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Encephalitis is an inflammation of the brain, which is usually due to infection of the brain by a virus. Only certain viruses have the capability to affect nerves and gain access to the central nervous system, and in most cases an infection by one of these viruses will not lead to encephalitis. Infection can involve one area of the brain (focal) or many scattered areas (diffuse). Encephalitis is a rare condition that affects approximately one person in every 100,000 in the UK each year.

What viruses cause encephalitis?

The viruses that commonly produce isolated or unrelated (sporadic) cases of viral encephalitis are:
• The herpes simplex virus (also causes cold sores)
• The measles virus
• The varicella virus (chickenpox)
• The rubella virus (German measles)
• Less commonly it is produced by the mumps virus, the Epstein-Barr virus(glandular fever), echovirus, coxsackie, HIV and CMV.

Herpes simplex encephalitis (HSE)

An infection with the virus that causes the common cold sore can very occasionally cause encephalitis or meningitis. Herpes simplex is one of the viruses that can affect nerves and remain dormant in the nerves for some time. The encephalitis is thought to be due to herpes simplex tracking along the olfactory nerves from the nose to the brain. Herpes simplex encephalitis affects the temporal lobes and less commonly the frontal lobes of the brain. The temporal lobes are the areas that control memory and speech and the frontal lobes control emotion and behaviour. If the inflammation and infection is severe, it can cause irreversible damage to part of the brain due to nerve cell death.

Measles

Encephalitis can occur in approximately one in a thousand cases of measles, usually four to six days after the start of the rash. The virus causes diffuse encephalitis. Most cases are mild, but 20 per cent of patients will be left with some complications affecting the nervous system. The disease is fatal in five to 10 per cent of cases.

Chickenpox

Encephalitis occurs rarely between the third and tenth days of infection with chickenpox. The virus affects the brain diffusely, but seems to particularly involve the cerebellum, producing problems with balance, coordination and speech. Infection is usually milder than with measles.

German measles

Rubella causes encephalitis in approximately one in 5000 cases. The encephalitis is diffuse and can be severe. Ten to 20 per cent of rubella encephalitis cases are fatal, but over 75 per cent of patients make a full recovery.

Other viruses

Most other common viruses can occasionally cause encephalitis. In acquired immuno-deficiency syndrome (AIDS), encephalitis due to the HIV virus itself, or more rarely, due to cytomegalovirus (CMV) occurs in more than 20 per cent of affected people.

Can the infection be passed on to others?

The common viruses which can cause encephalitis can be passed from person to person, but encephalitis is sporadic and cannot be passed on to others by direct contact. Epidemics of encephalitis do occur outside the UK, but these are due to the spread of the virus by insect bites.