Equine Encephalomyelitis


(Viral Encephalomyelitis of Horses)

Viral encephalomyelitis of horses is a group of closely related diseases of Equidae which are transmitted by arthropods (insects), and which cause moderate to high mortality after presenting clinical signs of dysfunction of the nervous system. They are caused by Alphavirus (family Togaviridae), of which three strains are commonly recognized: Eastern Equine Encephalomyelitis (EEE), Western Equine Encephalomyelitis (WEE), and Venezuelan Equine Encephalomyelitis (VEE), Human infection can occur, and the disease is of significant public health importance.

The viruses are restricted to the Americas, with cases reported from the USA, Canada, Venezuela, Brazil and Argentina. Outbreaks have occurred in Central America. In North America the disease tends to be seasonal due to the changes in mosquito and other insect populations. Spread of the disease between areas by migratory birds may be possible, and encephalitis has been detected in infected birds. In general, mortality is lower with WEE infection (20-30 %) than with EEE or VEE (40-80 %).

Wild birds act as a reservoir for the virus, and spread is by insects. Mosquitoes appear the main vector, but mites, ticks and lice may also be implicated. The movement of infected mosquioes between distant areas by high winds has been recorded. Rodents and other small mammals may serve as viral reservoirs for VEE.

Clinical signs with the different virus are indistinguishable. The incubation period may be 1-3 weeks after which there is a mild loss of appetite, low fever and depression. The animals may initially be hypersensitive to external stimuli, with periods of excitement and restlessness. Animals may circle and appear blind, or exhibit tremors. The disease progresses to a non-responsive "dummie" state, or to increasing paralysis.

Vaccines for WEE and EEE are available. Control of flies and insects, with provision of screened housing and control of access to wild bird populations help minimize risk of exposure

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References:

Surveillance: Special Issue - Exotic Diseases, Vol 23, 1996
MAF Regulatory Authority, Ministry of Agriculture
P.O. Box 2526, Wellington, New Zealand

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Ministry of Agriculture, Fisheries and Food, United Kingdom ISSN:0269 5545

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