New Variables Identified as West Nile Risk Factors

Based on a recent retrospective study performed at the Western College of Veterinary Medicine in Saskatchewan, Canada, three major risk factors that are completely outside of a horse owner's control contribute to the mortality rate of West Nile virus disease in horses: sex, time of infection, and coat color.

West Nile virus is transmitted to horses from mosquitoes. Once bitten, horses can either eliminate the virus uneventfully or become infected.

"To date, it is unclear why some horses develop disease after infection with the virus while others are able to mount a successful immune response and clear the infection without ever developing neurologic disease," reported Debra Sellon, DVM, PhD, Dipl. ACVIM, professor of Equine Medicine at Washington State University.

In 2003, 133 Saskatchewan-based horses were diagnosed with West Nile virus. Almost half of the infected horses (43.8%) died or were humanely euthanatized while the remaining 56.2% of horses recovered completely.

The fatality rate was higher in stallions (82%), than mares (36%) or geldings (45%), and horses with light colored coats were more likely to die or be euthanatized compared to those with dark coats. Horses infected in August had a greater chance of dying compared to horses infected in September.

While five of nine (44%) infected horses had been fully vaccinated against West Nile virus died, Sellon said the low number of vaccinated horses included in the study prevents one from making any sound conclusions regarding the impact of vaccination on West Nile virus disease based on this study.

"There is a tremendous body of evidence supporting the safety and efficacy of current vaccines for prevention of disease from West Nile virus infection," Sellon stated. "All horse owners who live in areas of virus activity should vaccinate their horses."

The study "Factors associated with West Nile virus disease fatalities in horses," was published in the Canadian Veterinary Journal in November, 2007. Contributing authors were Epp; Waldner; West; and Townsend.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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