Evaluation of management options for bison and brucellosis in Yellowstone National Park, Wyoming

Bison (Bison bison) in Yellowstone National Park, Wyoming, have been infected by the disease “brucellosis” since the early 1900's, when bison likely contracted the disease from infected cattle. Brucellosis, which causes abortion in cattle, bison, and elk, has been nearly eradicated from cattle throughout the United States; bison and elk populations in Yellowstone National Park and the surrounding areas are the last remaining wild reservoir of the disease. Although experts agree that the probability that bison will transmit the disease to cattle grazed on lands outside Yellowstone National Park is very low, if transmission does occur, the economic consequences to local ranchers are both certain and significant. In order to efficiently manage the disease transmission risk, it is critical to understand the relative outcomes of different disease and population management tools on the abundance of bison and the disease prevalence in the bison population.; Eight alternative management plans for reducing the risk of transmission from bison to cattle were evaluated using mathematical modeling. Models for nearly all management plans predicted that the bison population would asymptote at a fall population size of approximately 3500–3700 animals. Most models indicated that the management plans were capable of reducing the disease prevalence from 40% to 9–13% by 15 years after the management plan was implemented when the “most plausible” combination of values for percent vaccinated, vaccine efficacy, and reinfection by elk were used; these models also estimated that disease prevalence would be reduced to 25% after 25 years. The management plan focused on whole-herd test and slaughter within Yellowstone National Park was estimated to eradicate the disease within 7–15 years. However, all management plans, except whole-herd test and slaughter, rely heavily on the availability of a vaccine against brucellosis. Until a safe, effective vaccine is available for bison, test and slaughter of migrants which carry brucellosis is the only available tool for reducing disease prevalence, and this is not expected to reduce prevalence substantially below the current level.