In the event of an equine viral arteritis (EVA) outbreak, is it safe to vaccinate your pregnant mare so she develops the immunity she needs to prevent infection and resulting abortion? The results of a recent collaborative study completed by researchers at the University of Kentucky and Oklahoma State University indicate that the answer in most cases is yes.

Although cases of EVA do not commonly occur in the United States, outbreaks in 1984 in Kentucky and in 2006 across several states seriously affected the American horse breeding market. In addition to abortion (with rates sometimes as high as 50-60% in unvaccinated mares), the disease is characterized by signs of upper respiratory tract illness in adult horses, and pneumonia in young foals.

Stallions can become long-term carriers of the causative agent, equine arteritis virus (EAV), and transmit it during breeding; the disease can also be transmitted by the inhalation of the virus. In recent years, researchers also found a risk of transmission associated with embryo transfer from a donor mare inseminated with EAV-infective semen.

Current vaccination recommendations note that pregnant mares should not be vaccinated with the available vaccine, a modified-live product, unless at high risk of exposure to natural infection with EAV. It had been documented previously that maiden or non-pregnant mares can be vaccinated without resultant problems or abortions in future pregnancies.

In the current study the researchers separated a group of pregnant mares into three groups to test the safety of the vaccine at different stages in pregnancy: They vaccinated horses in the first group at approximately midgestation; the second group, within two months of foaling; and the third group, on Day 2 or Day 3 after foaling. The researchers measured virus and antibody levels in blood, milk, and nasopharyngeal samples from the immunized horses.

None of the 22 mares that were vaccinated midgestation aborted, and 21 of them developed antibodies to EAV (indicating they had built up immunity to protect them from natural exposure to the virus). Of the 19 mares vaccinated within two months of foaling, three aborted (15.7%) and all of them developed antibodies to the vaccine virus strain. All of the 28 mares vaccinated on Day 2 or Day 3 days after foaling developed antibodies to the virus, and none of their foals showed any adverse reactions to their dams' vaccination.

"This experimental study corroborates earlier findings from vaccination of thousands of mares in the aftermath of the 2006 multistate occurrence of EVA in the U.S.," said Peter J. Timoney, MVB, MS, PhD, FRCVS, of the Gluck Equine Research Center at the University of Kentucky and an author on the study. "Although vaccinating mares during the last two months of gestation can be associated with the risk of vaccine-induced abortion, owners must weigh this risk against the much greater risk of widespread abortions in unprotected populations of pregnant mares that are naturally exposed to EAV."

The study, "Evaluation of the safety of vaccinating mares against equine viral arteritis during mid or late gestation or during the immediate postpartum period," was published in the Journal of American Veterinary Medicine Association in March 2011. The abstract is available online.

About the Author

Nancy Zacks, MS

Nancy Zacks holds an M.S. in Science Journalism from the Boston University College of Communication. She grew up in suburban Philadelphia where she learned to ride over fields and fences in nearby Malvern, Pa. When not writing, she enjoys riding at an eventing barn, drawing and painting horses, volunteering at a therapeutic riding program, and walking with Lilly, her black Labrador Retriever.

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