Evaluation of the safety of vaccinating mares against equine viral arteritis during mid or late gestation or during the immediate postpartum period

Objective—To determine whether it is safe to vaccinate pregnant or postpartum mares with a commercial modified-live virus vaccine against equine viral arteritis (EVA). Design—Randomized controlled study. Animals—73 mares and their foals. Procedures—Mares were vaccinated during mid gestation, during late gestation, or 2 or 3 days after parturition with a commercial modified-live virus vaccine or were not vaccinated. Foaling outcomes were recorded, and serum, blood, milk, and nasopharyngeal samples were obtained. Results—All mares vaccinated during mid gestation foaled without any problems; 21 of 22 mares in this group had antibody titers against EAV at the time of foaling. Of the 19 mares vaccinated during late gestation, 3 aborted; antibody titers against EAV were detected in 13 of 15 mares from which serum was obtained at the time of foaling. All postparturient vaccinates were seronegative at foaling; all of them seroconverted after vaccination. No adverse effects were detected in any of their foals. Conclusions and Clinical Relevance—When faced with a substantial risk of natural exposure to EAV, it would appear to be safe to vaccinate healthy pregnant mares up to 3 months before foaling and during the immediate postpartum period. Vaccinating mares during the last 2 months of gestation was associated with a risk of abortion; this risk must be weighed against the much greater risk of widespread abortions in unprotected populations of pregnant mares naturally infected with EAV.