Abstract
Six pregnant mares were given equine viral arteritis virus intravenously. Tissues from genital tracts, placentae, and fetuses were examined by light and electron microscopy to study the mechanism of abortion. Four mares which died with acute disease had diffuse vacuolation of endometrial epithelium and systemic necrotizing vasculitis. Two of these mares had dead fetuses and two had live fetuses; virus was isolated from tissues of one live fetus. Placentae of mares dying from acute disease did not have lesions attributable to infection; virus was isolated from two of these placentae. One of the two mares which recovered from clinical disease aborted a dead fetus eight days after inoculation. The mare had severe necrotizing myometritis and virus was isolated from maternal ovaries and from fetal tissues. The fetus did not have lesions attributable to arteritis virus. These results suggest that although fetal death may occur in utero during acute equine viral arteritis, abortion probably is due to lesions in the uterus of the mare.