Equine herpesvirus-1 abortion: atypical cases with lesions largely or wholly restricted to the placenta

Equine herpesvirus-1 (EHV-1) abortion is diagnosed by detailed examination of the aborted fetus, using a combination of gross findings, histopathology, virus isolation, polymerase chain reaction (PCR) and immunostaining. Conventionally, these tests are applied to selected target organs, particularly liver, lung, thymus and spleen (review; Allen et al. 1999). The macroscopic and microscopic lesions in the fetus are well described (Westerfield and Dimock 1946). A feature of spontaneous EHV-1 abortions in mares is the often sudden and explosive nature of the event, the fetus being expelled without warning and still enveloped within the fetal membranes. Experimental studies carried out in ponies in the early 1990s (Smith et al. 1992, 1993) gave the opportunity to investigate the histological events taking place within the uterus of mares aborting following EHV-1 infection. A number of recently infected mares aborted virologically negative fetuses, and it was possible to confirm that these abortions were associated with virus-related thromboses and endometrial infarction, which had apparently precipitated a cascade of events leading to wholesale premature placental separation from the endometrium and expulsion of the fetus and placenta. These findings initiated concerns as to whether, in some spontaneous abortions, similar events might occur, giving rise to false negative diagnoses if only fetal tissues were screened for virus. This paper, based upon experiences in diagnostic pathology, describes data that provide strong support for this hypothesis