Most horses vaccinated for West Nile virus Most horse owners and breeders in Ohio say they are vaccinating their animals for the West Nile virus. Vaccinations a year ago are credited with helping reduce the number of horses infected by the virus from 600 to about 100 in 2003. But veterinarians are worried that some horse owners will overlook vaccinating this year because they may perceive that the threat is over. William Saville is a veterinarian with the Ohio State University Extension Services. He says they don't want them to think they have it under control. He estimates that about two out of three horses in the state are vaccinated.