Japanese encephalitis virus (JEV) is a member of the genus Flavivirus in the family Flaviviridae and causes encephalitis, principally in horses and humans. JEV also infects pigs where it causes abortions and stillbirths. JEV is maintained in nature between mosquitoes, pigs and water birds. The major vector of JEV throughout most of Asia is Culex tritaeniorhynchus, however other species may be locally important. Pigs act as important amplifiers of the virus, and birds can also be involved in its amplification and spread in the environment. The disease has been observed in large parts of Asia and recently in the western Pacific region. In horses, the infection is usually inapparent. Affected horses show clinical signs that include pyrexia, depression, muscle tremors, and ataxia. In pigs, abortions and stillbirths can occur when pregnant sows are infected with JEV for the first time. Infected pregnant sows usually show no clinical signs. Vaccines are commercially available in several Asian countries for humans and animals.