Infection within the brain, usually caused by a virus although it may also be the result of bacterial infection, for example in conjunction with MENINGITIS. It can occur as a complication of common viral disease such as measles, mumps, glandular fever, or chickenpox. It may occur with no evidence of infection elsewhere in the body apart from the skin, such as in HERPES SIMPLEX encephalitis, the most common form seen in Europe and America. RABIES is another form of viral encephalitis as is the HIV virus which causes AIDS can produce encephalitis (see AIDS/HIV). In some countries – North and South America, Japan, East Asia and Russia – there may be epidemics spread by the bite of mosquitoes or ticks.

The clinical features begin with influenza-like symptoms – aches, temperature and wretchedness; the patient then develops a headache with drowsiness, confusion and neck stiffness. Severely ill patients develop changes in behaviour, abnormalities of speech, and disorientation, sometimes with epileptic seizures. Some develop paralysis and memory loss. CT (see COMPUTED TOMOGRAPHY (CT)) and MRI (see MAGNETIC RESONANCE IMAGING (MRI)) brain scans show brain swelling, and damage to the temporal lobes if the herpes virus is involved. ELECTROENCEPHALOGRAPHY (EEG), is abnormal. Diagnosis is possible by an examination of the blood or other body fluids for antibody reaction to the virus, with modern laboratory techniques being very specific.

In general, drugs are not effective against viruses – antibiotics are of no use. Herpes encephalitis, however, does respond to treatment with the antiviral agent, ACICLOVIR. Fortunately, most sufferers from encephalitis make a complete recovery, but some are left severely disabled with physical defects, personality and memory disturbance, and epileptic fits. Rabies encephalitis is always fatal, and the changes found in patients with AIDS are almost always progressive. Except in very specific circumstances, immunisation against encephalitis is not possible.

Encephalitis lethargica is a rare variety that reached epidemic levels after the First World War. It was characterised by drowsiness and headache leading to COMA, sometimes lasting months or even years. The disease occasionally occurs as a complication after mumps and sometimes affected individuals subsequently develop postencephalitic PARKINSONISM.