Encephalitis - Causes of encephalitis

Encephalitis is usually the result of an infection. In many cases this is caused by a virus, but often no cause is found. 

In the UK, the most common viruses linked to encephalitis are:

• the herpes simplex viruses, which cause cold sores and genital herpes
• the varicella zoster virus, which is responsible for chickenpox and shingles

In rare cases, encephalitis is caused by a bacterial or fungal infection.

However, no cause can be identified in over half of all cases. This is thought to be due to the difficulties in diagnosing some types of infection in certain people, rather than the absence of an infection.

How the infection enters the brain

There are thought to be two main ways an infection can spread to the brain – the bloodstream and the nerves.

The brain is usually protected from infections by a thick membrane. In most cases, the membrane acts as a barrier by preventing foreign substances entering the brain. This is why encephalitis or other types of nervous system infections, such as meningitis, are so rare.

In a small number of people, an infection can pass through the barrier and into the brain tissue, affecting normal brain function. Further brain damage can occur as the brain swells and presses against the inside of the skull.

If left untreated, encephalitis can result in a coma and can be fatal.

Animal-related infectious encephalitis

It's possible to develop some types of encephalitis by coming into contact with infected animals. Three of the more common types are described below.

Tick-borne encephalitis

Tick-borne encephalitis (TBE) is a viral infection spread by tiny blood-sucking parasites called ticks. TBE is rare in the UK, but can be found in many other European countries.

In rare cases, an infection called Lyme disease, which is spread by ticks in the UK, can cause encephalitis.

Japanese encephalitis

Japanese encephalitis (JEV) is a viral infection spread by mosquitoes. It occurs throughout southeast Asia, the Far East and the Pacific islands. People in these parts of the world who work on farms are most at risk.

Rabies

Rabies is a very serious type of encephalitis you can get if you're bitten or scratched by an infected animal.

Most animals native to the UK are thought to be free of rabies, with the exception of a single species of bat. The majority of rabies cases occur in Africa and Asia, with half of all cases occurring in India.

Immune system problems

Autoimmune encephalitis and post-infectious encephalitis are caused by a problem with the immune system (the body's natural defence against infection).

The immune system mistakes healthy tissue in the brain as a threat and attacks it, causing the brain to become inflamed and swell. It's not always clear why the immune system malfunctions in this way.

Some cases of autoimmune encephalitis are caused by the immune system reacting to the presence of a tumour (an abnormal growth) inside the body.

Post-infectious encephalitis can be a rare complication of some common infections, such as:

• measles
• mumps
• influenza (flu)
• varicella zoster virus
• rubella
• Epstein-Barr virus, which causes glandular fever
• cytomegalovirus – a common infection that usually causes few or no symptoms
• HIV

In rare cases, post-infectious encephalitis has occurred following vaccination. However, it's important to stress that the risk of developing post-infectious encephalitis as a result of being vaccinated is far outweighed by the risk of developing a condition by not being vaccinated.

Chronic encephalitis

There are several types of chronic encephalitis including:

• subacute sclerosing panencephalitis – the inflammation occurs as a complication of a measles infection
• progressive multifocal leukodystrophy (PML) – caused by a usually harmless virus known as the JC virus
• chronic progressive HIV encephalitis – caused by HIV itself

Subacute sclerosing panencephalitis is extremely rare. This is partly due to the decrease in measles cases as a result of the MMR vaccine.

PML is also quite rare. It mainly only affects people with a severely weakened immune system due to factors such as having an end-stage HIV infection (AIDS).