Japanese encephalitis is a type of viral brain infection that's spread through mosquito bites. It's most common in rural areas throughout South East 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's more common in rural areas where there are pig farms and rice fields. It can't 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. This usually requires the person being admitted to hospital, so they can be given fluids, oxygen and medication to treat any symptoms.
Most people infected by the Japanese encephalitis virus have either no symptoms, or mild, short-lived symptoms, which are often mistaken for flu.
However, around 1 in every 250 people who become infected with Japanese encephalitis develops more severe symptoms, as the infection spreads to the brain. This usually happens 5-15 days after infection and symptoms can include:
- High temperature (fever)
- Seizures (fits)
- Stiff neck
- Inability to speak
- Uncontrollable shaking of body parts (tremor)
- Muscle weakness or paralysis
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
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.
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've been travelling, what you did on your trip and what vaccinations you've 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 that less than one in a million travellers develop Japanese encephalitis in any given year.
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.
Find out more about the causes of Japanese encephalitis and the countries and activities which have a higher risk of catching the disease.
The World Health Organization (WHO) estimates there are around 68,000 cases of Japanese encephalitis worldwide each year.
Causes of 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 can't be passed from person to person.
Japanese encephalitis is found throughout Asia and beyond. The area in which it's found stretches from the Western Pacific islands in the east, such as Fiji, across to the borders of Pakistan in the west. It's found as far north as parts of Russia and as far south as the north coast of Australia.
Most cases occur in:
- Myanmar (Burma)
- Sri Lanka
Despite its name, Japanese encephalitis is now relatively rare in Japan, due to mass immunisation programmes.
Visit the US Centers for Disease Control and Prevention (CDC) website for a map of Japanese encephalitis risk areas.
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're planning to visit before booking your holiday.
If you're planning a short visit to Asia, the risk of contracting Japanese encephalitis is very low, particularly if you're going to be staying in urban areas. Overall, it's estimated there's 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're more likely to come into contact with infected mosquitoes.
Preventing Japanese encephalitis
You can reduce your risk of Japanese encephalitis by getting vaccinated and taking precautions to avoid mosquito bites 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're visiting a high-risk area during the rainy season
- You're going to visit rural areas in a high-risk country – such as rice fields, marshlands
- You're taking part in any activities that may increase your risk of becoming infected – such as cycling or camping
- You work in a laboratory with potential exposure to the virus
For more information about at-risk areas, read about the causes of Japanese encephalitis.
There's 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.
People from the ages of 18 to 65 may be given the vaccine on an accelerated schedule, where the second dose is given seven days after the first.
Either course of vaccination should be completed at least seven days before potential exposure to the Japanese encephalitis virus.
The Japanese encephalitis vaccine isn't usually available on the NHS and the cost can vary between clinics. Each dose can cost more than £90 per person, so 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 12 to 24 months after you're first vaccinated.
Up to 40% of people who have the Japanese encephalitis vaccine experience mild and short-lived side effects, such as:
- 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.
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, due to the theoretical risk of problems resulting from the vaccine being passed to your baby.
The Japanese encephalitis vaccine isn't normally recommended for children less than two months old, because it's unclear how safe and effective it is for this age group.
You shouldn't have the vaccine if you've 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 isn't 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 isn't possible or you're 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)
Various types of insect repellent are available. Many contain diethyltoluamide (DEET), but some are available that contain dimethyl phthalate or eucalyptus oil, if you're 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'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 your GP, or a local healthcare professional if you're abroad.
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