EQUUS - - The Core -

� 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.

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