Health - VIRAL INFECTIONS - Japanese encephalitis

Introduction 

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.

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 develop 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
• confusion
• 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. There hasn't been a reported case in a traveller returning to the UK for more than 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.

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.

High-risk countries

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:

• 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.

Visit 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're planning to visit before booking your holiday.

High-risk activities

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

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've 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.