns of viral infection are acute and persistent. In an acute infection, there is rapid production of virus particles, producing symptoms after an incubation period (which may be as short as a few days, or last for weeks or months). Symptoms tend to resolve relatively quickly, and immunity may result. In a persistent infection, the primary infection is not cleared by the immune system for some reason, so virus particles continue to be produced. Sometimes the virus will be detectable, while at other times it may "hide" by remaining latent in certain cells. Persistent infections may be problematic as they can be reactivated, may be associated with immunopathological diseases, and may even be associated with certain cancers.
Understand the different types of persistent infections and be able to give examples of viruses that pertain to each type
The different types of persistent infection are latent, chronic, and slow.
Latent infections
In a latent infection, after an initial acute infection, the virus remains latent in cells of the body. The virus may reactivate every now and again throughout a person's lifetime, which may or may not cause symptoms. As mentioned in previous posts, herpes simplex virus and varicella zoster virus are good examples of latent infections.
Another example of a latent infection is cytomegalovirus (CMV), which can remain dormant in CD34+ myeloid progenitor cells and CD14+ monocytes. CMV is usually asymptomatic, but can be shed and spread in the urine and saliva of healthy carriers. This can be problematic, as while CMV is usually asymptomatic, it can have nasty consequences for the immunosuppressed or for foetuses.
Yet another example of a latent infection is Epstein-Barr Virus (EBV), which can remain latent in B-cells. It usually infects epithelial cells, such as the mouth mucosa, which shed the virus for months after infection. Like CMV, it is often carried asymptomatically. Reactivation is usually caused by immunosuppression, but unlike CMV, it usually will not cause symptoms. EBV is, however, associated with some cancers (*cough*Burkitt's lymphoma*cough*), so don't get too complacent.
Chronic infections
In a chronic infection, the virus continues to be produced, though usually there are long periods with no symptoms. The virus, however, can still be shed in the blood, which may be problematic in cases of blood transfusions.
Hepatitis B is one example of a chronic infection. The virus replicates in the liver, and infectious virus particles (Dane particles) and Hep B surface antigen (HBsAg) circulate in the plasma. Usually, virions and HBsAg are cleared (so no chronic infection), but around 5-10% of those infected with Hep B will have a chronic infection. A small proportion of the chronically affected may also experience liver cirrhosis and cancer later on. As Hep B can be shed in the blood, blood donations are screened for this virus.
Lymphocytic choriomeningitis virus (LCMV) is a chronic infection that mainly affects rodents. (It can also infect humans, but only rarely.) It is transferred from mother to child. Infant mice with LCMV tend to be normal, but have persistent viraemia. Some antibodies may be produced, and if antigen-antibody complexes are deposited in the kidney, kidney disease may result. No cell-mediated immunity develops, and no cellular dysfunction is evident as a result of the disease.
Slow infections
Slow infections normally start with an acute infection with symptoms, which is followed by a long period in which the virus is kept at low levels by the immune system, and finally a phase where viral load continues to rise until death of the host. One of the main causes of slow infection are lentiviruses, which is a group of virus including HIV, SIV (simian immunodeficiency virus- affects monkeys) and FIV (feline immunodeficiency virus- affects cats). Lentiviruses, which can also exist as an integrated DNA provirus, replicate in lymphocytes and macrophages.
As I just mentioned, HIV is an example of a slow virus. The acute phase is usually mildly symptomatic, with some flu-like symptoms. Following the acute phase, the patient can remain asymptomatic for up to 10 years. Eventually, the virus comes back with a vengeance and kills off a lot of CD4 T-cells, compromising the immune system to the point where opportunistic infections can kill. Nasty.
The measles virus can also cause a slow infection. In rare cases, someone who has been infected with measles may get a complication called subacute sclerosing panencephalitis (SSPE) 1-10 years after measles infection. In SSPE, virus is replicated slowly in the central nervous system, and nucleocapsids are transmitted from cell to cell. High levels of neutralising antibody are produced, but because a lot of viral replication is still occurring within cells, the infection can still progress until death occurs.
For some reason, this lecture also included transmissible spongiform encephalopathies, which are brain diseases caused by prions (self-replicating proteins). Scrapie is a prion disease that affects sheep, and can be transmitted from ewe to lamb. It progresses to paralysis and death. Creutzfeldt-Jacob Disease (CJD) is a prion disease in humans that causes a pre-senile dementia. A variant form (vCJD) can be caused by ingesting beef infected with bovine spongiform encephalopathy. (This is also known as "Mad Cow Disease," and since it's hard to test for, it's the reason why many Brits can't donate blood in Australia.) Another transmissible spongiform encephalopathy, Kuru, is confined mainly to the New Guinea Highlands. It is spread by ritual cannibalism, but education campaigns have seen a decrease in Kuru infection.
Understand the pathogenesis of persistent infection
Persistent infections hang around because they have properties that allow them to hang around and/or are able to avoid the host defences. Let's take a look at their strategies.
The main patterns of viral infection are acute and persistent. In an acute infection, there is rapid production of virus particles, producing symptoms after an incubation period (which may be as short as a few days, or last for weeks or months). Symptoms tend to resolve relatively quickly, and immunity may result. In a persistent infection, the primary infection is not cleared by the immune system for some reason, so virus particles continue to be produced. Sometimes the virus will be detectable, while at other times it may "hide" by remaining latent in certain cells. Persistent infections may be problematic as they can be reactivated, may be associated with immunopathological diseases, and may even be associated with certain cancers.
Understand the different types of persistent infections and be able to give examples of viruses that pertain to each type
The different types of persistent infection are latent, chronic, and slow.
Latent infections
In a latent infection, after an initial acute infection, the virus remains latent in cells of the body. The virus may reactivate every now and again throughout a person's lifetime, which may or may not cause symptoms. As mentioned in previous posts, herpes simplex virus and varicella zoster virus are good examples of latent infections.
Another example of a latent infection is cytomegalovirus (CMV), which can remain dormant in CD34+ myeloid progenitor cells and CD14+ monocytes. CMV is usually asymptomatic, but can be shed and spread in the urine and saliva of healthy carriers. This can be problematic, as while CMV is usually asymptomatic, it can have nasty consequences for the immunosuppressed or for foetuses.
Yet another example of a latent infection is Epstein-Barr Virus (EBV), which can remain latent in B-cells. It usually infects epithelial cells, such as the mouth mucosa, which shed the virus for months after infection. Like CMV, it is often carried asymptomatically. Reactivation is usually caused by immunosuppression, but unlike CMV, it usually will not cause symptoms. EBV is, however, associated with some cancers (*cough*Burkitt's lymphoma*cough*), so don't get too complacent.
Chronic infections
In a chronic infection, the virus continues to be produced, though usually there are long periods with no symptoms. The virus, however, can still be shed in the blood, which may be problematic in cases of blood transfusions.
Hepatitis B is one example of a chronic infection. The virus replicates in the liver, and infectious virus particles (Dane particles) and Hep B surface antigen (HBsAg) circulate in the plasma. Usually, virions and HBsAg are cleared (so no chronic infection), but around 5-10% of those infected with Hep B will have a chronic infection. A small proportion of the chronically affected may also experience liver cirrhosis and cancer later on. As Hep B can be shed in the blood, blood donations are screened for this virus.
Lymphocytic choriomeningitis virus (LCMV) is a chronic infection that mainly affects rodents. (It can also infect humans, but only rarely.) It is transferred from mother to child. Infant mice with LCMV tend to be normal, but have persistent viraemia. Some antibodies may be produced, and if antigen-antibody complexes are deposited in the kidney, kidney disease may result. No cell-mediated immunity develops, and no cellular dysfunction is evident as a result of the disease.
Slow infections
Slow infections normally start with an acute infection with symptoms, which is followed by a long period in which the virus is kept at low levels by the immune system, and finally a phase where viral load continues to rise until death of the host. One of the main causes of slow infection are lentiviruses, which is a group of virus including HIV, SIV (simian immunodeficiency virus- affects monkeys) and FIV (feline immunodeficiency virus- affects cats). Lentiviruses, which can also exist as an integrated DNA provirus, replicate in lymphocytes and macrophages.
As I just mentioned, HIV is an example of a slow virus. The acute phase is usually mildly symptomatic, with some flu-like symptoms. Following the acute phase, the patient can remain asymptomatic for up to 10 years. Eventually, the virus comes back with a vengeance and kills off a lot of CD4 T-cells, compromising the immune system to the point where opportunistic infections can kill. Nasty.
The measles virus can also cause a slow infection. In rare cases, someone who has been infected with measles may get a complication called subacute sclerosing panencephalitis (SSPE) 1-10 years after measles infection. In SSPE, virus is replicated slowly in the central nervous system, and nucleocapsids are transmitted from cell to cell. High levels of neutralising antibody are produced, but because a lot of viral replication is still occurring within cells, the infection can still progress until death occurs.
For some reason, this lecture also included transmissible spongiform encephalopathies, which are brain diseases caused by prions (self-replicating proteins). Scrapie is a prion disease that affects sheep, and can be transmitted from ewe to lamb. It progresses to paralysis and death. Creutzfeldt-Jacob Disease (CJD) is a prion disease in humans that causes a pre-senile dementia. A variant form (vCJD) can be caused by ingesting beef infected with bovine spongiform encephalopathy. (This is also known as "Mad Cow Disease," and since it's hard to test for, it's the reason why many Brits can't donate blood in Australia.) Another transmissible spongiform encephalopathy, Kuru, is confined mainly to the New Guinea Highlands. It is spread by ritual cannibalism, but education campaigns have seen a decrease in Kuru infection.
Understand the pathogenesis of persistent infection
Persistent infections hang around because they have properties that allow them to hang around and/or are able to avoid the host defences. Let's take a look at their strategies.
- Non-immunogenic agents: Not all pathogens are immunogenic (i.e. they do not induce immune responses, like production of type I interferon, and/or are not susceptible to the actions of the immune system). That makes things rather tricky...
- Integrated genomes: Retroviral DNA can be integrated into the host genome, allowing a virus to remain part of the host genome indefinitely. Viruses may also exist as episomes (separate segments of DNA within the host cell), which also stops them from being destroyed by the host.
- Antigenic variation: Some viruses, particularly lentiviruses, can undergo mutations that change their cell surface antigens, making it difficult to form antibodies against them.
- Growth in protected sites: Viruses can grow in sites where the immune system is not very strong. For example, HSV and VZV reside in neurons, which don't usually express MHC-I. Some other infections grow in epithelial cells and are shed in secretions, which may not provoke an immune response.
- Growth in macrophages: Viral growth in macrophages also impairs some macrophage functions, including antigen presentation, phagocytosis, and cytokine production.
- Non-neutralising antibodies: Viruses can induce the formation of non-neutralising antibodies, which can form complexes with viral antigens, leading to immune complex diseases. Non-neutralising antibodies can also block the binding of C1 antibody, which usually binds to and modulates lysis of infected cells.
- Immunological tolerance: Many viruses only induce a very weak antibody response, which is not effective at wiping out the virus.
- Suppression of cell-mediated immunity: Viruses may reduce MHC-I expression on cell surfaces, replicate inside and impair the function of immune system cells (e.g. macrophages), or have other immunosuppressive effects.
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