MCF results from infection by one of several members of a group of closely related ruminant gammaherpesviruses of the Rhadinovirus genus. Although the MCF group of ruminant rhadinoviruses currently comprises approximately 10 known members, only a few are known to be pathogenic under natural conditions. The principal carriers and their viruses are sheep (ovine herpesvirus-2), wildebeest (alcelaphine herpesvirus-1), and goats (caprine herpesvirus-2). Another strain of unidentified origin has caused MCF in white-tailed deer. Virtually all clinical cases are caused by the sheep or wildebeest viruses.