A recent report outlined a series of cases of West Nile virus infection in horses seen at the University of Pennsylvania’s New Bolton Center. The findings might help horse owners and veterinarians identify this infection more readily.

Eight horses from New York and New Jersey were referred to New Bolton Center with evidence of neurological disease. The predominant signs were ataxia and hypermetria (uncoordinated and exaggerated movements). Other signs included fever, head tremors, muscle twitching, and/or hypersensitivity to touch and sound. One animal was recumbent and experiencing seizures.

Three different tests were used to detect West Nile virus, and all eight horses were positive on at least one test. Three of the horses (39%) were euthanized, but the other five recovered. Three returned to racing after several weeks of rest, and two pregnant mares carried to term.

West Nile virus appears to infect all ages, sexes, and breeds of horses, although it is unclear whether all horses which are exposed become ill. In humans, only a modest percentage of those exposed get sick, and even fewer die. Birds are one of the few species in which West Nile virus is able to multiply in sufficient quantities to spread the virus. It is highly unlikely that an infected horse will transmit the virus to humans or other horses in the area. Still, multiple horses on one farm could become infected from the same source. At this time, the best course of action to prevent infection is vigorous mosquito control