Response of pregnant mares to equine herpesvirus 1 (EHV1).

Twenty-one pregnant mares were inoculated with EHV1. Nineteen became infected as evidenced by clinical signs and/or viremia but only one mare aborted a virus-infected fetus. The viremias were leukocyte-associated and appeared to be non-productive, latent infections of these cells. Infectivity, detectable by cocultivation, persisted in the circulating leukocytes for as long as 9 days without resulting in abortion. The data suggest that it is extremely difficult to evaluate the efficacy of vaccines in preventing EHV1 (Rhinopneumonitis) abortion due to the paucity of non-exposed mares, lack of tests which measure residual protection, and an incomplete understanding of the pathogenic mechanisms involved in abortion due to this virus.