Definition of Japanese encephalitis
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 Misspellings: japanese ensephilitis, japanese insephilitis
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