Simulation of host—parasite interactions within a resource management framework: impact of brucellosis on bison population dynamics

Abstract

We describe development of a simulation model representing bison (host)—bbrucellosis (parasite) interactions within a natural resource management framework. We document three phases in model development which focus on the simulation of bison population dynamics in (1) a brucellosis-free herd, (2) a brucellosis-infected herd, (3) and infected herd subjected to different vaccination schemes. The first phase represents bison populations dynamics based on age-specific rates of natural mortality and natality and rates at which animals are harvested. The second phase represents the influence of brucellosis-induced abortions, brucellosis transmission, and naturally acquired immunity. The third phase represents acquired immunity to brucellosis (or lack thereof) due to immunization, which is a function of vaccine efficacy and the percentage of bison to which the vaccine can be delivered. The model accurately simulated historical changes in herd size, annual recruitment, population age structure, and survivorship of individual cohorts over a 17-year period in a brucellosis-free bison herd on the National Bison Range, U.S.A. (Phase I). The model also accurately simulated historical changes in herd size, and predicted the most likely year of infection, in a bison herd in Grand Teton National Park, U.S.A., that contracted the disease during a 20-year period (Phase II). Finally, the model was used to evaluate a proposed bison brucellosis management plan for the Grand Teton National Park herd which involves annual vaccination of female calves (Phase III). Simulations suggested that, after 20 years, the proportion of the herd infected with brucellosis might be reduced from 69% (current level) to between 50% and 20%, depending on the (currently unresolved) vaccine efficacy.