Epizootic malignant catarrhal fever in three bison herds: differences fromcattle and association with ovine herpesvirus-2

Three bison herds in Colorado experienced high mortality from malignant cat arrhal fever (MCF). In comparison with cattle, the bison had a more rapidly progressive disease, fewer clinical signs, and milder inflammatory histolo gic lesions. There was consistent association with ovine herpesvirus-2 (OHV -2). Contact with sheep was not consistent. Of 17 animals in herd A, 15 die d of acute MCF; 1 was slaughtered while healthy; and 1 developed clinical s igns of MCF, was treated with corticosteroids and antibiotics, and died of fungal abomasitis and rhinitis after 5 months. In herds B and C, approximat ely 300 of 900 and 18 of 20 died of MCF following brief clinical disease. T he nearest sheep were 1 mile away from herd A, but direct contact with shee p could be documented in herds B and C. Complete gross and histologic exami nations were conducted on 34 animals, including all animals in herd A, and MCF was diagnosed in 31. In addition, field necropsies were performed on al l dead animals in herd B and most in herd C and MCF was diagnosed on the ba sis of the gross lesions in most animals. Clinical signs of each animal in herd A were recorded. Illness was brief, usually 8-48 hours. Clinical signs were subtle; separation from the herd was often observed. In all 3 herds, hemorrhagic cystitis and multifocal ulceration of the alimentary tract were consistently found at necropsy. Mild lymphocytic vasculitis was present in multiple organs. Ovine herpesvirus-2 was found by polymerase chain reactio n (PCR) in 71 of 105 formalin-fixed tissue specimens from 29 of 31 animals with MCF. In herd A, blood samples from 13 animals were collected at 5 time points and tested by PCR for the presence of OHV-2 viral sequences in peri pheral blood leukocytes. Nine bison with a positive PCR test and 4 with neg ative results prior to clinical illness died of MCF.