In 2002, West Nile Virus hit the upper midwest horse population with a vengeance. Our practice examined 10 cases in 7 days; 50% died. Those surviving battled an extremely difficult illness for 5-7 days with significant expenses to owners. West Nile cases, afflicting both horses and humans, are still sporadically reported. West Nile immunization is standard in our practice; thankfully we haven't seen a case since 2003.
In addition to West Nile, Eastern Equine Encephalomyelitis is another mosquito-borne encephalitis virus afflicting horses in the upper midwest. The American Association of Equine Practitioners lists these two viral diseases in its core vaccine recommendations. Similar diseases, rarely occurring in the United States, include Western Equine Encephalomyelitis and Venezuelan Equine Encephalomyelitis.
Another encephalitis, Japanese Encephalitis, as pointed out by Drs. Peter Timoney and Stacy Oke, may be the next mosquito-borne viral disease afflicting North American Horses; read "Equine Emerging and Surging Diseases: What's on the Horizon?".
Our best recommendation to horse owners? As we approach the height of the upper midwest's mosquito-borne disease season (late summer to early fall), make sure your horse's immunizations are up to date!