The harsh winter and lack of food have led the bison to emigrate in North America outside the borders of Yellowstone National Park. It has been the last trip for 750 visons for being killed outside the park.
The explanation given by U.S. health services has been as follows: bisontes carry a bacterium called Brucella, dangerous for livestock and human beings, and that is why they have been killed. For those of the national park, however, it makes no sense. Other species, such as the elk, also contain this bacterium, but circulate unhindered without danger of death.
In February 1996 at Yellowstone Park there were only 2,200 bison.