A bacterium, Brucella abortis, causes infection in a variety of mammals, including elk, bison, and cattle.

Elk, bison, and cattle, as well as many other mammals, are susceptible to infection by the bacteria Brucella abortus, which causes brucellosis. The Jackson bison and elk herds are chronically infected with the disease. Brucellosis has been present in elk on the National Elk Refuge since at least 1930, and even thought bison were declared brucellosis free in 1968 after several years of testing, samples collected in the late 1980s revealed that they had been reinfected either by the mid-1970s when they began wintering on the refuge, or possibly after they discovered the feedgrounds about 1980.

Although both sexes can contract the disease, transmission of brucellosis occurs by means of pregnant females when susceptible animals contact and ingest the bacterium. B. abortus from infected aborted fetuses, fetal fluids, fetal membranes, or vaginal discharges. Abortion is the characteristic sign of acute brucellosis, and there is no feasible treatment or cure for the disease. Abortion is characteristic of an acute brucellosis infection, and there is no feasible treatment or cure for the disease. Studies of elk and bison indicate between 50% of female elk and 90% of female bison abort their first calf after infection, but second and third pregnancies following infection progress normally. 

In general, brucellosis prevalence in bison and elk is more dependent on the intensity of a winter feeding program than on numbers of animals. When elk and bison are on feedlines, densities are much higher than what would be found on native winter ranges. Therefore, the primary management actions to reduce prevalence and transmission of brucellosis in these populations include greater dispersion of bison and elk through reductions in numbers or increasing movement and distribution.

The primary factor to consider when examining the risk for transmission of brucellosis from elk or bison to livestock is whether or not these species come into contact with each other or infectious birthing materials. In order to contract brucellosis, it is usually necessary for susceptible cattle to be present, or to arrive at the pace where infected bison or elk abort or give birth. Therefore, any management alternative that reduces the chance for contact between bison or elk and livestock will reduce the risk to livestock.