Japanese encephalitis (JE) is caused by infection with the japanese encephalitis virus (JEV), a mosquito-borne virus found predominately in Asia and the Pacific Islands. The epizootic/epidemic JE season in northern Asian temperate climates usually begins in May or June, and ends around September or October. Endemic JE virus circulates year-round in tropical areas of Asia among birds, swine and mosquitoes.
Three types of manifestations of JEV can occur---transitory, lethargic, or hyperexcitable. Clinical signs, if present, vary; disease usually presents itself in sporadic or localized clusters.
Mosquitoes belonging to the genera Culex and Aedes transmit the virus. Other genera have also been shown to harbor the virus but their role in transmission remains unconfirmed. Vertical transmission occurs in mosquitoes.
The incubation period is 8–10 days. Most infected horses do not show clinical signs of the disease.