winter and controversial brucellosis management strategies which sent hundreds to slaughter. The Yellowstone Park bison herd is believed to be one of only four free-roaming and genetically pure herds on public lands in North America. The other three herds are the Henry Mountains bison herd of Utah, at Wind Cave National Park in South Dakota, and in Elk Island National Park in Alberta. The relatively large bison populations are a concern for ranchers, who fear that the species can transmit bovine diseases to their domesticated cousins. About half of Yellowstone's bison have been exposed to brucellosis, a bacterial disease that came to North America with European cattle that may cause cattle to miscarry. The disease has little effect on park bison, and no reported cases of transmission from wild bison to domestic livestock have been filed. However, the Animal and Plant Health Inspection Service (APHIS) has stated that bison are the "likely source" of the spread of the disease in cattle in Wyoming and North Dakota. Elk also carry the disease and are believed to have transmitted the infection to horses and cattle. To combat the perceived threat of brucellosis transmission to cattle, national park personnel regularly corral bison herds back into the park when they venture outside of the area's borders. During the winter of 1996–97, the bison herd was so large that 1,079 bison that had exited the park were either shot or sent to slaughter. Animal rights activists argue that this is a cruel practice and that the possibility for disease transmission is not as great as some ranchers maintain. Ecologists point out that the bison are merely traveling to seasonal grazing areas that lie within the Greater Yellowstone Ecosystem that have been converted to cattle grazing, some of which are within National Forests and are leased to private ranchers. APHIS has stated that with vaccinations and other means, brucellosis can be eliminated from the bison and elk herds throughout Yellowstone. Starting in 1914, to protect elk populations, the U.S. Congress appropriated funds to be used for "destroying wolves, prairie dogs, and other animals injurious to agriculture and animal husbandry" on public lands. Park Service hunters carried out these orders, and by 1926 they had killed 136 wolves. Gradually, wolves were virtually eliminated from Yellowstone. Further exterminations continued until the National Park Service ended the practice in 1935. With the passing of the Endangered Species Act in