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Traveler Summary

Key Points

  • Japanese encephalitis (JE) is a viral infection acquired through the bite of an infected mosquito in many areas of south-central, southeastern, and eastern Asia.
  • Risk is high 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). Risk is negligible for short-stay travelers and persons who confine their travel to urbanized areas.
  • Symptoms include sudden high fever, nausea, headache, and altered mental status.
  • Consequences of infection may include convulsions, muscular paralysis, difficulty breathing, and coma.
  • 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 and at least 1 week prior to potential exposure. For imminent departures, the 2 doses can be administered 7 days apart in adults.
  • 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 11 months after the initial 2 doses). No further booster dose is recommended.

Introduction

JE is a viral infection acquired by the bite of infected mosquitos, resulting in fever, nausea, headache, and altered mental status. In local people, JE is a relatively common vaccine-preventable cause of encephalitis (brain inflammation) in Asia and parts of the western Pacific.

Risk Areas

JE occurs in a wide belt from Japan and northern coastal China, throughout Southeast Asia, and across India to Pakistan and is present through the Western Pacific islands from Indonesia to Papua New Guinea and as far north as the Philippines.

Risk is highest in rural agricultural areas that are often associated with rice cultivation and flood irrigation. However, in some areas, these ecological conditions may occur near (or occasionally within) urban centers.

Altitude 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.

Fewer than 100 cases of JE have been reported in travelers going to risk areas of Asia since the 1970s. No travel-related cases have been reported among exclusively urban travelers.

Transmission

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 Factors

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 (more than 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.

For U.S. travelers, all age groups are equally susceptible to JEV infection due to lack of prior exposure.

Symptoms

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.

Prevention

Nonvaccine

Mosquito precautions in the evening and nighttime are recommended when in risk areas, regardless of vaccination status. Travelers should be especially vigilant in applying repellent at dusk and again at dawn, when the mosquitoes that transmit JEV are most active. See Insect Precautions.

Vaccine

An injectable vaccine (Ixiaro) is available in the U.S. and is approved for persons 2 months and older. Several vaccines are available outside the U.S.

Persons with underlying medical conditions or those who have concerns about the vaccine should speak to their health care provider before vaccine administration.

Side Effects

The most common vaccine side effects 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.

Timing

The primary series consists of 2 doses, given 28 days apart and at least 1 week before potential exposure. An accelerated schedule for travelers aged 18-65 years consists of 2 doses given 7 days apart.

A booster dose may be given at least 11 months 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.