Prevalence
Japanese encephalitis (JE) is an acute viral infection of the central nervous system caused by JE virus which is a flavivirus. JE is endemic in parts of China, India, Korea, Japan, South East Russian Federation, islands in the Torres Strait of Australia, Nepal, Thailand, Vietnam, Cambodia, Laos PDR, the Philippines, Taiwan, Indonesia, Malaysia, and Sri Lanka. In Hong Kong, there had been 6 human cases of Japanese encephalitis reported in the past 10 years (1994 V 2003), including 2 local cases and 4 imported cases, and the range was 0 V 2 cases per year. In 2004, as of 12 July, three local cases have been reported.
Mode of Transmission
The virus is transmitted by the bite of infective Culicine mosquitoes. Culex tritaeniorhynchus 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.
Incubation period
The incubation period is usually 4 to 14 days.
Clinical Features
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 5% to 35%. Neurological and psychiatric sequelae are common among survivors.
Laboratory Diagnosis
Diagnosis of JE infections can be made by serological tests, such as haemagglutination inhibition test, by demonstrating a fourfold or greater rise in antibody titres in paired sera. This test is available at the Virology Division, Public Health Laboratory Services Branch, Centre for Health Protection. It is advisable to take the first blood specimen during the acute phase and the second at 2 weeks after onset. Occasionally, it may be necessary to take a third blood specimen at 3 weeks after onset to rule out the diagnosis of JE.
Treatment
Treatment for JE is supportive.
Prevention
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 Department of Health Travel Health Website at
http://www.chp.gov.hk/en/content/9/24/28.html