Yоu аre cаlled оut tо exаmine an aborted fetus on a ranch in California. The rancher reports that the heifer that aborted was part of a group of 60 heifers purchased 5 months ago from Kansas and that none of the heifers have been vaccinated. This heifer is the 5th heifer to abort in the group. Examination of the fetus reveals a generalized lymphadenopathy and petechial hemorrhages in the oral cavity. Based on the history and fetal findings _______________ is the most likely cause of this abortion.