NAS Report: Elk are Key Reservoir for Yellowstone Brucellosis

Elk herds in and around Yellowstone National Park have emerged as the key transmitters of brucellosis to cattle and bison, according to a new report from the National Academies of Sciences. The problem remains complex, with no simple solutions, but a coordinated, multi-agency approach, with emphasis on elk management coupled with vaccinations, enhanced surveillance and other measures could reduce the impact of Brucella abortus in the region’s cattle herds.

Researchers who conducted the extensive review, led by Terry F. McElwain, Regents Professor, School for Global Animal Health, Washington State University, presented their findings in a webinar on May 31.

The Greater Yellowstone Area (GYA), McElwain says, houses around 125,000 elk and 5,500 bison. The Academies last reported on brucellosis in 1998, and at that time, most of the surveillance and control strategies focused on bison moving in and out of the national park. Since then, elk numbers have grown and 22 cattle herds and five privately owned bison herds were affected with brucellosis in Idaho, Montana and Wyoming, the states that contain the GYA. Also, the disease has been detected outside the original designated surveillance areas (DSAs), resulting in expansion of surveillance boundaries. 

While bison management remains an important component in the control strategy, the report emphasizes a need to focus more study and control measures on elk and the interface between migrating elk and domestic livestock herds.

According to the Academies, brucellosis is found in cattle, bison, and elk and can result in late-gestation abortion, decreased milk production, and loss of fertility. Animals can be infected through contact with infectious materials such as aborted fetuses or afterbirth, and Brucella abortus – the bacterial pathogen that causes brucellosis – can spread to calves through nursing.  

The researchers suggest that reducing elk herd sizes and density could reduce prevalence of B. abortus over time. They also indicate that reducing or closing elk feedgrounds – areas designated for emergency feeding of elk during harsh winters – could reduce the spread of brucellosis among elk herds and over time, benefit the overall health of elk populations. McElwain stresses that closing feedgrounds or reducing elk populations are not stand-alone solutions, and should be carried out in a strategic, science-based manner, in coordination with other measures.

Naturally, discussions of managing elk populations generates controversy, as hunters, outfitters and wildlife enthusiasts generate significant revenue in the area, including on private ranches potentially affected by brucellosis. The authors suggest more study is needed to develop strategies for better “special and temporal separation” of elk and livestock herds, such as through adjustments to grazing allotments on public lands or strategic hazing of elk.

Cattle vaccination against B. abortus should continue in high-risk cattle herds, in conjunction with other risk-reduction strategies, the authors say. Vaccination does not provide 100 % protection against the pathogen, but significantly reduces abortion rates, helping prevent disease transmission. The authors encourage continued development for more-effective cattle vaccines and potentially for vaccines for use in elk.

The report also stresses a need for better coordination between federal, state and tribal agencies within the GYA, with expansion of DSA boundaries and uniformity in rules and standards in detecting infected cattle. Goals and strategies should address the entire GYA ecosystem, and agencies should adopt an “active adaptive management approach” – a decision-making process to reduce uncertainty of outcomes over time.

The committee also recommended developing a bio-economic model, which would provide a framework for decision making including the socio-economic costs and benefits of reducing transmission from wildlife to domestic cattle and bison.

The full report is available in print or electronic formats from the National Academies Press.