Male bison must be quarantined for a year before they can be deemed free of brucellosis, which is feared by the livestock industry because it causes cattle to abort their offspring. More than half of Yellowstone bison are believed to have been exposed to the disease.
Source: AgWeb, June 26, 2017.
There has been no documented case of bison transmitting the disease to cattle in the wild but that fear has driven efforts to control the Yellowstone population and limit where bison are allowed. Quarantining bison is seen as a way to reduce the number slaughtered each year and to establish more bison herds around the U.S.