Encephalitis

What is encephalitis?

Encephalitis means inflammation of the brain tissue (unlike meningitis which is inflammation of the membranes that cover the brain). Although it may be caused by a bacterial infection, it's usually due to a virus.

In the UK most cases are unrelated or sporadic. Although the underlying infection may be contagious, encephalitis itself cannot be passed from person to person.

Outbreaks or epidemics of encephalitis are more common elsewhere in the world. These outbreaks are often spread via insect bites.

Causes and risk factors

Only certain viruses are able to get through into the central nervous system, and fortunately even in these cases, encephalitis is rare, affecting about one person in 100,000 in the UK each year.

Causes include herpes simplex (the virus that causes cold sores) which usually lies dormant in the nerves until something triggers it to travel along the nerve to the skin and cause a cold sore. However, very occasionally it will travel in the opposite direction up to the brain, resulting in encephalitis. It usually affects the temporal lobes (affecting memory or speech) or the frontal lobes (affecting emotions and behaviour) and can cause severe damage.

Common childhood viruses can also cause the condition, such as:

• Measles. A diffuse encephalitis spreading across several areas of the brain may develop in about one in 1,000 cases, about four to six days after the rash appears. In most cases it's mild, but about 20 per cent have long term damage to the nervous system and in up to ten per cent it’s fatal.
• Chickenpox. Encephalitis is rare and usually mild, developing three to ten days after the onset of infection. It often affects the cerebellum causing problems with balance, co-ordination and speech.
• Rubella. About one in 5,000 affected by rubella go on to develop encephalitis which can be severe. Although fatal in up to 20 per cent, those who survive usually make a good recovery.
• Less common causes can be mumps, Epstein-Barr (the glandular fever virus), HIV, cytomegalovirus (CMV) and various other viral infections.

Some of those types seen more often outside the UK include:

• Australian encephalitis, which is passed from water birds to humans via mosquitoes. Symptoms are usually very mild but can be severe.
• California encephalitis covers a group of viruses carried by mosquitoes and some mammals, but primarily the La Crosse encephalitis which accounts for eight to 30 per cent of all cases of encephalitis in the US. Children are most often affected and in most cases there are minimal or no symptoms.
• Colorado tick fever, transmitted exclusively by female wood ticks.
• Eastern equine encephalitis which is spread from birds to humans and horses via mosquitoes. It mostly causes mild symptoms but in about two per cent of adults the central nervous system is affected and there is a high mortality.
• Japanese encephalitis which is transferred by mosquitoes from infected animals (usually pigs and wading birds) to humans. Cases mostly occur in the rural areas of China and Korea, but also throughout South East Asia. It’s usually mild but can in some cases be fatal.
• Western equine encephalitis.

Symptoms

The symptoms of encephalitis range from minimal to very severe. Initial symptoms may seem like flu, such as fever, headaches, neck stiffness, lethargy and a general sense of being unwell. The headache may grow more severe and vomiting may then develop. In severe cases the fever becomes very high and there may be increasing confusion, drowsiness, and seizures.

The elderly and the very young are most at risk.

Treatment

Treatment is mostly supportive with symptom relief such as painkillers for the headache, and tepid sponging for fever. Antiviral drugs may be given – for example in herpes simplex encephalitis treatment with Acyclovir should be started as soon as the condition is suspected and may reduce the risk of long term disability. In severe cases treatment in an intensive care unit may be needed to manage problems such as brain swelling or seizures.

In mild cases symptoms may settle in a matter of days but in severe cases it can take weeks or months to recover and the affected person may need long term help for complications such as epilepsy, or problems with memory, speech, balance or behaviour.