Federal officials consider low-risk bison to be untested bulls, yearlings, calves and cows that have completely passed a placenta. If the state adopted that definition, it would allow many more bison to remain outside the park unmolested. Brucellosis, which can cause animals to abort their fetuses and can cause undulant fever in humans, is passed primarily through contact with infected birthing materials or aborted fetuses. The goal is to prevent infected animals from spreading the disease to cattle. Many are killed.
Critics are harsh, calling the killing a brutal and needless slaughter of a national treasure.
They say it is ridiculous to think the bison pose any threat to cattle, who will not even enter the pastures near Yellowstone for months. But the National Academy of Sciences, after studying the issue, called the risk of the bison passing the disease small but real, and said the result to the cattle industry could be catastrophic if transmission indeed occurred. The state has refused to adopt the more liberal federal policy on low-risk bison unless the federal government promises to reimburse the livestock industry for losses that would occur if other states, concerned about more bison being allowed to roam outside the park, demand testing of all Montana cattle crossing their borders. The state also wants a national organization of state veterinarians to endorse the federal policy before it will be accepted by Montana livestock officials. The U.S. Animal Health Association opposes the federal policy.