Japanese encephalitis (JE) occurs throughout most of Asia, including India, China and Japan. In 1995, the virus was found in dogs, pigs, horses and people in the Torres Strait islands. The virus was detected at the top of Cape York Peninsula in 1998 and it now appears widespread in the Torres Strait.
In temperate areas, infection usually occurs in late summer and autumn, when mosquitoes are more active. Infection builds up in water birds and then spreads by mosquitoes to pigs during late spring and early summer and finally to humans and horses.
Cases in humans and horses tend to be sporadic or occur in small clusters, but serious outbreaks could occur in a large, susceptible population exposed to infected mosquitoes. Infected horses are dead-end hosts as there is not enough of the virus in their blood to infect mosquitoes.
Japanese encephalitis is caused by a virus related to West Nile virus and Murray Valley encephalitis virus. The virus cycles naturally between water birds (herons and egrets) and mosquitoes. Pigs can also be infected and spread disease causing abortions in pregnant sows and neurological signs in piglets.
Humans and horses may suffer severe disease from the virus, but they do not spread the disease. Infections without recognisable signs occur in other livestock and animals.
The disease has an incubation period of 8–10 days.
Signs of Japanese encephalitis
- vary from a passing fever through to violent neurological signs and death
- mild cases: off feed, sluggish and reddened or jaundiced (yellow) mucous membranes (gums)
- more serious cases: lethargic with a fluctuating fever, difficulty swallowing, jaundice (yellow), pinpoint haemorrhages in mucous membranes (gums)
- nervous signs such as lack of coordination, staggering, falling, aimless wandering and unpredictable behaviour may occur in serious cases
- severe cases: blindness with profuse sweating and muscle trembling before collapsing and dying.