The facts about Japanese encephalitis

Japanese encephalitis (JE) is the leading cause of viral encephalitis in Asia, occurring in almost all countries in the region. It is estimated that at least 1 million infections occur yearly. From the high number of infections, about 35,000 to 50,000 clinical cases are reported yearly and fatality can be as high as 60%. In addition, 30-50% of survivors develop severe neurological sequelae or long term complications that require rehabilitation and continued care.

JE is caused by a single stranded RNA virus, which is a member of the Flavivirus genus in the Flaviviridae family, which includes other human pathogens such as yellow fever virus, West Nile virus, St. Louis encephalitis virus, Murray Valley encephalitis virus and dengue virus. JE is endemic throughout large parts of east, south and southeast Asia. Approximately 3 billion people currently live in JE endemic areas, where more than 70 million children are born each year, putting around 1 billion children at risk.

The virus is transmitted between a wide range of vertebrate hosts, including birds and mammals. Mosquitoes transfer the virus to humans from the vertebrate hosts, so the risk of infection is highest in rural areas as the vector species, Culex tritaeniorhyncus, breeds in water pools and rice fields. In most areas of Asia, seasonal variation affects the transmission of JE virus. In temperate locations, the period of transmission starts around April or May and lasts until September or October. In subtropical and tropical areas, transmission intensifies with the rainy season and the migratory patterns of the amplifying hosts. Hence, the incidence of JE is mainly influenced by vector abundance, which is dependent on other factors including temperature, rainfall and agricultural practices. Domestic animals, especially pigs, are generally implicated as amplifier hosts and reservoirs for the virus in terms of human infection. Given these factors, the disease periodically becomes hyperendemic in areas like northern, central and southern India, southern Nepal, northern Vietnam and Southeast Asia.

The age distribution of JE varies between regions. In general, children under the age of 14 are principally at risk from the JE virus. The highest age specific attack rates usually occur in children aged 3-6 years. In highly endemic areas of JE, half of the cases occur before the age of 4 years and almost all before 10 years of age.

More Japanese encephalitis Facts

Centers for Disease Control and Prevention:
http://www.cdc.gov/ncidod/dvbid/jencephalitis/index.htm