Encephalitis simply means inflammation of the brain.  It is usually caused by a viral infection but can also be caused by parasites, bacteria and fungi.  Inflammation of the brain can also result from metabolic disturbances, be a side effect of some drugs and a symptom of a type of cancer.

They are two mechanisms by which a viral infection can cause encephalitis:

• Direct - invasion of the brain parenchyma (substance) by a virus, example herpes simplex encephalitis
• Indirect - inflammation triggered by the immune system, example is ADEM

A number of different viruses can cause encephalitis, examples include the herpes simplex virus (in the UK), japanese virus (in the far east), west nile virus (in Africa and North America) and tic-borne (in forested areas of Europe). Others include herpes simplex types 5 and 6 (in children), varicella-zoster virus (chicken pox), Enteroviruses (tummy bugs), Influenzaviruses, Measles, Mumps and Rubella. However in most cases in the UK the causative agent is not found.

Some types of encephalitis have very distinctive patterns and include Rasmussen’s encephalitis, Brain Stem Encephalitis, Hashimoto’s encephalitis and SSPE (Sub-Sclerosing Pan Encephalitis).

Acute Disseminated encephalomyelitis (ADEM) is also called post-infectious encephalitis or para-infectious encephalitis. Nerve cells themselves are not the major target of damage but rather their "electrical insulation coat" (myelin sheath) which results in abnormal brain function.

Last modified: 27/06/2006