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 encephalitiscan'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 asparts ofRussiaand 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 theUS 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 thanone 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 canmean you're more likely to come into contact with infected mosquitoes.

Content supplied by the NHS Website

Medically Reviewed by a doctor on 24 Nov 2016