The late 1990s saw a dicult problem arise between the agencies responsible for protecting bison and the ranchers who graze herds of cattle on private and public property all around Yellowstone. The basic problem is that cattle in Montana have been certified free of brucellosis, a disease that can cause a pregnant cow to abort her calf, but does not cause abortion in bison. Ranchers fear the disease could be transmitted to cattle when the park's brucellosis-infected bison migrate to winter feeding grounds outside the park. Following the signing of an interagency agreement in 2000, the National Park Service and the Montana Department of Livestock now herd bison back into the park when they stray to the north and west.
Some animals are penned and tested, and those that test positive for brucellosis are slaughtered.
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In 2007 there were well over 4000 bison. During the next winter legally regulated slaughter and winterkill combined to reduce the herd to about 3000. Factors complicating the story include the strong historical possibility that brucellosis was originally introduced into Yellowstone by infected cows brought in to supply milk for the early hotels and the fact that elk also carry brucellosis.