Japanese encephalitis

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Japanese Encephalitis

Overview - Japanese encephalitis

Japanese encephalitis is a viral brain infection that's spread through mosquito bites. It's most common in rural areas in southeast Asia, the Pacific islands and the Far East, but is very rare in travellers.

The virus is found in pigs and birds, and is passed to mosquitoes when they bite infected animals. It cannot be spread from person to person.

There's currently no cure for Japanese encephalitis. Treatment involves supporting the functions of the body as it tries to fight off the infection.

The person usually needs to be admitted to hospital so they can be given fluids, oxygen and medication to treat any symptoms.

Symptoms

Most people infected by the Japanese encephalitis virus have either no symptoms or mild short-lived symptoms, which are often mistaken for flu.

But around 1 in every 250 people who become infected with Japanese encephalitis develop more severe symptoms as the infection spreads to the brain.

This usually happens 5 to 15 days after infection.

Symptoms can include:

  • a high temperature (fever)
  • seizures (fits)
  • a stiff neck
  • confusion
  • the inability to speak
  • uncontrollable shaking of body parts (tremor)
  • muscle weakness or paralysis

Up to 1 in every 3 people who develop these more serious symptoms will die as a result of the infection.

In those who survive, these symptoms tend to slowly improve.

But it can take several months to make a full recovery, and up to half of those who do survive are left with permanent brain damage.

This can lead to long-term problems, such as tremors and muscle twitches, personality changes, muscle weakness, learning difficulties and paralysis in 1 or more limbs.

When to get medical advice

You should get immediate medical advice if you have any of the symptoms of Japanese encephalitis and you have recently visited, or are still in, an area where the infection is found.

GOV.UK has information about who to contact when you need immediate medical help abroad.

If you're already back in the UK, see your GP.

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

If necessary, they may carry out a blood test to see if you have an infection.

How common is Japanese encephalitis?

It's very rare for travellers visiting risk areas to be affected by Japanese encephalitis.

It's estimated less than 1 in a million travellers get Japanese encephalitis in any given year.

The World Health Organization (WHO) estimates there are around 68,000 cases of Japanese encephalitis worldwide each year.

The people most at risk are those who live and work in rural areas, such as on pig farms and in rice fields, where the condition is widespread.

Around 75% of cases involve children under the age of 15.

Find out more about the causes of Japanese encephalitis, and which countries have the highest risk.

Preventing Japanese encephalitis

The best way to prevent Japanese encephalitis is to be vaccinated against the infection before you visit a part of the world where there's a risk of catching it.

The risk is greater if you're planning to visit rural areas or go hiking or camping.

The vaccine, which is usually only available privately, gives protection against Japanese encephalitis in more than 9 out of 10 people who receive it.

Even if you have been vaccinated, you should still take precautions to reduce your risk of being bitten by an infected mosquito, such as:

  • sleeping in rooms with close-fitting gauze over the windows and doors – if you're sleeping outside, use a mosquito net that's been impregnated with insecticide
  • covering up with long-sleeved tops, trousers and socks
  • applying a good-quality insect repellent to exposed areas of skin

Find out more about how to prevent Japanese encephalitis

Accessing healthcare abroad

It's a good idea to keep a list of important telephone numbers with you when travelling abroad.

These should include numbers for:

  • local emergency services
  • a representative of the travel company you booked your visit with
  • your travel insurer
  • the British consulate or embassy in the area you're visiting – GOV.UK has a directory of British consulates and embassies

Find out more about accessing healthcare abroad

Page last reviewed: 1 February 2019
Next review due: 1 February 2022

Causes - Japanese encephalitis

Japanese encephalitis is caused by a flavivirus, which can affect both humans and animals. The virus is passed from animals to humans 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 you get bitten by an infected mosquito, it can pass on the virus.

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

They usually feed between sunset and sunrise.

Japanese encephalitis cannot be passed from person to person.

High-risk countries

Japanese encephalitis is found throughout Asia and beyond.

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 as a result of mass immunisation programmes.

The US Centers for Disease Control and Prevention (CDC) website has a map showing Japanese encephalitis risk areas.

When you're most at risk

The risk of becoming infected with Japanese encephalitis is highest during and just after rainy seasons.

This is because mosquito populations tend to increase suddenly around this time.

There's also a risk of getting Japanese encephalitis in countries with a year-round tropical climate.

TravelHealthPro has more information about the country you're travelling to.

High-risk activities

If you're planning a short visit to Asia, the risk of getting Japanese encephalitis is very low, particularly if you're going to be staying in urban areas.

Overall, it's estimated there's less than 1 case of Japanese encephalitis for every million travellers.

But there are certain activities that can increase your risk of becoming infected, such as:

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

These activities can mean you're more likely to come into contact with infected mosquitoes.

Page last reviewed: 1 February 2019
Next review due: 1 February 2022

Prevention - Japanese encephalitis

You can reduce your risk of getting Japanese encephalitis by being vaccinated and taking precautions to avoid mosquito bites.

Japanese encephalitis vaccination

The vaccine gives protection for more than 9 out of every 10 people who have it.

You should get vaccinated if you're:

  • planning a long stay in a high-risk country (usually at least a month)
  • visiting a high-risk area during the rainy season or where there's a year-round risk because there's a tropical climate
  • visiting rural areas in a high-risk country, such as rice fields, marshlands or somewhere close to pig farms
  • taking part in activities while in a high-risk country that may increase your risk of becoming infected, such as cycling or camping
  • working in a laboratory with potential exposure to the virus

The vaccine is not available on the NHS, so you'll need to pay for it.

You can either go to a private travel clinic or a pharmacy that offers travel healthcare services.

The cost can vary between clinics – each dose can cost more than £90 per person.

The vaccine

There's currently 1 vaccine for Japanese encephalitis permitted for use in the UK. This is for adults and children aged 2 months and older. 

The vaccine is given as an injection. You need 2 doses for full protection. The second dose is given 28 days after the first.

People aged 18 to 64 may be given the second dose 7 days after the first. This is called an accelerated schedule.

Both doses of the vaccine should be completed at least 7 days before you visit an area where there's a risk of Japanese encephalitis.

If you continue to be at risk of infection, a booster dose of the vaccine should be given 12 to 24 months after you're first vaccinated.

Side effects

Up to 40% of people who have the Japanese encephalitis vaccine experience mild and short-lived side effects.

These include:

  • soreness, redness or swelling at the site of the injection
  • a headache
  • muscle pain

More serious side effects, such as a raised, itchy red rash (urticaria or hives), swelling of the face and difficulty breathing, are rare.

If you develop any worrying symptoms after being vaccinated, contact your GP as soon as possible or call NHS 111 for advice.

Precautions

Most people can have the Japanese encephalitis vaccination safely, but you should tell the doctor or nurse before being vaccinated if you have a high temperature (fever), or if you're pregnant or breastfeeding.

The vaccination may need to be postponed if you have a fever.

It may also not be recommended if you're pregnant or breastfeeding as there's a theoretical risk of problems resulting from the vaccine being passed to your baby.

The Japanese encephalitis vaccine is not usually recommended for children less than 2 months old because it's unclear how safe and effective it is for this age group. 

You should not have the vaccine if you have had a severe allergic reaction (anaphylaxis) to it or any of its ingredients in the past.

Avoiding mosquito bites

As the vaccination against Japanese encephalitis is not 100% effective, you should protect yourself against mosquito bites while travelling or staying in at-risk areas by:

  • sleeping in rooms with close-fitting gauze over the windows and doors
  • if this is not possible or you're sleeping outside, use a mosquito net that's been impregnated with an insecticide, such as permethrin
  • spraying your room with insecticide in the early evening to kill any mosquitoes that have got in during the day
  • covering up with long-sleeved tops, trousers and socks – mosquitoes that carry the Japanese encephalitis virus are usually most active at dusk and enjoy warm, humid conditions
  • wearing loose-fitting clothes, as mosquitoes can bite through skin-tight clothing 
  • applying a good-quality insect repellent to any exposed areas of skin

Insect repellent

Various types of insect repellent are available. Many contain diethyltoluamide (DEET), but some contain dimethyl phthalate or eucalyptus oil if you're allergic to DEET.

When using insect repellent, make sure you:

  • do not use it on cuts, wounds or irritated skin
  • do not get it in your eyes, mouth and ears
  • do not spray it directly on your face – spray it on your hands and then apply it to your face
  • do not allow young children to apply it themselves – put it on your hands and then apply it to your child
  • apply it after applying sunscreen, not before
  • wash your hands thoroughly after use, and wash the repellent off your skin with soap and water when it's no longer needed
  • always follow the manufacturer's instructions

If you or your children have a reaction to an insect repellent, such as redness, stop using it.

Wash it off and contact a GP or a local healthcare professional if you're abroad.

Page last reviewed: 1 February 2019
Next review due: 1 February 2022