What is encephalitis?|
Encephalitis is infection or inflammation of the substance of the brain, as opposed to meningitis which is infection of the lining of the brain.
What causes the disease?
There are several different types of encephalitis. The most common type is acute viral encephalitis, which is a sudden onset viral infection of the brain. Less commonly, encephalitis can occur after a viral infection elsewhere in the body! This is because the body's defense system becomes confused and mistakenly affects the brain. Encephalitis is also a rare side effect of some childhood vaccinations. It is important to note that the risk of encephalitis from vaccination is LESS THAN the risk of encephalitis from the illnesses being vaccinated against.
Viruses can get to the brain by two possible routes. Either by getting into the bloodstream, or by getting into nerves and traveling up them to reach the brain. Viral infections involve the virus taking over some functions of cells in our body. In encephalitis this means the virus gets into our brain cells and uses them as a 'factory' to make more viruses. The body's defense against this involves killing both the virus itself and also the brain cells which are infected. This leads to swelling of the brain in the affected area (the blood vessels become leaky to allow the body's defence force to travel from the bloodstream into the brain).
There are many different viruses which can cause encephalitis:
- Herpes simplex - this is the virus which causes cold sores and genital warts. It is a very common virus with up to 80% of the world's population having been exposed to it. Many people have herpes simplex virus living in the nerves of their face, however other than causing the occasional cold sore this presents no problem. Encephalitis is caused by the virus traveling up the facial nerves to infect the brain. The reason for this occurring is unknown. Cold sores are very common but herpes simplex encephalitis is not. Herpes simplex encephalitis is thought to affect one to two people per half million per year. It is a severe type of encephalitis. Untreated 70% of people die and only 2.5% recover without any brain damage. Treated, 50 to 65% of people die or have severe brain damage. 13-38% return to normal or have only minor brain problems. This is the most important cause of encephalitis in New Zealand.
- Varicella zoster - this is the virus responsible for both chickenpox and shingles. It is unknown exactly how it causes encephalitis. Some researchers think the virus affects the brain directly whereas others think the virus tricks the body's defence system into damaging the brain. Encephalitis results from approximately 0.1% of cases of chickenpox and is less common from shingles. The estimated death rate is 15 to 25%.
- Cytomegalovirus (CMV) - this is a very common virus and most people (especially those with poor hygiene) will have been exposed to it. It rarely causes any problems though and the risk of leading to encephalitis is unknown.
- Epstein Barr - this is the virus which causes glandular fever (infectious mononucleosis). Virtually everyone has had it although many experience only minor or no symptoms. It is a disease of adolescence (ie. young adults) and is thought to cause brain related problems in approximately 5% of severe cases. This figure includes all brain related problems though, not just encephalitis. The encephalitis which does occur is generally quite mild and recovery is complete in almost all cases.
- Mumps: prior to vaccinating against mumps this was the leading cause of encephalitis in children. It is now a relatively uncommon disease thanks to immunisation. Encephalitis occurs in less than 1% of cases and the outcome is generally good.
- Measles - prior to vaccination 99% of the population had had measles by the age of twenty. Up to 10% died from the illness. Thanks to immunisation measles is now consi