Yellowstone National Park does not have a fence so the bison can migrate outside of the park.  About half of Yellowstone’s bison test positive for exposure to brucellosis, a disease that can cause susceptible domestic cattle to abort their first calf. Since Yellowstone’s bison migrate into Montana, their exposure to brucellosis concerns the cattle industry in that state.  Montana has spent a lot of money trying to get rid of brucellosis in cattle.