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.