West Nile Virus (WNV), Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE) are zoonotic diseases transmitted by mosquitos. Birds amplify the viruses, which allows them to be spread to other species. They cause brain and spinal cord inflammation. Many horses infected with WNV show no clinical signs of illness following their first exposure. Clinical signs that may occur include muzzle and lower lip twitching, twitching of muscles in the neck, shoulder, and pectoral region, behavior abnormalities, incoordination, weakness, trouble swallowing, impaired vision, convulsions, wandering, paralysis, coma, recumbency, and death. Horses with WNV illness have a 60 percent chance of survival, while horses infected with EEE or WEE usually die or are euthanized

The symptoms of WNV, EEE, and WEE can be similar to other equine neurological diseases, such as rabies, botulism, and equine herpesvirus–1. WNV is usually diagnosed by connecting clinical signs with the presence of antibodies in the horse’s blood.