� Definition: Eastern and western equine encephalomyelitis are a pair of closely related viral diseases that affect the horse’s central nervous system. Eastern equine encephalomyelitis (EEE) is more deadly than western equine encephalomyelitis (WEE). (A related disease, Venezuelan equine encephalomyelitis [VEE], occurs in Central and South America. The last recorded cases in the United States occurred in southern Texas in 1971.)
� Transmission: EEE and WEE are caused by alphaviruses of the family Togaviridae. The two forms are separated by geography. EEE occurs in the Southern and Eastern United States, and WEE in the West. However, outbreaks of both viruses have occurred outside of their normal ranges. The viruses that cause EEE and WEE are carried by birds and spread by mosquitoes. Birds that carry the viruses do not become seriously ill. However, a mosquito who feeds on an infected bird can transmit the virus to its next host. Horses are considered dead-end hosts, meaning that they cannot pass the virus on to mosquitoes or other animals once infected.
� Signs: Five to 10 days after a horse is bitten by an infected mosquito, the virus passes through the bloodbrain barrier to infect the central nervous system---the brain and spinal cord---where it multiplies and begins to kill nerve cells. The earliest signs of EEE are listlessness, fever and loss of appetite, but within the next 24 hours the horse will develop more significant
neurological signs, including incoordination, sensitivity to sound and touch, muscle twitching in the shoulder and flank, head pressing and seizures. Within another day the horse will develop paralysis and become recumbent; coma and death follow.
The signs of WEE are similar--including fever, depression, ataxia0 and head pressing---but tend to be milder.
� Treatment: The only treatment is supportive therapy. The horse might receive intravenous fluids and corticosteroids to help reduce edema in the brain. Even with treatment, about 90 percent of horses who develop EEE will die within two to four days; those who survive are likely to have lifelong neurological impairment. The mortality rate for WEE is estimated to range from 20 to 40 percent if the case progresses to full-blown encephalomyelitis, but milder cases may never even be diagnosed if the horse recovers before developing serious signs of infection.