Because a number of factors can affect the risks of falling victim to potentially deadly brain-swelling diseases carried by mosquitoes, Purdue University experts recommend that all horses be vaccinated and that people take precautions to avoid mosquito bites. The number of cases of West Nile virus, and Eastern and Western equine encephalitis, can vary due to temperatures, rainfall, regional seasonal differences and bird migratory patterns, according to veterinarians and entomologists. Encephalitic diseases are spread when mosquitoes suck blood from infected birds and then bite people, horses or other mammals. In addition, areas that don’t experience insect-killing frost have a threat of these illnesses almost year-round. In the Midwest, West Nile virus is the most common mosquito-transmitted disease, but outbreaks of the other diseases are possible.
Hope says horses must have inoculations early enough in the spring and fall to provide immunity throughout mosquito season. There is no indication that the West Nile shots are harmful to pregnant mares. However, it is best to vaccinate mares before breeding season, Hope says.
The disease now occurs in almost every area of North America.