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. 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 the north coast of Australia.

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, due to mass immunisation programmes.

See the US Centers for Disease Control and Prevention (CDC) website for a map of Japanese encephalitis risk-areas.

Rainy seasons

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 point.

Therefore, it may be useful to find out when the wet seasons are for the areas you are planning to visit before booking your holiday.

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. Overall, it is estimated that there is less than one case of Japanese encephalitis for every million travellers.

However, 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 (see above)
• fieldwork, camping or cycling in rural areas

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

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 person to person.

Read more about the causes of Japanese encephalitis.

Signs and symptoms

Most people infected by the Japanese encephalitis virus have either no symptoms, or mild symptoms that are short-lived and flu-like.

In less than 1 in every 250 cases, the infection can spread to the brain (encephalitis) and cause more serious symptoms, such as:

• seizures (fits)
• confusion
• inability to speak
• paralysis (inability to move certain body parts)

There is currently no cure for Japanese encephalitis. Treatment involves supporting the functions of the body as it tries to fight off the infection. This will usually require the person being admitted to hospital, where you can be given fluids, oxygen and medication to treat any symptoms.

Up to one in every three people who develop more serious symptoms will die, and many of those who survive are left with permanent brain damage.

Read more about the symptoms of Japanese encephalitis.

When to seek medical advice

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

For information about who to contact when you need immediate medical help abroad, visit the GOV.UK website. If you are 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 from outside risk areas to be affected by Japanese encephalitis. It is estimated that less than one in a million travellers will develop Japanese encephalitis in any given year. There has not been a reported case in a traveller returning to the UK for over 10 years.

The people most at risk are those who live and work in rural areas where the condition is widespread. Around 75% of cases involve children under the age of 15.

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

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 infection. 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, provides 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 mosquito nets that have been impregnated with an insecticide
• covering up with long-sleeved tops, trousers and socks
• applying a good quality insect repellent to exposed areas of skin

Read more about preventing Japanese encephalitis.

Encephalitis
Encephalitis is inflammation of the brain. This can develop as a result of infection (usually viral) or when the immune system attacks the tissue of the brain by mistake (post-infectious encephalitis).

Inflammation
Inflammation is the body’s response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.

Immune system
The immune system is the body’s defence system, which helps to protect it from disease, bacteria and viruses.

Seizures
The electrical impulses in the brain are disrupted, causing both the brain and the body to behave strangely.

You can reduce your risk of Japanese encephalitis by getting vaccinated and by taking precautions to avoid mosquito bites when in at-risk areas.

Japanese encephalitis vaccination

Vaccination against Japanese encephalitis is recommended for anyone who may be at risk of the infection through their work or travels. It provides protection for more than 9 out of every 10 people who have it.

Vaccination is particularly important if you are visiting a high-risk area during the rainy season and you are going to visit rural areas (such as rice fields or marshlands), or take part in any activities that may increase your risk of becoming infected (such as cycling or camping).

If you are travelling to a country where Japanese encephalitis is found, visit your GP or practice nurse at least six to eight weeks before you leave, to discuss whether you should have the vaccination. For more information about at-risk areas, read about the causes of Japanese encephalitis.

The vaccine

There is one vaccine for Japanese encephalitis currently licensed in the UK for use in adults and children over two months old. 

The vaccination is given as an injection and requires two doses for full protection, with the second dose given 28 days after the first. The second dose should ideally be completed at least a week before travelling.

The Japanese encephalitis vaccine is not usually available on the NHS, and each dose will normally cost about £70-75 per person. It’s a good idea to include this when budgeting for your trip.

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

Side effects

Up to 40% of people who have the Japanese encephalitis vaccine experience mild and short-lived side effects, such as soreness, redness or swelling at the site of the injection, a headache and 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 are pregnant or breastfeeding.

The vaccination may need to be postponed if you have a fever. It may also not be recommended if you are pregnant or breastfeeding, due to the theoretical risk of problems resulting from the vaccine being passed to your baby.

The Japanese encephalitis vaccine is not normally recommended for children less than two months old because it is 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 are sleeping outside, use mosquito nets that have 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
• cover 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 (see below)

Insect repellent

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

When using insect repellent, make sure you:

• don’t use it on cuts, wounds or irritated skin
• don’t get it in your eyes, mouth and ears
• don’t spray it directly onto your face – spray it onto your hands and then apply it to your face
• don’t 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 is 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 your GP, or a local healthcare professional if you are abroad.

In most people, the Japanese encephalitis virus causes either no symptoms or mild, short-lived symptoms, which are often mistaken for flu.

Only around 1 in every 250 people who become infected develops more obvious symptoms of Japanese encephalitis. This usually happens between 5 and 15 days after infection.

Initial symptoms of infection can include:

• a high temperature (fever)
• headache
• feeling and being sick
• diarrhoea
• muscle pain

In some cases, these initial symptoms may be followed a few days later by more serious problems, as the infection spreads to the brain. This is known as encephalitis and it can cause more wide-ranging symptoms, such as:

• seizures (fits)
• stiff neck
• muscle weakness
• changes in mental state – which can range from mild confusion to being very agitated or slipping into a coma
• uncontrollable shaking of body parts (tremor)
• inability to speak
• paralysis (inability to move certain body parts)

Up to one in every three 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. However, 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 one or more limbs.

When to seek medical advice

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

Visit the GOV.UK website for information about who to contact when you need immediate medical help abroad. If you are already back in the UK, see your GP.

Encephalitis
Encephalitis is inflammation of the brain. This can develop as a result of infection (usually viral) or when the immune system attacks the tissue of the brain by mistake (post-infectious encephalitis).

Inflammation
Inflammation is the body’s response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.

Seizures
The electrical impulses in the brain are disrupted, causing both the brain and the body to behave strangely.