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Page last reviewed: 13/07/2011

Encephalitis is inflammation of the brain tissue. It is caused either by infection, which is usually viral, or by autoimmune conditions (conditions that cause the immune system to malfunction and attack healthy tissue).

Encephalitis is a complex condition that has a number of possible causes. It is often referred to as a 'rare complication of common infections'.

The initial stage of encephalitis is usually serious and severe, and it is often followed by an injury to the brain (which can vary in degree and severity). Encephalitis usually begins with flu-like symptoms, such as:

  • high temperature (fever) of 38ºC (100.4ºF) or above
  • headache
  • nausea and vomiting
  • joint pain

More serious symptoms may then begin to develop, including:

  • changes in mental state, such as confusion, drowsiness or disorientation
  • seizures (fits)
  • changes in personality and behaviour

See encephalitis - symptoms for more information.


Encephalitis is usually regarded as a medical emergency, and it typically requires immediate admission to an intensive care unit.

Treatment options for encephalitis include:

  • anti-viral medication
  • steroid injections (the medical term for this is intravenous corticosteroids)
  • immunosuppressants (medicines that stop the immune system from attacking healthy tissue)

For more information, see encephalitis - treatment.


However, even with the best available treatment, deaths from encephalitis remain high. Some forms of encephalitis will kill as many as 1 in 3 people with the condition.

The inflammation that is associated with encephalitis can also result in an acquired brain injury, which can lead to long-term complications. These can include:

  • memory loss
  • epilepsy - a condition that causes repeated seizures
  • personality and behavioural changes
  • developmental difficulties
  • problems with attention, concentration, planning and problem solving
  • physical and motor difficulties
  • fatigue (extreme tiredness)

For more information see encephalitis - complications.

How common is encephalitis?

All types of encephalitis are relatively rare.

People of all ages and both sexes can be affected by encephalitis, but the very young and the very old are most at risk because their immune system (the body's natural defence against infection and illness) tends to be weaker.


A number of vaccinations can help protect your child against encephalitis. The MMR vaccine is the best way to protect your child against it. Before the MMR vaccine was introduced, measles, mumps and rubella were leading causes of encephalitis in children.

See encephalitis - prevention and the Health A-Z topic about MMR for more information and advice.

Types of encephalitis

Encephalitis can be broadly divided into four main types:

  • infectious encephalitis – the inflammation occurs as a direct result of an infection, which is often viral
  • post-infectious – the inflammation caused by the immune system reacts to a previous infection or vaccine; this can occur many days, weeks or sometimes months after the initial infection
  • autoimmune – the inflammation is caused by the immune system reacting to a non-infectious cause, such as a tumour or antibodies
  • chronic – the inflammation develops slowly over many months; it can be the result of a condition such as HIV, though in some cases there is no obvious cause

See encephalitis - causes for more information.

Page last reviewed: 13/07/2011

Encephalitis is inflammation of the brain which is often a life-threatening. It usually begins with flu-like symptoms such as:

  • high temperature (fever) of 38ºC (100.4ºF) or above
  • headache
  • nausea and vomiting
  • joint pain

After this initial stage, more serious symptoms can begin to develop, which may include:

  • changes in mental state, such as confusion, drowsiness or disorientation
  • seizures (fits)
  • changes in personality and behaviour

Other symptoms of encephalitis can include:

  • sensitivity to bright lights (photophobia)
  • inability to speak
  • inability to control physical movements
  • stiff neck
  • hallucinations - seeing and hearing things that are not actually there
  • loss of sensation in certain parts of the body
  • partial or total vision loss
  • involuntary eye movements, such as moving the eyes from side to side
  • involuntary movements of the face, arms and legs

Some types of autoimmune encephalitis can cause changes to your mental health, such as

  • anxiety
  • unusual behaviours
  • delusions - believing things that are obviously untrue
  • paranoia - a belief that people are 'out to get you' and are actively plotting against you

When to seek medical advice

Flu-like symptoms that rapidly get worse and change a person's mental state should be treated as a medical emergency. In such circumstances, you should dial 112/999 immediately and request an ambulance.

Page last reviewed: 13/07/2011

Infectious encephalitis

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

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).

Page last reviewed: 13/07/2011

Accurately and promptly diagnosing suspected cases of encephalitis is essential. Two main tests are used to diagnose encephalitis. They are:

  • brain scans - such as computerised tomography (CT) scans and magnetic resonance imaging (MRI) scans
  • lumbar punctures

These procedures are briefly described below.

Brain scans

CT scans or MRI scans can be used to highlight the extent of brain inflammation and help to distinguish encephalitis from other conditions, such as stroke, brain tumours and aneurysms (a swelling in the wall of an artery).

However, in the early stages of encephalitis, brain scans do not always rule out a diagnosis. This is because the swelling inside the brain that can be detected with a brain scan often only occurs as the symptoms of encephalitis progress to a more advanced stage.

See the Health A-Z topics about CT scan and MRI scan for more information about these procedures.

Lumbar puncture

A lumbar puncture, also known as a spinal tap, is a procedure that is used to test a sample of spinal cord fluid. The fluid, called cerebrospinal fluid (CSF), surrounds your brain and spinal cord and supports and protects them.

A lumbar puncture is performed by inserting a hollow needle into the lower part of your spinal canal so that a sample of CSF can be drawn out. The fluid is subjected to a series of tests, which can help to confirm a diagnosis of encephalitis, such as checking it for the presence of infection.

See the Health A-Z topic about lumbar puncture for more information about the procedure.

Other tests

Other tests that are occasionally used to help confirm or rule out a diagnosis of encephalitis include the laboratory screening of blood, urine and other body fluids. Screening these substances can help to detect and identify infections of the brain or spinal cord. Results from these tests can also help to exclude other conditions that mimic encephalitis. 

Page last reviewed: 13/07/2011

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

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:

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

Page last reviewed: 13/07/2011

Some people will make a good recovery after having encephalitis, particularly if they received a prompt diagnosis and treatment. However, in some cases, a person will develop one or more long-term complications due to the underlying injury to the brain.
The most common complications that occur following encephalitis are:

  • memory problems, which affect 70% of people who have complications
  • personality and behavioural changes, which occur in just under half of all people
  • aphasia - speech and language problems, which occur in around 1 in 3 people
  • epilepsy, which occurs in 1 in 4 affected adults and 1 in 2 affected children
  • changes in emotions, such as anxiety and anger; mood swings are also common
  • problems with attention, concentrating, planning and problem solving
  • physical and motor difficulties
  • low mood and a sense of feeling different
  • fatigue (extreme tiredness)

Individual care plans

Due to the complications described above, if you are recovering from encephalitis you may need specialised services, including:

  • a neuropsychologist - a healthcare professional who specialises in brain injury and cognitive rehabilitation
  • an occupational therapist - they can identify problem areas in a person's everyday life, such as dressing or getting to the shops, and help them to work out practical solutions
  • physiotherapist - a therapist who uses physical methods, such as massage, manipulation and exercise, to promote health and wellbeing
  • a speech and language therapist - they use specialist techniques to improve all aspects of communication

Before being discharged from hospital, your health and social care needs will be fully assessed. An individual care plan that is designed to meet those needs will be drawn up for you.

If you are the primary carer of someone who is recovering from encephalitis, such as their spouse or parent, you should be invited to take part in discussions about the care plan, and your own circumstances and requirements should be taken into account. You should also be given information about the support services that are available in your local community.

Seeking further help

Seek additional help if you are experiencing problems after having encephalitis. Many healthcare professionals are unaware of the problems following encephalitis, and it can sometimes be a struggle to find the right help for you.

Page last reviewed: 13/07/2011

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 Ireland, these types of encephalitis are virtually non-existent, but they are widespread in certain parts of the world, such as some countries in Europe, 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 Health A-Z topics about Japanese encephalitis - vaccination and tick-borne encephalitis - prevention for more information and advice about taking precautionary measures against these types of encephalitis.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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