Rabies in Two Bison from Colorado

Two adult female bison, housed in an outdoor research facility and observed daily, died suddenly three days apart. Minimal coordination and behavioral changes were observed in one animal the evening before being found in a moribund state. Malignant catarrhal fever was suspected in both bison due to a recent confirmed MCF case with similar course. The cause of death was not apparent from necropsy, but brains of both animals were strongly positive for rabies virus antigen by fluorescent antibody and/or immunohistochemical tests. Minimal to mild encephalitis with Negri bodies was observed on histopathology. The bison were located in an area that had not been endemic for skunk rabies; however, a case of rabies in a skunk had been discovered 1.6？km north of the bison paddock two months prior to the bison cases. 1. Introduction There are few reported cases of rabies in bison (Bison bison). A single case occurred in North Dakota in 1998 and was described in a case report [1]. Additionally, 3 cases in South Dakota bison have been reported in surveillance summaries since 1960 [2, 3], and a case in a European bison (Bison bonasus) in Russia has been reported [4]. We report here the occurrence of rabies in two bison due to a presumed skunk exposure at an outdoor research facility in Colorado. 2. Case Presentation The Colorado State University (CSU) Animal Population Health Institute Wildlife Research Facility (APHIWRF) occupies 2.6 hectares and is located on the foothills campus of CSU in Fort Collins, Colorado. At the time of the rabies cases, 60 bison, 12 white-tailed deer, and a colony of 13 feral swine were contained in the facility. The two bison reported here had been onsite for 9 months. On the morning of June 18, 2012, an adult female bison over 5 years of age was found dead. Disturbance of the soil near the carcass indicated leg paddling prior to death. No abnormalities had been noted in the animals at feeding time the prior evening. At necropsy, the bison had poor body condition, thoracic contusions, and focal fibrinous peritonitis and pleuritis. Rumen contents were considered dry. The cause of death was not apparent based on gross examination. The primary differential diagnosis was malignant catarrhal fever (MCF) due to ovine herpesvirus-2 (OHV-2) as a previous case had occurred in the same herd two months earlier. Tissues were fixed in 10% buffered formalin for histopathology and fresh spleen was submitted to CSU Veterinary Diagnostic Laboratory (CSU-VDL) for OHV-2 PCR. On the evening of June 20, 2012, a second, adult, female bison from the same