Brucellosis in Yellowstone

Rates of Infection

About 60% of adult female bison in Yellowstone test positive for exposure to the Brucella bacteria. However, testing positive for exposure does not mean an animal can transmit the disease. For example, anyone who received a smallpox immunization will test positive for smallpox antibodies even though they are not infected with the disease and cannot transmit it. The number of infectious female bison that could transmit live bacteria varies from 10% to 15% of the population.


There have been no documented cases of bison transmitting brucellosis directly to cattle, in part due to efforts by federal and state agencies to maintain separation between these animals. There have been many occasions where elk transmitted brucellosis to cattle. In both cases, transmission is possible and the likelihood increases during late winter when bison, elk, and cattle are more likely to share low elevation grasslands along the river valleys outside Yellowstone.

Prevention & Testing

Currently, there is no vaccine that’s 100% effective against brucellosis. Local cattle operations vaccinate their animals with the best available vaccine, but it doesn’t eliminate the potential for brucellosis infection from wildlife. The lack of an effective vaccine also makes it even more difficult to eliminate or suppress the disease in wild bison (which park officials studied in 2014).

Testing for brucellosis also lacks certainty. Blood tests on live bison can only identify which animals have been exposed to the bacteria. To determine that an animal is truly brucellosis-free requires months to years of quarantine depending on an animal’s age, sex, and reproductive status. Quarantine is an alternative for bison management that the National Park Service studied and is implementing through its Bison Conservation Transfer Program.

Risk to People

People can get brucellosis if they come into contact with infected animals or animal products contaminated with the bacteria. The most common way to be infected is by eating or drinking unpasteurized/raw dairy products. Brucellosis cannot be contracted by eating cooked meat from an infected animal. More information is available on the Centers for Disease Control and Prevention website.

Last updated: December 21, 2023