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