Japanese encephalitis
Cause
Japanese encephalitis virus belongs to the mostly vector-borne Flaviviridae family.
Transmission
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 or encephalitis; 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.
Geographical distribution
Japanese encephalitis (JE) is the leading cause of viral encephalitis in Asia and occurs in almost all Asian countries (Map). Largely as a result of immunization, its incidence has been declining in Japan and the Republic of Korea and in some regions of China, but the disease is increasingly reported from Bangladesh, parts of India, Nepal, parts of Pakistan, northern Thailand and Viet Nam. Transmission occurs principally in rural agricultural locations where flooding irrigation is practised – some of which may be near or within urban centres. Transmission is mainly related to the rainy season in southeast Asia, but year-round transmission occurs, 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.
Risk for travellers
The risk for 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 in countries or areas at risk, particularly where flooding irrigation is practised. Whereas Japanese encephalitis in countries or areas at risk is primarily a disease of children, it can occur in travellers of any age. Prevention is by avoidance of mosquito bites (Chapter 3) and by vaccination.