Cause
Japanese encephalitis (JE) virus, which is a flavivirus.
Transmission
The virus is transmitted by various mosquitoes of the genus Culex. It infects pigs and various wild birds as well as humans. Mosquitoes become infective after feeding on viraemic pigs or birds.
Nature of the disease
Most infections 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 30% of severe clinical cases have a fatal outcome.
Geographical distribution
JE occurs in a number of countries in Asia (see Map) and occasionally in northern Queensland, Australia.
Risk for travellers
Japanese encephalitis (JE) is the leading cause of viral encephalitis in Asia and occurs in almost all Asian countries (see Map). Its incidence has been declining in Japan and the Korean peninsula and in some regions of China, but is increasing in Bangladesh, India, Nepal, Pakistan, northern Thailand and Viet Nam. Transmission occurs principally in rural agricultural locations where fl ooding irrigation is practised – some of which may be near or within urban centres. Transmission is seasonal and mainly related to the rainy season in south-east Asia, but all-year transmission occurs. 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. Vaccination is not recommended for all travellers to Asia because of the low incidence of the disease in travellers and potential (although rare) adverse events of current inactivated vaccine: it should be based on individual risk assessment, taking into account the season, the type of accommodation and the duration of exposure, as well as the travel itinerary. The risk to short-term travellers and those who travel mainly to urban areas is very low. Vaccination is recommended for travellers with extensive outdoor exposure (camping, hiking, bicycle tours, outdoor occupational activities, in particular in areas where fl ooding irrigation is practised) in rural areas of an endemic region during the transmission season. It is also recommended for expatriates living in endemic areas through a transmission season or longer.
Vaccine
Two types of JE vaccine are widely available: inactivated mouse-brain-derived vaccine (IMB) and cell-culture-derived live attenuated SA 14-14-2 vaccine. Until recently, the IMB vaccine was the most widely available commercially, but its production is being discontinued by several manufacturers. New vaccines including cell-culture derived, inactivated vaccines are in the process of being licensed or in the fi nal stage of development.
Precautions and contraindications
A hypersensitivity reaction to a previous dose is a contraindication. The live attenuated vaccine should be avoided in pregnancy unless the likely risk favours its administration. Rare, but serious, neurological side-effects attributed to IMB vaccine have been reported from endemic as well as non-endemic regions. Allergic reactions to components of the vaccine occur occasionally. As such reactions may occur within 2 weeks of administration, it is advisable to ensure that the complete course of vaccine is administered well in advance of departure.
Type of vaccine: Inactivated mouse-brain-derived or live attenuated
Schedule: For the inactivated vaccine: 3 doses at days 0, 7 and 28;
or 2 doses given preferably 4 weeks apart (0.5 or 1.0 ml
for adults, 0.25 or 0.5 ml for children depending on the
vaccines)
For the live attenuated SA 14-14-2 vaccine equally good
protection is achieved with a single dose followed, as
required, with a single booster dose given at an interval
of about 1 year
Booster: After 1 year and then 3-yearly (for IMB only) when continued
protection is required
Contraindications: Hypersensitivity to a previous dose of vaccine, pregnancy
and immunosuppression (live vaccine)
Adverse reactions: Occasional mild local or systemic reaction; occasional
severe reaction with generalized urticaria, hypotension
and collapse
Before departure: Inactivated vaccine, at least two doses before departure.
Live attenuated vaccine, one dose is enough