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Encephalitis: Mosquito-borne Disease
Encephalitis in Nepal
Japanese Encephalitis
Japanese encephalitis (JE), a mosquito-borne flaviviral infection, is the leading cause of childhood encephalitis in Asia, where up to 50,000 cases may be reported annually. Most infections are asymptomatic, but when encephalitis develops, the case-fatality rate can be as high as 30%. Neuropsychiatric sequelae are reported in 50% of survivors. Although children are at greatest risk of infection in endemic areas, outdoor occupation, recreational exposure, and male gender are also risk factors for infection.

Immunity to JE virus from previous vaccination or naturally acquired immunity reduces the risk of illness. Although most adults living in endemic areas have acquired natural immunity and older persons rarely develop illness, a high case-fatality rate has also been reported in the elderly

JE virus is transmitted chiefly by mosquitoes in the Culex vishnui complex; the specific species depends on the geographic area. In China and other endemic areas in Asia, C. tritaeniorhyncus is the principal vector. This species feeds outdoors beginning at dusk and during evening hours until dawn. Larvae are found in flooded rice fields, marshes, and other small stable collections of water found around cultivated fields. In temperate zones, this vector is present in greatest density from June through September; it is inactive during winter months.

It has a wide host range that includes domestic mammals, birds, and humans. Swine and certain species of wading birds are the amplifying hosts in an enzootic transmission cycle. Because JE virus primarily cycles among animals and mosquitoes and because national JE vaccination programmes are present in many affected countries, the absence of human infections alone should not be used to gauge a traveller's risk for infection.

The disease and how it affects people
Japanese encephalitis (JE) is a disease caused by a flavivirus that affects the membranes around the brain. Most JE virus infections are mild (fever and headache) or without apparent symptoms, but approximately 1 in 200 infections results in severe disease characterized by rapid onset of high fever, headache, neck stiffness, disorientation, coma, seizures, spastic paralysis and death.

The case fatality rate can be as high as 60% among those with disease symptoms; 30% of those who survive suffer from lasting damage to the central nervous system. In areas where the JE virus is common, encephalitis occurs mainly in young children because older children and adults have already been infected and are immune.

An effective killed vaccine is available for Japanese encephalitis, but it is expensive and requires one primary vaccination followed by two boosters.
Sources: BBC and CDC

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Mosquito-born disease: Encephalitis in Nepal