Epidemiology of anthrax outbreaks in wood bison (Bison bison athabascae) of the Mackenzie bison population

• Main Author: New, Dallas
• Other Authors: Epp, Tasha, Woodbury, Murray, Elkin, Brett, Shury, Todd
• Format: Thesis
• Language: English
• Published: University of Saskatchewan 2014
• Subjects: anthrax; conservation; epidemiology; Northern Canada; wood bison; zoonosis; Canada; Bison bison athabascae; Wood Bison; Bison bison bison
• Online Access: http://hdl.handle.net/10388/ETD-2014-12-1895

Wood bison (Bison bison athabascae) conservation in Northern Canada is negatively affected by diseases that kill these animals, such as anthrax caused by the bacterium Bacillus anthracis. Although this disease is considered ancient and was identified more than 2000 years ago in Egypt, little is still known about this disease in wild bison, such as why adult males are often predominantly affected and if the reason there are mortalities in some years and not in others is due to environmental, pathogen, or host factors. The overall objective of this thesis was to use descriptive and serological epidemiology to provide evidence needed to enhance our understanding of anthrax in wild wood bison. The first chapter explored the 2012 anthrax outbreak in bison of the Mackenzie bison population using descriptive epidemiology. Field crews discovered 451 bison carcasses during the outbreak. The carcasses were found between late June and early August, and it was estimated that the epidemic peaked between July 13-19 based on the date carcasses were found and the estimated length of time the animal had been deceased. A unique feature of this outbreak compared with the two previous outbreaks in the same population, as well as outbreaks in other wild wood bison herds, is that numerous calves, yearlings and adult females died rather than mostly adult males. Three separate geographic regions were identified by a field wildlife veterinarian, and examined for differences in outbreak characteristics. One region had proportionally more male carcasses than the others, and one had more calf deaths. Lack of complete data made it difficult to ascertain if the outbreak truly started in one of these regions before the others, or if it began simultaneously in all three. The second component of this project used serological epidemiology of anthrax in the Mackenzie bison population to gain an understanding of wood bison exposure to the bacterium. Serological samples were collected through various sources between 1986 and 2009, and later tested for anti-PA antibodies. Of the 278 samples tested, 191 (69%) were positive, indicating previous exposure to B. anthracis. Of the samples with a recorded gender, approximately 18.2% of those from females and 35.5% from males tested positive. The dataset spanned only one anthrax outbreak year in this population of animals, and the year with the highest proportion of positive samples was the year following this known epidemic (1994, 90% positive submissions). Adults had a higher prevalence of being seropositive than any of the other age categories, for both sexes. This research has revealed that in some outbreak years, all age classes and both genders of bison are affected by anthrax unlike in most outbreaks where predominantly adult males succumb to disease. Furthermore, bison are likely exposed to B. anthracis in non-outbreak years, indicating that they either experience subclinical disease or recover from clinical disease.