Japanese encephalitis: A mosquito-borne
viral infection, the leading cause of
viral encephalitis in Asia. Japanese
encephalitis virus cannot be transmitted from person-to-person.
Number of cases: About 50,000 cases of Japanese encephalitis are reported annually from the People's Republic of China, Korea, Japan, Southeast Asia, the Indian subcontinent, and parts of Oceania.
The virus: The Japanese encephalitis virus is related to the
viruses of
St. Louis encephalitis and Murray Valley encephalitis and to the West Nile virus.
Infection leads to overt encephalitis in only 1 of 20 to 1,000 cases.
Symptoms: The
incubation period (the time from contract with the virus to when symptoms surface) is usually 5 to 15 days. Mild infections occur without apparent symptoms other than
fever with
headache. More severe infection is marked by quick onset, headache, high fever,
neck stiffness, stupor, disorientation,
coma, tremors, occasional convulsions (especially in infants) and spastic (but rarely flaccid)
paralysis.
Severe encephalitis: Japanese encephalitis usually is severe, resulting in a fatal outcome in 25% of cases and
residual neuropsychiatric problems in a further 30% of cases.
Infection in pregnancy: Limited
data indicate that Japanese encephalitis acquired during the first or second trimesters of pregnancy causes
intrauterine infection and
miscarriage (
spontaneous abortion). Infections that occur during the third
trimester of pregnancy have not been associated with adverse outcomes in newborns.
The mosquito and host animals: Mosquitoes become infected by feeding on domestic pigs and wild birds infected with the Japanese encephalitis virus. Infected mosquitoes then transmit the Japanese encephalitis virus to humans and animals. The Japanese encephalitis virus is amplified in the blood systems of domestic pigs and wild birds, chiefly Ardeid (wading) birds.
Culex mosquitoes are the principal vectors.
Viral infection rates in the mosquitoes range from less than 1% to 3%. These species are prolific in rural areas where their larvae breed in ground pools and especially in flooded rice fields. All elements of the transmission cycle are prevalent in rural areas of Asia, and human infections occur principally in this setting. Because vertebrate-amplifying hosts and agricultural activities may be situated within and at the periphery of cities, cases of Japanese encephalitis are occasionally reported from urban locations.
Seasonality of the disease: The Japanese encephalitis virus is transmitted seasonally. In temperate regions, it is transmitted during the summer and early fall, approximately from May to September. In subtropical and tropical areas, seasonal patterns of viral transmission are correlated with the abundance of
vector mosquitoes and of vertebrate-amplifying hosts. These, in turn, fluctuate with rainfall, with the rainy season, and with migratory patterns of avian-amplifying hosts. In some tropical locations, however, irrigation associated with agricultural practices is a more important factor affecting vector abundance, and transmission may occur year-round.
Common in children and the elderly: Children less than 15 years of age are principally affected and there is nearly universal exposure to the virus by adulthood. In developed countries of Asia and in areas where children are protected by
immunization, an increase in Japanese encephalitis occurs in the elderly, consistent with waning
immunity with age.
Risk for travelers: Overall, the risk of acquiring Japanese encephalitis while traveling in Asia is extremely low. However, the risk for an individual traveler is highly variable and depends on factors such as the season, locations and duration of travel, and activities of the person. Travel during the transmission season and exposure in rural areas, especially for extended periods of time, are the principal factors contributing to risk. The extent and nature of outdoor activity, use of protective clothing, bed nets and repellents, and lodging in air-conditioned or well-screened rooms are additional factors that affect exposure. Residents of developed countries usually have no natural immunity to the Japanese encephalitis virus and travelers of all ages are equally susceptible to infection with the virus. The elderly may be more susceptible to developing neuroinvasive disease.
Treatment: There is no specific
therapy. Intensive supportive therapy is indicated.
Prevention: Japanese encephalitis vaccine is recommended for persons who plan to reside in areas where Japanese encephalitis is prevalent (endemic or epidemic) during a transmission season). The incidence of Japanese encephalitis in the location of intended residence, the conditions of housing, nature of activities, and the possibility of unexpected travel to high-risk areas are factors that should be considered in the decision to seek vaccination.
Japanese encephalitis vaccine is NOT recommended for all travelers to Asia. In general, vaccine should be offered to persons spending a month or longer in endemic areas during the trans-mission season, especially if travel will include rural areas. Under specific circumstances, vaccine should be considered for persons spending
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