There were nearly 14,000 reported cases of West Nile virus (WNV) in the United States in 2002 by the end of November, and many broodmares were exposed to the virus even if not clinically affected. As the country begins its fifth year of handling the disease, broodmare owners have many questions about how to vaccinate mares and foals appropriately.

Rob Holland, DVM, PhD, a private practitioner in Kentucky and a technical services veterinarian for the Intervet pharmaceutical company, explained the protocol for vaccinating broodmares. “Vaccinate four to six weeks before foaling,” he said, “and what you’re doing is boostering their IgG (a type of antibody) and all their immunological parameters. In the case of the mare and the (unborn) foal, there’s a six-layer placenta that does a very good job of protecting the foal against potential disease that affects the mare, and doesn’t allow any antibodies to cross it.”

According to Holland, very few Central Kentucky foals have been clinically affected by the disease, probably due to a combination of the inability of WNV to get to the foal via the placenta, antibodies passed to the foal from the mare in the colostrum, and because of the timing of foaling season. “We’re very lucky from an epidemiological standpoint because most foals are born when there are no mosquitoes,” he said.

Maternal vs. Foal Antibodies

W. David Wilson, MS, BVMS, MRCVS, of the Department of Medicine and Epidemiology in the School of Veterinary Medicine at the University of California, Davis, hopes to determine the best WNV vaccination protocol for foals in a research study he has submitted to the Grayson-Jockey Club Research Foundation for review.

“We do not yet have any concrete information on which to base recommendations for vaccination of foals against WNV. However, it seems reasonable to use infor