Encephalitis is usually the result of an infection. In many cases this is caused by a virus, but often no cause is found.
The most commonly identified causes of encephalitis in the UK are:
In rare cases, a bacterial or fungal infection is the cause of encephalitis.
However, no cause can be identified in more than 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.
Usually, the brain is protected from infections by a thick membrane barrier. In most cases this barrier prevents foreign substances from entering the brain, which is why encephalitis or other types of nervous system infections, such as meningitis, are so rare.
However, in a small number of people the infection can pass through the barrier and infect the brain tissue, affecting normal brain function. Further brain damage can occur as the brain swells and presses against the hard inside surface of the skull.
If left untreated, encephalitis can result in a coma and can be fatal.
Animal-related infectious encephalitis
It is possible to develop some types of encephalitis by coming into contact with infected animals. Three of the more common types are briefly described below.
Tick-borne encephalitis (TBE)
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. Read more about tick-borne encephalitis.
In rare cases, an infection called Lyme disease - which is spread by ticks in the UK - can cause encephalitis. Read more about Lyme disease.
Japanese encephalitis (JEV)
Japanese encephalitis is a viral infection spread by mosquitoes. It occurs throughout southeast Asia, the Far East and the Pacific islands. People involved in farming in these parts of the world are most at risk. Read more about Japanese encephalitis.
Rabies is a very serious type of encephalitis usually spread when a person is bitten or scratched by an infected animal.
All native UK animals are thought to be free of rabies, with the exception of a single species of bat. Most cases of rabies occur in Africa and Asia, with half of all cases occurring in India. Read more about rabies.
Problems with the immune system
In cases of autoimmune encephalitis and post-infectious encephalitis, the condition is caused by a problem with the immune system (the body's natural defence against infection).
This occurs when the immune system attacks healthy tissue in the brain that it mistakes for a threat, which then causes the brain to become inflamed and swell.
It is often not clear exactly 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 may be a rare complication of some common infections, such as:
In rare cases, post-infectious encephalitis has developed following vaccination. However, it should be stressed 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.
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) - this is caused by a usually harmless virus known as the JC virus
- chronic progressive HIV encephalitis - the infection is caused by HIV itself
Subacute sclerosing panencephalitis is extremely rare. This is partly due to the fall 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).