Disease Ecology and Adaptive Management of Brucellosis in Greater Yellowstone Elk

Brucellosis is a bacterial infection that primarily affects livestock and can also be transmitted to humans. In the Greater Yellowstone Ecosystem (GYE), elk (Cervus canadensis) and bison (Bison bison) are habitual carriers of Brucella abortus, which arrived to the region with cattle over a century ago. The disease was eliminated from cattle in the United States through widespread control efforts, but is now periodically transmitted back to cattle on open rangelands where they can come into contact with fetal tissues and fluids from disease-induced abortions that occur among elk during the late winter and spring. In Wyoming, south of Yellowstone National Park, there are 23 supplemental feedgrounds that operate annually and feed the majority of the region’s elk during a portion of the winter. The feedgrounds are controversial because of their association with brucellosis and may be shuttered in the future in part due to the arrival of chronic wasting disease. Using data collected at these feedgrounds, this study investigates the role of winter feedgrounds in the ecology of this host-pathogen relationship: it evaluates the full reproductive costs of the disease to affected elk, how herd demography influences pathogen transmission, and assesses management strategies aimed at reducing pathogen spread among elk. Using blood tests for pregnancy status and brucellosis exposure in female elk, I demonstrated a previously undocumented fertility cost associated with the pathogen which is not due to abortions, but which nearly doubles the estimated fertility cost to affected individuals. I also built mechanistic transmission models using time-series disease and count data from feedgrounds. Within that framework, I assessed various management actions including test-and-slaughter of test-positive elk, which I found to be counterproductive due to rapid recovery times and the protective effects of herd immunity. The overall picture that emerges of winter feedgrounds is one of imperfect practicality driven by social and political consideration, not pathogen control. These results illustrate the underappreciated importance that recruitment and population turnover have on the transmission dynamics of brucellosis in elk, a pathogen which itself flourishes in the reproductive tracts of individual animals and thus impacts vital rates at the population level. Together, this study contributes to the field of disease ecology using a unique long term disease data set of free-ranging wild ungulates

R code. Function to create a smoother line using generalised additive models. from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

This file is called in to other code files and is necessary for the generation of smoothed brucellosis prevalence estimates, including in years for which there is no data

R code for seroprevalence of unfed herds (fig 3) from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

Reads in serology data from 3 unfed elk populations and generates a plot illustrating brucellosis prevalence, sample sizes, and trends over time

Test and slaughter serology data. from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

This is the serology data used to generate figure 4 in .csv format

R code. Function to create prevalence estimates by location and year. from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

This file is called in to other code files and is necessary for the generation of brucellosis prevalence estimates by site and year (and 95% confidence intervals)

Unfed elk herd serology data. from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

This is the serology data used to generate figure 3 in .csv format

R code for seroprevalence at test and slaughter feedgrounds (fig 4) from Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics

Reads in serology data from 3 sites and generates a plot illustrating brucellosis prevalence, strength of the estimates, and trends over time in relation to when experimental treatment occurred