Study Concludes West Nile Virus Vaccine Not Harmful to Pregnant Mares

By John V. Wood
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Researchers at Texas A&M University have just completed a retrospective study into the safety of administering the killed West Nile virus vaccine to pregnant broodmares, the first study on this topic in equine reproductive research.

The study looked at 595 mares from four different farms in Texas and Kentucky. The authors of the study, which was published in the Journal of the American Veterinary Medical Association, hoped to resolve the dispute of whether administration of the vaccine resulted in reproductive losses in pregnant broodmares or not.

"The main reason we did the study was to investigate the validity of anecdotal claims made by lay journals and web sites concerning infertility, pregnancy loss, and fetal abnormalities associated with administration of this vaccine to pregnant mares," said Dr. Joey Vest, lead author of the study and equine theriogenology resident at Texas A&M.

"Given the widespread concerns expressed and possible adverse outcomes of vaccinating pregnant mares, we felt it would be important to systematically evaluate whether there was any association of administration of the killed WNV vaccine and adverse outcomes during pregnancy," said Dr. Noah Cohen, the author responsible for statistical and epidemiological data analysis.

"We got so many phone calls and e-mails from horse owners, wanting to know if it was safe to vaccinate their broodmares," said Dr. Terry L. Blanchard, one of the study's authors and Texas A&M professor of theriogenology. "We felt it was important to get the best safety information we could. After reviewing our results, we feel comfortable using the vaccine in pregnant mares, but you must realize this study was not a vaccine safety trial, as such."

The authors reviewed the mares' medical histories to include date(s) of vaccinations and whether a pregnancy loss occurred or not. According to the study, "the overall incidence of pregnancy loss in (vaccinated) mares fell within the normal range of typical losses and that mares vaccinated during any period of pregnancy were not any more likely than mares not vaccinated during that same period of pregnancy to lose their foals," said Vest.

Researchers hope this study gives veterinarians more of a comfort zone with the decision of whether or not to vaccinate mares.

"This study should alleviate some of the horse owners' fears that there are any adverse reproductive side effects to the WNV vaccine," said Blanchard.