|MadSci Network: Virology|
Hi Amanda, To answer your question I need to tell you something basic about pathology. Your teacher is absolutely right to tell you that, in short, encephalitis is an inflammation of the brain. Viral encephalitis is endemic in some parts of the world; it may occur epidemically or sporadically. But what is “inflammation” anyways? Often in everyday term “inflammation” is used as a blanket term to denote the type of tissue reaction you would expect when e.g. a wound get infected. Indeed, inflammation is the stereotyped tissue reaction against many different kinds of tissue insults. It could be acute or it could be chronic, depending not just on the time course but also in the range of cells involved in the process. No matter it is a physical injury e.g. burn, frost bite; chemical injury e.g. chemical burn; allergic reaction, bacterial, viral or parasitic infection, or the presence of foreign body or foci of necrosis (dead tissues) the tissues concerned respond locally in a stereotyped way (and normally backed up by systemic changes as well!). Several different events operate in concert in acute inflammation to produce a sequence of events which is broadly similar regardless of the site in the body involved. In essence, this includes hyperaemia (increased local blood flow) largely due to local vasodilatation; exudation (accumulation of protein-rich fluid in the extracellular space to cause “swelling”, due to increased vascular permeability); and leukocyte emigration (mostly polymorphic neutrophils, but also cells from the monocyte/macrophage lineage). The purpose of acute inflammation is to generate an innate immediate defence against injury and to localise and eliminate e.g. bacterial infection. Most aspects of acute inflammation is in turn driven by chemical mediators, for e.g., mast cells release histamine, resident macrophages release prostaglandins, leukotrienes, platelet activating factors, interleukins, tumour necrosis factor-alpha and nitrous oxides, etc. What causes these cells to initiate the release then? Aha!! Many of these cells have surface receptor that could facilitate them to recognise invaders either directly or via the help of some serum proteins. Now back to your question. Viruses are obligate intracellular parasites and are distinguishable from all other biological entities by their lack of metabolic machinery. Different viruses could have different nucleic acids which constitute their genomes. This necessarily means that different viruses must use different strategies to invade the host. If you think logically, all invaders must have a route of entry and a route of exit, and these two routes to a large extent determine the transmission route. Using your example of encephalitis, a number of viruses and bacteria could cause encephalitis, a common sporadic type being herpes simplex viruses (nonetheless, encephalitis can also be part of an allergic response to a systemic viral infection). If we put all the above together, let’s say, a patient contracted a viral infection via the mucosa lining the conjunctiva (herpes simplex maybe, herpes can cause herpes encephalitis). The body immediate launches an acute inflammation in the hope to control the spread of the infection. Unfortunately, the virus this time is smart enough to escape host response and get into the blood (viraemia). The viruses thus get carried by the blood to all over the body. Different viruses have their preference target tissue and let’s say this virus XYZ expresses a surface receptor that facilitates them to invade nervous tissues (brain, spinal cord, etc.). This could result in the viruses causing an acute inflammatory response in the brain. Remember that the brain also has its vascular component (i.e. blood supplies) and resident surveillance cells (microglia – acts just like macrophages) and they can sense the presence of invaders and launch an inflammatory response just like your skin get “inflamed” after a superficial cut – hence encephalitis! So vasodilatation happens and local oedema occurs due to exudation (see above). But this is VERY bad if it happens in the brain because the brain is contained inside the cranium which cannot expand to accommodate the increased volume, instead the brain gets compressed by the cranium. The patient thus may feel headache at first, then drowsiness, and may progress to coma and finally death… I am omitting all the details in the explanation because I think your teacher might have already explained the mechanism of viral infection and stuff. Moreover, general principles are far more important than specific details at this stage. So I am just trying to put them into context. However, if you are really into the details of virology, you can consult any of the standard virology textbook or general pathology textbooks in your school library. I hope this can help your revision and good luck with your test!! Joshua Chai Medical Student University of Cambridge, UK
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