Detection and Transmission Dynamics of Brucella abortus in the Greater Yellowstone Area

Wild, free-ranging, bison and elk in the greater Yellowstone area (GYA) are the last reported reservoir of Brucella abortus in the United States. Diagnosis of B. abortus infection, potentially leading to brucellosis, is challenging, as there is no perfect reference test. An evaluation of the fluorescence polarization assay (FPA) was performed using serum and tissues from the known B. abortus-infected bison herd in Yellowstone National Park (YNP) and serum from privately-owned bison. While the FPA and five other tests had perfect sensitivity in screening of B. abortus antibodies in bison, all tests had substantially lower specificity in the YNP herd. However, a Bayesian analysis showed that 59-74% of the culture-negative animals were most likely truly infected. A decision-tree analysis illustrated that the expected cost of FPA testing was comparable to the cost of other serologic tests. The FPA was shown to be highly sensitive but may not be able to differentiate culture-positive and culture-negative animals. For evaluation of tests under field conditions, longitudinal studies should be performed, testing animals throughout the study period and harvesting a subset of subjects at various points to determine their culture status. These studies could further facilitate sound, adaptive management decisions for the GYA. The ability of bison and elk to concomitantly serve as hosts of B. abortus increases the complexity of the risk of transmission to cattle. This multi-reservoir system poses significant challenges for comprehensive disease management. To address these intricacies, the first spatially-explicit risk assessment of B. abortus transmission among elk, bison, and cattle in the northern portion of the GYA was performed. The model used for this assessment was based on spatio-temporal probabilities of bacterial shedding by bison and elk on the northern GYA landscape. Although the model estimated substantial shedding of Brucella bacteria from bison in some winters, the most substantial risk of B. abortus transmission to cattle was from elk. The estimated percentage of cattle exposure risk from the Yellowstone bison herd was small (0.0-0.3% of total risk) compared with elk, which contributed 99.7-100% of the total risk. Increasing population size resulted in higher herd densities and increased bacterial shedding. Interactive effects between population size and winter severity were major determinants influencing bison movements to lower elevation winter grazing areas, overlapping with federally-regulated domestic cattle grazing allotments. Median total annual risk to cattle from elk and bison was 3.6 cattle-exposure event-days (95% P.I. 0.1-36.6). Natural herd migration and boundary management operations were important in minimizing the contribution of bison to cattle exposure risk, which supports continued boundary management operations for spatio-temporal separation between bison and cattle. Under current management practices, bison risk to cattle grazing in the northern portion of the GYA is expected to be minimal. The comingling of cattle and elk, especially during the late gestation period for elk, should be reduced when spontaneous elk abortions pose a risk for interspecies disease transmission. The inter- and intra-species contact rates required to maintain brucellosis in the GYA were previously uncharacterized. Without this knowledge, the likely effects of risk mitigation strategies could not be adequately evaluated. The wildlife risk model described above was used to estimate the spatio-temporal distribution of B. abortus shedding events from bison and elk populations in the northern GYA. The percentage of B. abortus infectious events in overlapping wildlife populations was calculated, and the risk of B. abortus transmission within and between populations was estimated. Bison risk from other bison and from elk showed almost 100% adequacy to transmit the organism once spatio-temporal overlap occurred; however, contact within elk populations was only sufficiently able to produce disease 34% of the time. Transmission risks to elk from elk in other populations or from bison were very small. Minimal opportunity exists for B. abortus transmission from bison to elk under current natural conditions in the northern GYA. Under current conditions, management alternatives that reduce bison seroprevalence are unlikely to substantially reduce transmission risk from elk to cattle. Strategies that decrease elk herd densities and group sizes and reduce elk-to-elk transmission could reduce the overall risk to cattle grazing in the northern portion of the GYA. This research has substantially filled gaps in the understanding of B. abortus diagnostics, transmission risk from wildlife to cattle, and transmission dynamics within and between wildlife populations in the GYA. It highlights the need for longitudinal studies of B. abortus infection and directs future management actions toward mitigating transmission risks within elk populations as well as from elk to cattle in the northern portion of the GYA. This work should help to justify the most efficient allocation of research and management funding and aid eventual eradication of B. abortus from the United States