"Once staring into the oblivion that is extinction, bison have rebounded in number and prestige to be called the United States’ national mammal. Yet those in Yellowstone National Park don’t fully carry the prestige that distinction bestows.
Though believed to have descended from pocket herds in the upper headwaters of the Yellowstone River that escaped the great slaughter of the late 19th century that veritably annihilated the species, and prized for their unsullied genetics, half or more of the park’s bison carry a cattle-introduced disease that, frankly, scares the hell out of ranchers.
Brucella abortus, a bacterium thought to have reached the country from European livestock, can cause spontaneous abortions or stillbirths in bison and cattle. Until 2010, if two or more herds in a state contracted the disease, or if a single herd detected to carry the disease was not sent to slaughter, all herds in that state were blacklisted from markets. Today that blacklisting applies only to the affected herd. Nevertheless, the risks of the disease infecting cattle have stigmatized Yellowstone bison and impacted their instincts to head outside the park for calving and prevented the spread of their genes to other bison herds in the West. ...
Yet the fact of the matter, according to the National Academy of Sciences, is that elk, not Yellowstone bison, are the culprit when it comes to spreading brucellosis. But, as Doug Smith, Yellowstone’s senior wildlife biologist, points out, 'elk are really gods around here.'"