- Japanese encephalitis (JE) is a viral infection occurring in many areas of Asia, as well as in a few areas of the western Pacific, and is acquired through the bite of an infected mosquito.
- Risk is extremely low for short-stay travelers and all those who stay in urban areas or have brief daytime excursions to rural farming areas but is increased for long-stay travelers (with extensive unprotected outdoor, evening/nighttime exposure) going to rural areas in affected countries, especially during JE virus (JEV) transmission season (May to October in most affected countries).
- Symptoms include sudden high fever, nausea, headache, and altered mental status.
- Consequences of infection may include convulsions, muscular paralysis, difficulty breathing, coma, and death.
- Prevention includes wearing long sleeves and long pants and observing personal protective measures against mosquito bites.
- JE vaccine (Ixiaro) is given as 2 doses (28 days apart in children aged 2 months through 17 years and 7 to 28 days apart in adults aged 18-65 years) at least 1 week prior to potential exposure.
- Vaccine side effects are most commonly injection-site reactions, fever, headache, and muscle aches.
- Duration of vaccine protection is 10 years or longer following an initial booster (given at least 1 year after the initial 2 doses). No further booster dose is recommended.
JE is a potentially severe viral infection occurring throughout south-central, southeastern, and eastern Asia and in parts of the western Pacific and is transmitted via the bite of infected mosquitoes. Fewer than 100 cases of JE have been reported in travelers going to endemic areas of Asia since the 1970s, although JE is the leading cause of mosquito-borne encephalitis (brain inflammation) in local populations in rural farming areas of these countries.
JE occurs in a wide belt from Japan and northern coastal China, throughout Southeast Asia, and across India to Pakistan and is present throughout the Western Pacific islands from Indonesia to Papua New Guinea and as far north as the Philippines.
Risk is highest in rural agricultural areas (and occasionally near/within urban centers) that are often associated with rice cultivation and flood irrigation. No travel-related cases have been reported among exclusively urban travelers.
Elevation and local variations in rainfall and temperature affect mosquito breeding and seasonality of transmission. In temperate areas of Asia, the prevalence of human JEV infection increases toward the end of the summer rains and usually peaks in the summer and fall. In the subtropics and tropics (including Indonesia, the Philippines, southern Thailand, and southern Vietnam), seasonality varies with monsoon rains and irrigation practices and may be prolonged or even occur throughout the year.
JEV is transmitted to humans through the bite of infected evening-biting and night-biting mosquitoes that breed in rice fields. Mosquitoes acquire the virus when they bite wading birds or pigs (in rural farms) that carry JEV.
Risk is very low for short-stay travelers and persons who confine their travel to urbanized areas or who have brief daytime exposures during common tourist excursions. However, cases may be sporadic and have been reported (albeit rarely) in short-stay visitors traveling out of season whose only rural travel had been to beach resorts.
Risk is highest for expatriates and long-stay travelers (> 1 month) in rural areas where JE is prevalent. Persons with extensive unprotected outdoor, evening, or nighttime exposure (e.g., biking, hiking, camping, and certain occupational activities) in rural areas might be at high risk even if their trip is brief.
Symptoms most commonly appear 5 to 15 days following exposure and include sudden high fever, abdominal pain, nausea, vomiting, headache, and altered mental status.
Consequences of Infection
JEV infection can lead to convulsions, muscular paralysis, breathing difficulties, seizures, mild tremors, poor concentration, memory problems, and coma. Death occurs in about 30% of symptomatic cases.
Need for Medical Assistance
Travelers who develop symptoms (especially altered mental status) within 15 days of leaving a risk area should seek urgent medical attention.
Personal protective measures are the main prevention strategy. Mosquitoes that transmit JEV (Culex spp.) are generally night biters but have peak biting activity at dusk and again at dawn. Regardless of vaccination status, travelers should be especially vigilant in applying repellent during peak biting activity times. Treat outer clothing, boots, tents, and sleeping bag liners with permethrin (or other pyrethroid) when traveling in a very high-risk area for JE. See Insect Precautions.
An injectable vaccine (Ixiaro) is widely available in many countries and is approved for persons 2 months and older. Several single-dose live vaccines are available in endemic areas of Asia as well as in Australia.
The most common vaccine side effects of Ixiaro are injection-site reactions, fever, headache, and muscle aches. Fever, irritability, and diarrhea are most common in infants and children younger than 12 years. Allergic reactions (both immediate and delayed) to the vaccine have occurred.
Persons with underlying medical conditions or those who have concerns about the vaccine should speak to their health care provider before vaccine administration.
Ixiaro vaccine is given to travelers as follows:
- Children aged 2 months through 17 years: 2 doses, given 28 days apart.
- Adults aged 18-65 years: 2 doses, given 7 to 28 days apart.
- Doses should be completed at least 1 week before potential exposure.
A booster dose may be given at least 1 year after completion of the primary immunization series if ongoing exposure or re-exposure to JEV is expected. Duration of protection is 10 years or longer.