Brucellosis and tuberculosis as factors limiting population growth of northern bison

Abstract

Tuberculosis (Mycobacterium bovis) and brucellosis (Brucella abortus) were introduced to Wood Buffalo National Park (WBNP), Canada in the late 1920s. In the last thirty years, the bison population has declined to less than 25% of its former size. My main objectives were to assess the: (1) prevalence of tuberculosis and brucellosis in bison; (2) impacts of tuberculosis and brucellosis on bison reproduction and survival; (3) disease predation hypothesis as the cause of population decline. Serological testing indicated prevalence of 49% and 31% for tuberculosis and brucellosis, respectively, in 1997-1999. Prevalence for both diseases increased with age and males were more likely to test positive for tuberculosis. Historical data indicates that prevalence of neither disease is a direct function of bison density. These diseases are endemic and unlikely to disappear as the population of bison in WBNP declines. Brucellosis and tuberculosis interacted to affect survival and reproduction of bison. Among female bison captured in the Delta and Hay Camp populations, bison that tested positive for tuberculosis and had a high titre for brucellosis were less likely to be pregnant relative to bison with one or neither disease. In the Nyarling River population, tuberculosis was associated with a significant reduction in pregnancy rate. Annual survival was lowest in the Delta population and highest in the Nyarling River population. Wolf predation rate on bison >1 year of age was highest in the Delta population, but was a minor source of mortality elsewhere in the park. Tuberculosis-infected bison with a high brucellosis titre were 2.5 and 3.7 times more likely to die during early and late winter, respectively, than other bison. Simulation indicated that in the absence of tuberculosis and brucellosis, there is a high likelihood of bison persisting at high densities. In contrast, tuberculosis and brucellosis resulted in a high probability of bison persisting at low densities. I conclude that the decline in bison abundance in WBNP can be attributed to the presence of tuberculosis and brucellosis, and that the population will likely persist at low densities with the continued presence of these introduced diseases and natural levels of wolf predation.