Infectious encephalitis

Research carried out in 2010 found that the most commonly identified causes of infectious encephalitis in England are:

• 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 England, TBE is rare but it can be found in many other European countries.

See the 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

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 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.

See the topic about Rabies for more information.

Post-infectious encephalitis

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:

• measles
• mumps
• influenza (flu) virus – it is thought that seasonal flu and swine flu can trigger post-infectious encephalitis
• varicella zoster virus
• rubella
• Epstein-Barr virus, which is the virus responsible for glandular fever
• cytomegalovirus – a very common infection that usually causes no or very few symptoms
• HIV

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

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.

Chronic encephalitis

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).

Aims of treatment

There are three important objectives in the treating encephalitis. Treatment aims to:

• halt and reverse the process of infection by using anti-viral medication or other appropriate medication
• control any immediate complications of encephalitis, such as seizures or dehydration, that are caused by fever
• prevent long-term complications developing, such as injury to the brain

Most people being treated for encephalitis will be admitted to an intensive care unit (ICU). An oxygen mask will be used to help with breathing. Feeding tubes will provide nutrition and help to keep the body hydrated.

Infectious encephalitis

In England, a medicine called aciclovir is the most widely used treatment for infectious encephalitis. However, it is only effective in treating cases of encephalitis that are caused by herpes viruses.

In encephalitis, the earlier that aciclovir is used the lower the risk of death and long-term complications. Therefore, treatment with aciclovir is usually started before all the diagnostic tests are complete. If tests reveal that encephalitis is being caused by something different, such as another type of infection, the use of aciclovir may be stopped and alternative treatments considered.

Aciclovir works by directly attacking the deoxyribonucleic acid (DNA) inside viral cells. This stops the virus from reproducing and spreading further into the brain. It is given intravenously via a tube into the body, usually three times a day for 14 to 21 days.

Common side effects of aciclovir include:

• headache
• nausea (feeling sick)
• vomiting (being sick)
• diarrhoea

Less common side effects include:

• liver damage
• hallucinations (seeing things that are not real)
• a decrease in the number of white blood cells that are produced by the bone marrow, which can make you more vulnerable to infection

If you fail to respond to treatment with aciclovir, or you have had a previous allergic reaction after taking the medication, an alternative anti-viral medication may be considered.

Post-infectious encephalitis

Post-infectious encephalitis is usually treated with injections of high-dose corticosteroids. This may last several days, depending on the severity of the condition.

Corticosteroids work by calming the immune system (the body’s natural defence against infection and illness). This reduces the levels of inflammation inside the brain.

Some people’s symptoms may improve a few hours after treatment. But in most cases it will take a few days before symptoms start to improve.

Side effects of corticosteroids include:

• nausea and vomiting
• indigestion
• increased appetite
• skin irritation at the site of the injection
• rapid mood changes, such as feeling very happy one moment and very depressed the next

See the Health A-Z topic about corticosteroids for more information.

If your symptoms fail to respond to treatment with corticosteroids, an additional medication called  immunoglobulin therapy may be used. This is a type of donated blood product that contains a number of specific antibodies, which help to regulate the abnormal function of the immune system.

If your symptoms do not improve despite having treatments such as those described above, a therapy called plasmapharesis may be considered. Plasmapharesis involves removing the blood and discarding the parts that contain antibodies. After the blood has been treated, it is returned to your body.

Autoimmune encephalitis

Autoimmune encephalitis can be treated with corticosteroids, immunoglobulin therapy and plasmapharesis. An additional medication known as an immunosuppressant may also be recommended.

As the name suggests, immunosuppressants suppress the workings of your immune system, which should help to prevent your immune system from attacking healthy tissue.

Ciclosporin is a widely used immunosuppressant in the treatment of autoimmune encephalitis.

Common side effects of ciclosporin include:

• numbness or tingling in different parts of your body
• loss of appetite
• nausea and vomiting
• high blood pressure
• headaches
• tremor (uncontrollable shaking or trembling)
• muscle pains or cramps
• increased growth of hair on your skin

Chronic encephalitis

There is currently no cure for the subacute sclerosing panencephalitis type of chronic encephalitis. Anti-viral medication can slow the progression of the disease but this condition inevitably proves fatal within 1-2 years after receiving a diagnosis.

The recommended treatment for progressive multifocal leukoencephalopathy type of chronic encephalitis will usually depend on what is causing the immune system to weaken.

If the immune system is weakened due to a complication of a treatment such as chemotherapy or taking immunosuppressants, then treatment may have to be temporarily withdrawn.

If your immune system is weakened due to a HIV infection, a type of medication known as highly active antiretroviral therapy may be effective. For more information see HIV – treatment

MMR vaccine

The most effective way to protect against encephalitis is to make sure that you receive the MMR vaccine (for measles, mumps and rubella).

The first MMR vaccination should be given to all children at around 13 months of age, with a booster dose given before they start school (between three and five years old). Between 5 and 10% of children are not fully immune after the first dose, so the booster jab increases protection and results in less than 1% of children remaining at risk.

Other vaccinations

Vaccinations are also available for Japanese encephalitis and tick-borne encephalitis, which are two types of encephalitis that are spread by insects (for more information see encephalitis - causes).

In England, these types of encephalitis are virtually non-existent, but they are widespread in certain parts of the world, such as in Africa and Asia. Therefore it is a good idea to discuss vaccinations for these types of encephalitis with your GP before travelling to these areas.

See the topics about Japanese encephalitis - vaccination and tick-borne encephalitis - prevention for more information and advice about taking precautionary measures against these types of encephalitis.