Japanese Encephalitis

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Japanese encephalitis is the leading cause of viral encephalitis (brain infection) in Asia. The disease occurs chiefly in China, Korea, Southeast Asia and the Indian subcontinent (see map showing distribution of Japanese encephalitis in Asia). Japanese encephalitis is transmitted by Culex mosquitoes, which breed in ground pools, especially flooded rice fields, and bite primarily after dusk. The virus lives principally in domestic pigs and Ardeid (wading) birds. Transmission of Japanese encephalitis is therefore greatest in rural, agricultural areas where rice paddies and pig farming co-exist. The risk may be increased by heavy rainfall and irrigation. Most infections are asymptomatic. But encephalitis, when it occurs, is severe and frequently leads to death or permanent brain damage.

The risk of Japanese encephalitis for most travelers appears to be very small. Since 1981, only five cases have been reported among American travelers (not counting military personnel). Japanese encephalitis vaccine (JE-VAX; Aventis Pasteur Inc.) (PDF) is recommended only for long-term (more than 1 month) travelers to rural areas where Japanese encephalitis is reported and for travelers making shorter trips who may engage in extensive unprotected outdoor activities, especially in the evening, in such areas. The vaccine is administered as a series of three 1.0 ml injections given subcutaneously on days 0, 7, and 30. The dosage is reduced to 0.5 ml for those 1-2 years of age. (No data are available on children less than one year of age.) Immunization should be completed at least 10 days prior to departure.

About 20% of those vaccinated experience mild reactions, including discomfort at the injection site, headaches, muscle aches, rash, nausea, vomiting, chills, dizziness, and malaise. Approximately 0.6% of vaccinees develop severe allergic reactions, including hives, facial swelling (angioedema), respiratory distress, and anaphylaxis, which may occur as long as one week after vaccination. Erythema multiforme, erythema nodosum, and joint swelling have also been reported. These reactions are treated with epinephrine, antihistamines, and/or steroids. Reactions may occur after any of the three doses, even if previous doses were well-tolerated. Vaccinees should be observed for 30 minutes after immunization and should remain in areas with access to medical care for at least ten days after vaccination. Those with a past history of hives are more likely to be allergic to Japanese encephalitis vaccine. No further vaccine should be given if an allergic reaction occurs.

When the full series is completed, Japanese encephalitis vaccine appears to reduce symptomatic infections by 80-90%. Protective levels of antibody persist for at least three years. The full duration of protection is unknown. There are no recommendations concerning booster doses, but they may be given after three years. No information is available on the safety of the vaccine in pregnant women or in children less than one year of age. Japanese encephalitis may cause fetal death in the first and second trimesters, so the risk of the vaccine must be balanced against the risk of illness in a pregnant woman traveling to an endemic area. A small study of children with altered immune function implied no increased risk from the vaccine in this population.

Insect protection measures are essential in areas where Japanese encephalitis is known to occur.

From the World Health Organization (WHO)

Japanese encephalitis (background information and vaccine position paper)

Safety of Japanese encephalitis vaccine

Japanese encephalitis vaccines: WHO position paper (PDF)

Japanese encephalitis: disease burden and vaccines

Japanese encephalitis (background)

From the Centers for Disease Control (CDC)

Japanese encephalitis

Japanese encephalitis fact sheet

Japanese encephalitis frequently asked questions

Inactivated Japanese Encephalitis Vaccine: Recommendations of the Advisory Committee on Immunization Practices (PDF) (most complete summary)

From the National Travel Health Network and Centre (U.K.)

Japanese encephalitis

From the New England Journal of Medicine

T.Solomon, "Control of Japanese Encephalitis — Within Our Grasp?" (NEJM 2006; 355:869-871)

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