Japanese encephalitis virus belonging to the mostly vector-borne Flaviviridae family.
Pigs and various wild birds represent the natural reservoir of this virus, which is transmitted to new animal hosts and occasionally humans by mosquitoes of the genus Culex.
Nature of the disease
Most infections in humans are asymptomatic. In symptomatic cases, severity varies: mild infections are characterized by febrile headache or aseptic meningitis followed by an uneventful recovery; severe cases have a rapid onset and progression with headache, high fever and meningeal signs. Permanent neurological sequelae are common among survivors. Approximately 25% of severe clinical cases have a fatal outcome.
Japanese encephalitis virus is the leading cause of viral encephalitis in Asia and occurs in almost all Asian countries (see map). Transmission occurs principally in rural agricultural locations where flooding irrigation is practised – some of which may be near or within urban centres. Transmission is related mainly to the rainy season in south-east Asia but may take place all year round, particularly in tropical climate zones. In the temperate regions of China, Japan, the Korean peninsula and eastern parts of the Russian Federation, transmission occurs mainly during the summer and autumn.
Largely as a result of immunization, the incidence of Japanese encephalitis has been declining in Japan and the Republic of Korea, in some regions of China, and more recently in Nepal, Sri Lanka, Thailand and Viet Nam. However, transmission of the virus remains unaffected by immunization, and non-immunized individuals remain at risk. The disease is also reported from Bangladesh, parts of India and Pakistan, and from Cambodia, the Lao People’s Democratic Republic, the Philippines and other countries in the region (see map).
Risk for travellers
The risk of Japanese encephalitis is very low for most travellers to Asia, particularly for short-term visitors to urban areas. However, the risk varies according to season, destination, duration of travel and activities. Vaccination is recommended for travellers with extensive outdoor exposure (camping, hiking, working, etc.) during the transmission season, particularly in endemic countries or areas where flooding irrigation is practised. In areas at risk, Japanese encephalitis is primarily a disease of children, but it can occur in travellers of any age. Prevention is by avoidance of mosquito bites (Chapter 3) and by vaccination.