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What is Japanese encephalitis?

Japanese Encephalitis (JE) is a viral disease which, like malaria, is transmitted by mosquitoes. In many patients, Japanese Encephalitis symptoms remain mild and the infection may even go unnoticed. However, in some cases, the virus causes severe symptoms and leads to serious complications. The disease is transmitted to humans by Culex spp. mosquitoes in predominantly rural parts of Asia and the Pacific Rim where rice cultivation and pig farming are common.


JE virus is transmitted to humans from animals and birds via the bite of an infected Culex spp. mosquito. These mosquitoes feed predominantly during the night, between dusk and dawn; pigs and wading birds are the principal hosts.

Culex spp. mosquitoes become infected by biting JE infected animals (particularly pigs) or birds and are prolific in rural areas where flooded rice fields and marshes provide breeding grounds. However, they are also found in cities and urban locations. In temperate regions of Asia, most cases occur in the warm season, when large epidemics can occur.

Cases of JE can also occur outside the normal high transmission season. In the tropics and subtropics, JE can occur year-round but transmission often intensifies during the rainy season and pre-harvest period in rice-cultivating regions.



Most cases of JE are asymptomatic (without symptoms) or present with non-specific flu-like symptoms. The incubation period is 5-15 days, and initial symptoms include fever, headache and nausea progressing to meningoencephalitis with convulsions and altered consciousness. Of those who develop encephalitis, approximately 30% die. In those who survive, it is estimated that 30-50% will develop long term neurological or psychiatric problems. Typical encephalitis symptoms include nausea and vomiting, seizures, headaches and confusion. The Japanese encephalitis virus affects the human brain and can lead to inflammation and swelling in the brain. In very severe cases, the infection can lead to brain damage.

There is no specific medication to treat and cure this type of encephalitis. Treatment usually focuses on relieving symptoms and supporting the immune system in its effort to fight the virus. A Japanese encephalitis vaccine provides medical protection for travellers at risk of catching the virus.


Side effects

Allergic reactions are rare but can be serious. Common, less worrying side effects of the Japanese encephalitis vaccine, are redness and swelling at the site of injection, high temperature, nausea, dizziness and vomiting as well as abdominal pain. You may also experience muscle pain. These side effects should pass quickly.


Risk Areas

Japanese encephalitis is primarily found on the Asian continent. High risk countries include China, Cambodia, India, Indonesia, Korea, Laos, Malaysia, Myanmar, Nepal, Philippines, Singapore, Sri Lanka, Thailand and Vietnam. Your risk of encephalitis in some of these countries will vary depending on the time of year of your visit. Due to the increased number of mosquitoes, the risk of encephalitis can be higher during the rainy seasons.

The risk to most travellers to Asia is very low, especially for short-term travellers visiting urban areas. The overall incidence of JE in travellers from non JE risk countries visiting Asia is estimated to be less than one case per one million travellers. The risk varies on the basis of destination, duration, season, and activities. Typically, the risk increases for persons who intend to live or travel for long periods of time in JE risk areas or during outbreaks and have rural trips. Certain activities, even during short trips where there is significant rural, outdoor or night time exposure e.g. fieldwork, camping, or cycling can increase the traveller’s risk, as do children. Staying near areas of pig farms or rice fields also increases your risk of contracting the virus, as mosquitoes thrive in damp areas and can pass the virus from animals to humans.



The risk of acquiring JE can be reduced by insect bite avoidance methods, particularly between the hours of dusk and dawn, when the Culex spp. mosquito vector is most active. Travellers are at increased risk of infection when visiting rural areas. Short trips (usually less than a month) especially if only travelling to urban areas, are considered lower risk.


A JE vaccine is available and is recommended for those intending to stay for long periods in regions where JE is known to occur or whose planned activities will increase their risk. As with other mosquito transmitted illnesses, you can protect yourself from Japanese encephalitis by avoiding mosquito bites. Prevention steps such as insect repellent, mosquito nets, long-sleeved clothing etc. will help you stay safe when travelling to an area of known risk. Travelling outside the rainy season is also another way to limit exposure to these infectious mosquitoes.


Vaccination Schedule 

Age 65 years and over

  • 2 doses - 0 and 28 Days

  • Long-term exposure require a booster at 12 months

  • Length of protection is uncertain

Age 18-64 years

  • 2 doses - 0 and 28 Days

  • Long-term exposure require a booster at 12 months

  • First booster dose at 12-24 months following the primary course prior to re-exposure to JE virus.

  • A 2nd booster (4th dose) should be offered at 10 years for those who remain at risk.

Age 3-17 years

  • 2 doses - 0 and 28 Days

  • Long-term exposure require a booster at 12 months

  • First booster dose at 12-24 months following the primary course prior to re-exposure to JE virus.

  • Duration of protection, beyond two years after the first booster is uncertain.


Accelerated schedule

Adults aged 18-65 years

  • Licensed accelerated schedule: first dose at day 0, second dose: 7 days after first dose.

3-17 years and >65 years

  • Off license when time is short.




Additional Information

In situations where the primary course (days 0 and 28 or days 0 and 7) plus the first booster has been interrupted, the schedule should be resumed, and not restarted.

With both schedules, the primary immunisation schedule (first and second dose) should be completed at least one week prior to potential exposure to JE

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