Research carried out in 2010 found that the most commonly identified causes of infectious encephalitis:
- the herpes simplex virus - the virus that causes cold sores and the sexually transmitted infection (STI), genital herpes
- the varicella zoster virus, which is responsible for chickenpox in children and shingles in adults
However, many studies have discovered that no cause can be identified in over half of all cases of infectious encephalitis. This is thought to be due to the difficulties in diagnosing some types of infection in certain people, rather than the actual absence of an infection.
How the infection enters the brain
There are thought to be two main ways that an infection can spread to the brain. These are via:
- the bloodstream
- the nerves
Usually, the brain is protected from infections by the blood-brain barrier, a thick membrane that prevents foreign substances from entering the brain. In most cases, the blood-brain barrier is very effective, which is why encephalitis or other types of nervous system infections, such as meningitis, are so rare.
However, in a small number people, often for unclear reasons, the infection can pass through the blood-brain barrier and infect the brain tissue. Once an infection has penetrated the blood-brain barrier, it can enter brain cells and damage them, leading to a loss of normal brain function. In addition, 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, eventually, death.
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 of animal-related encephalitis are:
- tick-borne encephalitis
- Japanese encephalitis
- rabies encephalitis
These are briefly described below.
Tick-borne encephalitis (TBE)
Tick-borne encephalitis (TBE) is a viral infection that is spread by tiny blood-sucking parasites called ticks. In Ireland, TBE is rare but it can be found in many other European countries.
See the Health A-Z topic about tick-borne encephalitis for more information.
Japanese encephalitis (JEV)
Japanese encephalitis is a viral infection that is spread by mosquitoes. The condition occurs throughout South East Asia, the Far East and the Pacific islands. People who are involved in farming in these parts of the world are most at risk.
See the Health A-Z topic about Japanese encephalitis for more information.
Rabies is a very serious type of encephalitis that is usually spread when a person is bitten or scratched by an infected animal.
All native Irish animals are thought to be free of rabies. Most cases of rabies occur in Africa and Asia, with half of all cases occurring in India.
See the Health A-Z topic about Rabies for more information.
Post-infectious encephalitis is thought to occur as a complication of a number of common infections.
Days, or possibly weeks, after the initial infection, the immune system (the body's natural defence against infection) starts to produce antibodies that are meant to fight the infection. But instead they are sent to the brain where they cause inflammation of the brain's tissue. Exactly why the immune system malfunctions in this way is unclear.
Some of the infections that may result in post-infectious encephalitis include:
In some very rare cases, post-infectious encephalitis has been known to develop after a person has been vaccinated.
However, it should be stressed that the risk of someone developing post-infectious encephalitis as a result of being vaccinated is far outweighed by the risk of them developing the condition by not being vaccinated.
Autoimmune encephalitis is occurs when the immune system mistakes substances inside the brain as a threat and starts attacking them.
The immune system sends infection-fighting antibodies to the brain, which causes the brain to swell. The swelling then disrupts the normal working of the brain. This can lead to changes in a person's thinking and behaviour, such as:
- delusions - believing things that are obviously untrue
- paranoia - a belief that people are 'out to get you' and are actively plotting against you
Some cases of autoimmune encephalitis are caused by the immune system reacting to the presence of a tumour (an abnormal growth) inside the body.
In many cases, no reason can be found as to why the immune system is attacking the brain.
There are two main types of chronic encephalitis:
- subacute sclerosing panencephalitis - the inflammation occurs as a complication of a measles infection
- progressive multifocal leukodystrophy (PML) - the inflammation is caused by a usually harmless virus known as the JC virus
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).