Bovine brucellosis, caused by Brucella abortus, is a global zoonotic disease primarily infecting cattle, in which it produces abortions, retained placentas, male reproductive tract lesions, arthritis, and bursitis. In humans, brucellosis can cause recurrent fever, night sweats, joint and back pain, other influenza-like symptoms, and arthritis. In animals and humans, it can persist for long periods. During the 1930s, a state–federal cooperative effort was begun to eliminate the disease from livestock in the United States. From an initial estimated prevalence in 1934 of ≈15%, with nearly 50% of cattle herds having evidence of infection (1,2), the United States now has no known infected livestock herds outside of portions of Idaho, Wyoming, and Montana, adjacent to Grand Teton and Yellowstone National Parks. This area, referred to as the Greater Yellowstone Area (GYA), also encompasses state and federal feeding grounds in Wyoming where elk are fed during the winter. Considered a spillover disease from cattle to elk and bison, brucellosis now regularly spills back from elk to cattle. Although bison-to-cattle transmission has been demonstrated experimentally and in nature (3,4), it has not been reported in the GYA, probably because of ongoing rigorous management actions to keep cattle and bison spatially and temporally separated.