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Japanese encephalitis


Japanese encephalitis is a type of viral brain infection that is spread through mosquito bites. It’s common in rural areas throughout Southeast Asia, the Pacific islands and the Far East.

The virus is found in pigs and birds, and is passed to mosquitoes that bite the infected animals. It’s more common in rural areas where there are pig farms and rice fields. It cannot be spread from human to human.

Read more about the causes of Japanese encephalitis.

Most people who are infected by the Japanese encephalitis virus do not develop any symptoms, or they get only mild, flu-like symptoms.

However, around one person in 250 infected by the virus has serious and severe symptoms, including:

  • being sick
  • seizures (fits)
  • headache
  • mental confusion

The virus can cause swelling inside the brain (encephalitis), leading to increased pressure in the brain. This can cause permanent brain damage. Around one in three cases of encephalitis is fatal.

Read more about the symptoms of Japanese encephalitis.


There is currently no cure for Japanese encephalitis so treatment involves supporting the functions of the body as it tries to fight of the infection. This will usually involve admission to hospital.

A medication called mannitol can be used to reduce pressure inside the brain, which can sometimes prevent permanent brain damage.

Read more about the treatment of Japanese encephalitis.

Who is affected?

It’s rare for travellers from outside the region to be affected by Japanese encephalitis. It is estimated that only one in 1 million travellers will develop Japanese encephalitis in any year.

People who live and work in rural areas in parts of the world where the condition is widespread are most at risk. Around 9 out of 10 cases involve children under the age of 15 years.

There are an estimated 70,000 new cases every year, half of which occur in China.


Two vaccines are available for Japanese encephalitis. Vaccination is normally recommended if:

  • You are planning to spend a month or longer in a high-risk country during the rainy season.
  • You are planning to visit any rural areas in high-risk countries, even if only for a short time.
  • You will be taking part in any activities that may increase your risk of becoming infected, such as visiting rice fields or pig farms, cycling or camping.

You will normally have to pay for the vaccine, which costs around £160–180 for the complete course.

Read more about vaccines for Japanese encephalitis.

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In most cases, the symptoms of Japanese encephalitis (JE) are mild. There may be no symptoms at all. However, in about 1 in 200 cases, the symptoms of JE are severe.

The virus affects the membranes around the brain. After a bite from an infected mosquito, it will take between 5 and 16 days for symptoms to appear.

Mild symptoms

For most people, JE has no symptoms or only mild, flu-like symptoms, such as headache or slightly high temperature.

Severe symptoms

More serious symptoms of JE include:

  • high temperature (fever) of 38°C (100.4°F) or over,
  • headache,
  • vomiting,
  • neck stiffness,
  • confusion,
  • feeling weak,
  • coma (a sleep-like state when someone is unconscious for a long time),
  • seizures (fits or convulsions),
  • focal neurological deficits (a problem with a nerve in a specific location, such as the tongue),
  • movement problems, such as shaking, stiffness, slowness of movement or paralysis, and
  • meningitis (an infection of the meninges, which are the protective membranes that surround the brain and spinal cord).

These symptoms can appear suddenly and get rapidly worse. In around 1 in 300 cases of JE, the symptoms develop into encephalitis (inflammation of the brain). The rate of death among people who develop severe symptoms is around 30%.

As JE is caused by bites from infected mosquitoes, outbreaks of JE are usually linked to the mosquito population. A sudden increase in the mosquito population could cause a sudden outbreak of JE in humans. In some cases, the fatality rate during such an outbreak could be as high as 60%, particularly if many young children are affected.

Around 30% of those who survive a severe case of JE may be left with permanent damage to their central nervous system (the brain, nerves and spinal cord), such as:

  • paralysis (loss of strength in a muscle so that you are unable to move it),
  • ataxia (loss of physical coordination), or
  • speech difficulties.
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Japanese encephalitis is caused by a flavivirus. This type of virus can affect both animals and humans. The virus is passed on to humans from animals through the bite of an infected mosquito.

Pigs and wading birds are the main carriers of the Japanese encephalitis virus. A mosquito becomes infected after sucking the blood from an infected animal or bird. If an infected mosquito bites you, it can pass the virus on to you. The virus cannot be passed from person to person.

Risk factors

Several factors can increase your risk of catching Japanese encephalitis, including:

  • the areas you visit
  • the time of year you visit
  • the activities you do

High-risk countries

Japanese encephalitis is found throughout Asia and beyond. The area in which it is found stretches from the Western Pacific islands in the east, such as Fiji, across to the borders of Pakistan in the west. It is found as far north as Korea and as far south as Papua New Guinea.

Most cases occur in

  • China
  • Myanmar (Burma)
  • Thailand
  • Vietnam
  • Cambodia
  • Laos
  • Nepal
  • India
  • Philippines
  • Sri Lanka
  • Malaysia
  • Indonesia

Despite its name, Japanese encephalitis is now relatively rare in Japan because of mass immunisation programmes.

Cases of Japanese encephalitis have also occurred on the Torres Strait Islands off the north coast of Australia. It is thought that migrating birds have introduced the disease to the area and it is now found in pigs there.

The mosquitoes that carry Japanese encephalitis usually breed in rural areas, particularly where there are flooded rice fields or marshes. The infected mosquitoes have also been found in urban areas. They usually feed between sunset and sunrise.

Rainy seasons

A sudden increase in the mosquito population, which can occur during the rainy season, could cause a sudden outbreak of Japanese encephalitis in humans:

  • May to September is the peak time for cases of Japanese encephalitis in Korea and Japan.
  • April to October is the peak time for cases of Japanese encephalitis in areas of Southeast Asia, such as Thailand, Cambodia and Vietnam.
  • September to December is the peak time for cases of Japanese encephalitis in Nepal and India.
  • Japanese encephalitis occurs all year round in Malaysia, Indonesia and the Philippines, where rain falls throughout the year.

This pattern can vary. For example, infected birds can change their migration patterns, so cases of Japanese encephalitis may be seen at other times of the year or in different places.

High-risk activities

If you are planning a short visit to Asia, the risk of contracting Japanese encephalitis is very low, particularly if you are going to be staying in urban areas. It is estimated that there is one case of Japanese encephalitis for every 1 million travellers.

Activities that may increase your risk of developing Japanese encephalitis include:

  • living or travelling in high-risk areas for a long time
  • visiting rural areas, particularly during the rainy season
  • fieldwork, camping or cycling in rural areas

Among travellers who visit rural areas and stay for long periods, the risk of catching Japanese encephalitis may increase. For example, one estimate has suggested that there could be up to two cases a week for every 100,000 travellers.

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If you have any of the symptoms of Japanese encephalitis and have recently visited or are still in a country where it is a risk, get medical help immediately.

For information about how to seek medical help abroad, read our travel health topic or the advice on the Foreign & Commonwealth Office website. If you are back in the UK, see your GP.

Your GP or the healthcare professional treating you will ask about your symptoms, what you did on your trip and what vaccinations you have had.


Your GP or healthcare professional will take a sample of blood from your arm and test it for antibodies. Antibodies are proteins that are produced by the body to neutralise or destroy disease-carrying organisms.

If you have been infected with the Japanese encephalitis virus, your body will start to produce a specific type of antibody to destroy the virus.

Around seven days after the start of your symptoms, the antibody should be noticeable in your blood.

The antibodies are also present in your cerebrospinal fluid (CSF), which surrounds your brain and spinal cord. In some cases, a sample of CSF may be taken and tested instead.

A sample of CSF is taken using a procedure called a lumbar puncture or spinal tap. A hollow needle is inserted between the vertebrae (back bones) in your lower spine and some of the fluid is drawn out. 


If your GP suspects that Japanese encephalitis may have spread to your brain, they may recommend a brain scan to check for any damage and to confirm the diagnosis. This could be:

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There is no specific treatment for Japanese encephalitis, although measures can be taken to control your symptoms and prevent complications from developing.

If your symptoms are severe, you may be admitted to hospital, often to an intensive care unit (ICU) or high dependency unit. The healthcare professionals treating you will try to reduce your high temperature and ease any pain.

You will be given an oxygen mask to help you breathe and a feeding tube to provide you with nutrition and keep you hydrated.

If you develop swelling inside your brain, you may be given an injection of a medication called mannitol.

This medication causes water to flow out of brain cells into surrounding blood vessels. This helps reduce pressure inside the brain, which can sometimes help reduce the risk of permanent brain damage.

Before leaving the UK, make sure you know how to access medical treatment in the country you are visiting.

Read about travel health for more information and advice.

The Foreign & Commonwealth Office website provides comprehensive travel advice by country, including information about UK embassies and consular offices. It also has a travel checklist you should use before you leave.

Make sure you have travel insurance that covers both the cost of medical treatment in the country of your visit as well as the potential cost of flying you back to Wales for treatment.

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A vaccine against Japanese encephalitis is available for people who are visiting a country where it is a risk. After a full course of the vaccine, you will have around 98% protection against Japanese encephalitis. This reduces to around 83% after 12 months.

For up-to-date information about which countries are considered high risk for Japanese encephalitis, check the NHS Fit for Travel website or the National Travel Health Network and Centre (NaTHNaC) website.

If you are travelling to a country where the vaccination is recommended, visit your GP or practice nurse at least six to eight weeks before you leave. They will discuss your trip with you and determine whether you should have the vaccination.

When is the vaccine recommended?

The Japanese encephalitis vaccine is recommended if:

  • You are planning to spend a month or longer in a high-risk country during the rainy season.
  • You are planning to visit any rural areas in high-risk countries, even if only for a short time.
  • You will be taking part in any activities that may increase your risk of becoming infected, such as visiting rice fields or pig farms, cycling, camping or fieldwork. 

The vaccine

Two vaccines for Japanese encephalitis are currently available in the UK:

  • Green Cross

Both vaccines are given as a course of injections. Ideally, this should be completed at least one month before you travel. This gives enough time for your body to develop full immunity (resistance to the virus).

IXIARO is given as two injections, with the second dose given 28 days after the first. A booster dose is required after 12 months. This vaccine is suitable for people who are 18 years of age and over.

Green Cross is given as three injections. The second dose is given seven days after the first, and the third dose is given 28 days after the first. A booster dose is required after 12 months, then once a year if you are still at risk of catching Japanese encephalitis. This vaccine is suitable for children who are over one year of age.

The Green Cross vaccine must be completed at least 10 days before you travel because an allergic reaction to the vaccine can take this long to show up.

After each injection, you will be monitored for 30 minutes to check for any side effects (see below).

Rapid course

If there is not enough time before you travel to complete a full course as described above, you may be able to have a rapid course of the vaccination.

You can receive three doses of the Green Cross vaccine with seven days between each dose, or you can receive two doses seven days apart with a booster dose three months later.

The two dose rapid course of the Green Cross vaccine will provide less protection than the three dose course. These will still need to be completed at least 10 days before you leave in case you have a reaction.

Side effects

Some people get side effects after having the vaccine. Between 10% and 20% of people may experience:

  • soreness, redness or swelling at the site of the injection
  • a high temperature of 38C (100.4F) or over
  • dizziness
  • headache
  • feeling sick
  • vomiting
  • abdominal pain
  • aching muscles

More serious side effects occur in around 0.6% of people. These include:

  • urticaria or hives (a raised, itchy red rash)
  • swelling of the face
  • difficulty breathing

This type of reaction usually occurs within minutes of having the vaccine, although it could appear up to two weeks later.

If you have any of these symptoms, see your GP immediately or call NHS Direct Wales on 0845 4647.

If you have the vaccine while you are travelling abroad, stay in an area where you have access to a doctor or hospital for at least 10 days in case you have a bad reaction.

When is the vaccine not recommended?

The Japanese encephalitis vaccine is not suitable for children under one year of age.

If you have previously had a severe allergic reaction (anaphylaxis) to a Japanese encephalitis vaccine or are allergic to any of the ingredients in the vaccine, do not have it.

You may be more at risk of having an adverse reaction to the vaccine if:

  • You have other allergies, such as allergic rhinitis or asthma.
  • You have a condition that affects your nervous system (your brain, spinal cord and nerves) such as multiple sclerosis.
  • You have had convulsions (fits) in the previous year.

In all cases, the risks of catching the Japanese encephalitis virus will be weighed up against the risks of having an adverse reaction to the vaccine.

Will I have to pay for the Japanese encephalitis vaccine?

The vaccination for Japanese encephalitis will cost around £150. It is, therefore, a good idea to include this in the budget for your trip.

Where can I have the vaccine?

The Japanese encephalitis vaccine may be available at your GP surgery. If not, your GP may be able to recommend a specialist vaccination clinic in your area.

What if I’m pregnant?

There is no evidence that the Japanese encephalitis vaccine presents any risks for pregnant women or women who are breastfeeding.

Is there anything else I should do?

Take out adequate travel insurance for the countries you are visiting. Check the advice of the Foreign & Commonwealth Office about what your insurance should cover.

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It can be hard to predict the long-term effects of having this type of brain infection. They can range from full recovery to being severely disabled.

One study from Malaysia, which looked at just over 100 people who were admitted to hospital for Japanese encephalitis, found that:

  • 41% of patients made a full recovery
  • 3% had mild disability
  • 26% had moderate disability
  • 31% had severe disability

Mild disability can take the form of:

  • uncontrollable shaking of the hands
  • changes to personality
  • muscle weakness and twitching in the arms and hands

Moderate disability can take the form of:

  • mild learning difficulties
  • paralysis of a single limb
  • weakness on one side of the body

Severe disability after Japanese encephalitis usually takes the form of a severe learning difficulty as well as muscle spasms and stiffness of the arms and legs.

The Encephalitis Society is a charity that provides information and support for people affected by encephalitis.

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The vaccination against Japanese encephalitis is not completely effective, so you should also protect yourself against mosquito bites in high-risk areas.

Follow the advice below to prevent being bitten by infected mosquitoes:

  • If possible, sleep in rooms with close-fitting gauze over the windows and doors.
  • If this is not possible or you are sleeping outside, use mosquito nets that have been impregnated with an insecticide, such as permethrin. 
  • Spray the room with insecticide in the early evening to kill any mosquitoes that have got in during the day.
  • Mosquitoes that carry the Japanese encephalitis virus are usually most active at dusk and enjoy warm, humid conditions. If you go outside after sunset, cover up with long-sleeved tops, trousers and socks.
  • Mosquitoes can bite through skin-tight clothing so, if possible, wear loose-fitting clothes.
  • Apply a good-quality insect repellent to any exposed areas of skin.

Insect repellent

Various types of insect repellent are available. Many contain diethyltoluamide (DEET). If you are allergic to DEET, other insect repellents are available, including dimethyl phthalate (contained in Shoo!) or eucalyptus oil (contained in Mosi-Guard).

When using insect repellent, follow the guidelines below:

  • Do not use the insect repellents on cuts, wounds or irritated skin.
  • Do not get insect repellent in your eyes, mouth and ears.
  • Do not spray the insect repellent directly onto your face. Spray it onto your hands and then apply it to your face.
  • Do not allow young children to apply insect repellent themselves. Put it on your hands and then apply it to your child.
  • Wash your hands thoroughly after applying insect repellent.
  • Wash off the repellent with soap and water when it is no longer needed.
  • Always follow the manufacturer's instructions.

If you or your children have an adverse reaction to insect repellent, such as redness, stop using it. Wash it off and contact your GP or a local healthcare professional if you are abroad.

Is DEET safe?

DEET is considered very safe when used according to the manufacturer’s instructions:

  • Do not spray directly onto your face.
  • Use only on exposed skin.
  • Avoid contact with lips and eyes.
  • Wash hands after application.
  • Do not apply to cuts, abrasions, broken or irritated skin.
  • Wash off with soap and water when it is no longer needed.
  • Avoid sunscreen containing repellent.
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