Infections of the Respiratory System

Another post, another body system!

The organisation and innate defences of the respiratory tract

For the purposes of this lecture, the respiratory tract was divided into upper and lower. The larynx marks the boundary between these two sections. The upper respiratory tract is colonised by microbes, which are mostly anaerobic, and help protect us from pathogens, just like other commensal bacteria. The lower respiratory tract is, however, sterile.

Aside from commensal bacteria, we have several other defences in the respiratory tract. Physical defences include our nasal hair, mucus, and ciliated cells. Chemical defences include α-trypsin and lysozyme. We also have immune defences via our innate immunity, alveolar macrophages, and secretory IgA. Finally, if all else fails, we can cough stuff out.

Different types of respiratory tract infections

Respiratory tract infections can be divided into upper respiratory tract infections (URTI) and lower respiratory tract infections (LRTI), depending on the area that they infect. URTIs include pharyngitis (inflammation of the pharynx, a.k.a. sore throat), laryngitis, tonsillitis, sinusitis, epiglottitis, and otitis (ear inflammation). LRTIs include bronchitis (a cough that brings up thick mucus) and pneumonia (rapid breathing with wheeze as well as chest or abdominal pain).

Bacterial infections of the respiratory tract – Streptococcus pyogenes, Corynebacterium diphtheriae, otitis media

Streptococcus pyogenes

Yup, it's S. pyogenes again! (I also wrote about it in my post about skin and soft tissue infections.) It's mentioned here because it causes strep throat. Strep throat and skin infections by S. pyogenes can also cause scarlet fever, which is a high fever with a red skin rash. It is caused by erythrogenic toxins which are carried on bacteriophages and released by the bacteria. S. pyogenes antigens can be detected on throat swabs, making diagnosis relatively easy.

Corynebacterium diphtheriae

C. diphtheriae is a gram-positive rod that causes- you guessed it!- diphtheria. Symptoms of diphtheria include pharyngitis, fever, neck swelling, and the formation of a pseudomembrane in the throat. This pseudomembrane consists of fibrin, dead tissue, and bacterial cells, and can cause asphyxiation. The diphtheria toxin alone, which is carried on β-phages and ω-phages, is pretty nasty and can cause cardiac arrest, and sequelae of diphtheria include heart disease. Thankfully, this disease is extremely rare in developed countries thanks to the diphtheria toxoid vaccine.

Otitis media

Otitis media, or middle ear infection, can be caused by a range of bacteria, including H. influenzae, S. pneumoniae, M. catarrhalis and S. pyogenes. Many of these bacteria cannot be cultured, and thus detection requires microscopy or PCR. Virulence factors for these bacteria include motility, adhesins, and formation of a biofilm. Symptoms of otitis media include vomiting, dizziness, sore throat, and pus. In extreme cases, it can progress to hearing loss, facial paralysis, meningitis, and brain abscesses. Ear infections can be treated with grommets (little tubes that can relieve the pressure in the ear), and sometimes antibiotics.

Bordertella pertussis

B. pertussis causes whooping cough, in which the patient coughs a lot, followed by a whooping sound as they try to get their breath back. Patients can die if their breathing is interrupted for too long, and it's also possible to break ribs (particularly in young children). Thankfully, this disease isn't too common due to the vaccine.

B. pertussis is a gram-negative coccobacillus that is difficult to culture due to its fastidious nature. A PCR for the pertactin gene can be done instead in order to aid in diagnosis. Virulence determinants include filamentous haemagglutinin (FHA), which aids in adhesion, cytotoxins that kill certain cell types, and pertussis toxin, which contributes to mucus production.

Bacterial pneumonias

A range of bacteria can cause pneumonia. I'm not going to list all of them here (since there are a lot), so I'll just focus on the most common: S. pneumoniae, which can also cause otitis, sinusitis, bacteremia and meningitis. S. pneumoniae is a gram-positive diplococcus that undergoes alpha-haemolysis on blood agar and is sensitive to optochin. Its virulence factors include its capsule and pneumolysin (a haemolysin that can damage ciliated mucosal cells). The pneumonia vaccine is based on the capsule polysaccharides.

Pneumocystis carinii

P. carinii is a weird one. It has both fungal and protozoan characteristics, but it can be treated by an antibiotic, cotrimoxazole. It can only be detected by microscopy. P. carinii is common in immunocompromised patients.

Legionella spp.

Legionella are poorly-staining gram-negative bacteria. It is associated mainly with water, like the water in air conditioning systems.

Viral infections of the respiratory tract – Rhinovirus, coronavirus, viral pneumonia

Common cold

The common cold is caused mainly by rhinoviruses and to a lesser extent coronaviruses, adenoviruses, and other viruses. There are so many agents that cause the common cold that developing a vaccine simply isn't practical. However, we develop immunity (based on IgA antibodies) to cold viruses throughout our lifetime, so we get fewer colds as we get older. Rhinoviruses thrive at temperatures below body temperature, which is probably why colds seem to go around more during the cooler months.

Viral pneumonia

Many viral pneumonias are a complication of influenza, measles, or chicken pox.

Respiratory Syncytial Virus (RSV)

RSV is the most common cause of viral respiratory disease in infants, but it can also affect others and become potentially deadly in the elderly. It usually causes coughing and wheezing lasting over a week. If there are bacterial complications, fever may also occur. Acquired immunity to RSV is poor, which I suppose means that getting it once is no guarantee that you won't get it again. Generally only supportive care is required, but in severe cases, the antiviral drug ribivarin may help to deal with the symptoms.

Influenza

Influenza is characterised by chills, fever, headache, and muscle pain. The mortality rate is low, but since so many people are infected by it, a lot of people die of the flu every year (12 000-56 000 deaths in the US alone).

Influenza's structure consists of 8 separate RNA segments enclosed by an inner layer of protein. Covering the outside of this is a lipid bilayer, which has H spikes and N spikes embedded in it. H (haemagglutinin) spikes allow the virus to recognise and attach to body cells. Antibodies to the flu usually form against H spikes. N (neuraminidase) spikes help the virus to separate from an infected cell after it has replicated.

Influenza strains are usually named after the form of H and N (e.g. H1N1). Changes in H and N are called antigenic shifts, as they are great enough to evade immunity. Antigenic shifts are probably caused by major genetic recombination between more than one strain. This recombination may occur in another animal- for example, pigs or birds. Migratory birds can carry the virus over a large area. Aside from antigenic shifts, there are also antigenic drifts, which are smaller variations due to random genetic mutation.

Influenza can also be classified according to the antigens in the protein coats. These groups are A, B, and C. A is the most severe and is associated with major pandemics, whereas B and C are milder.

There is no specific treatment for flu, but antiviral drugs (such as amantadine and rimantadine) and neuraminidase inhibitors (such as zanamivir) can reduce symptoms if taken early.

SARS

SARS (Severe Acute Respiratory Syndrome) is a respiratory illness that was first reported in Asia, but spread over the world. It begins with a high fever, which may be accompanied by headaches and body aches. After 2-7 days, patients may develop a dry cough and difficulty in breathing. SARS is caused by SARS-associated coronavirus (SARS-CoV).

MERS

MERS (Middle East Respiratory Syndrome) is characterised by fever, cough, and shortness of breath. It is caused by the MERS-associated coronavirus (MERS-CoV).