Epidemics occur in late summer in temperate regions, but the virus is enzootic and occurs throughout the year in many tropical areas of Asia. JE virus is maintained in a natural enzootic cycle involving culicine mosquitoes and water birds. The virus is transmitted to humans by Culex mosquitoes, primarily Cx. tritaeniorhynchus and related species, which breed in rice fields. Pigs are the primary amplifying hosts in the peridomestic environment.
Most human infections are asymptomatic or present as a nonspecific viral syndrome.
In severe cases, the onset of symptoms is usually sudden, with fever, headache, and vomiting. The illness resolves in 5 to 7 days if there is no central nervous system involvement. In those patients with CNS involvement, lethargy is common, faces are expressionless, and there are sensory and motor disturbances affecting speech, eyes, and limbs. There may be confusion and delirium progressing to coma, with convulsions in children as a presenting sign. Weakness and paralysis may affect any part of the body.