Brucellosis is caused by bacteria from the genus Brucella, and primarily affects ungulates (domestic livestock, bison, elk). The incurable disease localizes in the reproductive organs and/or the udder, resulting in abortion. Bacteria are transmitted through direct contact with infected animals or an environment contaminated with discharges from infected animals, particularly during abortion or calving. Although almost eradicated in domestic livestock in the United States through efforts by USDA-APHIS and state animal health agencies, brucellosis is a problem in wild bison and elk in the Greater Yellowstone Area of Wyoming and Idaho. Humans are susceptible to infection, called undulant fever, via direct contact with infected carcasses, afterbirth, and uterine discharge, as well as consumption of unpasteurized goat milk, and antibiotic therapy is the treatment of choice.
There are no specific signs of brucellosis infection in an animal's appearance. Obvious signs in pregnant animals are abortion or production of weak calves, lowered fertility, retained afterbirths resulting in uterine infections, and occasionally enlarged, arthritic joints. Utah currently has a brucellosis-free status.