In his comments on "Diseases," Dan Flores speaks of the possible role of anthrax and bovine tuberculosis, in the decline the bison during the 19th century. But it was the bison's susceptibility to another disease that has recently corralled it into the political arena again—"brucellosis," named for the Scottish physician Sir David Bruce, who identified the bacterial disease agent (Brucella abortus) in 1887. Recently, strong circumstantial evidence has suggested that bison and possibly elk can transmit the disease to domestic livestock such as cattle or horses. Ironically, the American bison, in all probability, originally became infected with the brucellosis organism after contact with infected cattle originating from Europe. Brucellosis was first diagnosed in the Yellowstone National Park's herd in 1917.
In cattle, brucellosis causes partial sterility, decreased milk supply, and abortion. A vaccine for calves was developed in the 1950s.
In humans, the same disease is called undulant fever, because the symptoms include recurrent fever, as well as joint pains and central nervous-system damage. The pasteurization of milk prevents the transmission of it; if contracted, it can be treated with certain antibiotics, at least for the time being.
During the 1960s a federally-supported campaign to eradicate brucellosis from domestic cattle began to take effect, and state after state has since earned, and rigorously guarded, its brucellosis-free status, without which interstate transportation of cattle is prohibited. All private, and most state and federal bison herds, such as those at Washita and Moiese, are tested regularly for brucellosis, but the Yellowstone National Park's herd roams a much more extensive area, making effective disease control difficult.
The problem in Yellowstone National Park came to a head in 1996 with the midwinter migration of more than a thousand head of bison from Yellowstone National Park onto state and private land north of the Park. For a number of years a few animals had crossed the park boundary each winter, and the Montana Fish, Wildlife and Parks department harvested them, first by permitted hunting, and later by game wardens.
The mass exodus of 1996 was prompted by a combination of circumstances. The herd had grown beyond the carrying capacity of the park's limited habitat, nourished by an increase in forage immediately following the fires of 1988, encouraged by a short succession of comparatively mild winters, and favored by the absence of wolves, their principal predators.