Japanese encephalitis (JE) is an acute viral infection of the central nervous system caused by JE virus which is a flavivirus. According to the World Health Organization, JE occurs in a large number of countries/areas of Asia, including Cambodia, China, Indonesia, Japan, Lao People's Democratic Republic, Malaysia, Myanmar, the Philippines, Republic of Korea, Thailand, Viet Nam, south-eastern Russian Federation and the Indian subcontinent. In recent decades, JE has gradually spread to previously non-affected Asian regions, and a small outbreak was recently reported from islands in the Torres Strait off the Australian mainland. In Hong Kong, about 0 - 2 cases of human JE were reported each year. Both local and imported cases have occurred.
Mode of Transmission
The virus is transmitted by the bite of infected Culex mosquitoes. Culex tritaeniorhychus is the principal vector of the disease. The mosquito becomes infected by feeding on pigs and wild birds infected with the JE virus. The infected mosquitoes then transmit the virus to humans and animals during the feeding process. The transmission reaches its dead end in human. The disease is not directly transmitted from person-to-person.
The incubation period is usually 4 to 14 days.
Mild infections may occur without apparent symptoms other than fever with headache. More severe infection is marked by rapid onset, headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions (especially in infants) and paralysis. Case fatality rates range from 10% to 35%. Neurological and psychiatric sequelae are common among survivors.
Diagnosis of JE infections can be made by serological tests, such as haemagglutination-inhibition test, by demonstrating a fourfold rise in antibody titres in paired sera. This test is available at the Government Virus Unit of the Department of Health (DH).
Treatment for JE is supportive.
As JE is a mosquito-borne disease, measures should be taken to eliminate mosquito breeding sites and prevent mosquito bites. Vaccination is indicated mainly for persons spending 30 days or more in a rural agricultural endemic area during the transmission season. Currently one inactivated JE vaccine is licensed in Hong Kong. For initial immunization, usually two doses are administered at an interval of 1 - 2 weeks. Immunity may take one month to develop. Common reported side effects include local reactions at the injection site, and mild systemic symptoms such as headache, myalgia, gastrointestinal symptoms and fever. Further information is available in the DH's Travel Health Website at http://www.chp.gov.hk/en/content/9/24/28.html
Department of Health
9 July 2003