The greater Yellowstone region covers about 20 million acres overlapping Idaho, Montana, and Wyoming. Commonly called the Greater Yellowstone Area, Greater Yellowstone Ecosystem, or just Greater Yellowstone, the region is home to numerous species of wildlife, including bison. It encompasses two national parks—Yellowstone and Grand Teton—as well as several other federally managed areas, including the Gallatin, Custer, Shoshone, Bridger-Teton, Caribou-Targhee, and Beaverhead national forests; the National Elk Refuge; and the John D. Rockefeller, Jr., Memorial Parkway. Privately owned lands, including the Royal Teton Ranch north of Yellowstone National Park and west of the Yellowstone River, are interspersed throughout these federal lands, and a number of towns dot the landscape (see fig. 1). The Greater Yellowstone Area has also earned the distinction, according to APHIS, of being the nation’s last known reservoir of the brucellosis bacterium, Brucella abortus, which is present in the region’s bison and elk populations. Bison, which are native to the Greater Yellowstone Area, were observed by early travelers both before and after Yellowstone National Park was created in 1872. Bison hunting and poaching in the late 1800s substantially diminished the Yellowstone herd, and, by 1901, fewer than 25 animals remained (see fig. 2). To save this wild herd from extinction, park managers in 1902 imported 21 bison from captive herds in Texas and Montana. Although the wild and captive herds were initially kept separate, they began to intermingle between 1915 and 1920, and after the 1920s, little or no effort was made to keep the two populations separate. By the 1930s, the park’s wintering bison had increased to more than 1,000 head, and the park began managing the herd by shipping animals to public parks, zoos, and privates estates around the country. From the 1920s through the late 1960s, the bison herd was managed through culling, supplemental feeding, and other activities until the park instituted a policy of natural regulation of wildlife populations, whereby a species’ population size is left to respond to environmental conditions like climate or food supply without human interference. By the early 1980s, the bison herd had grown to more than 2,300 head, and bison were increasingly reported moving beyond the park’s boundaries into Montana. During the mid-1980s through 1999, a total of 3,176 bison that crossed outside the park’s northern or western boundaries were killed, under a series of park boundary area management approaches implemented by the Park Service and Montana. Since 2000, boundary area management action by federal and state agency officials have lethally removed more than 1,900 bison, but the herd has continued to grow, with nearly 4,700 head counted in late summer 2007. Bison, as well as wildlife such as elk, may carry the bacterial disease brucellosis, which is also contagious to humans and domestic animals. Known in humans as undulant fever, brucellosis can be hard to diagnose because early symptoms, such as intermittent fever and joint pain, are shared with several other diseases; although rarely fatal, the disease can be debilitating. However, thanks to widespread pasteurization of dairy products since the early twentieth century, very few cases of undulant fever have occurred in the United States, and the disease today poses a risk mainly to hunters, large-animal veterinarians, and ranchers. In some animals, including cattle, the disease can cause abortions, infertility, reduced milk production, lameness, and swollen joints. In the Yellowstone area, bison and elk are the disease’s principal wildlife hosts. Results of blood tests done by the Park Service over the past several decades indicate that about half of the Yellowstone bison have been exposed to brucellosis. Some bison may develop immunity or have a natural resistance to the disease, while other animals may develop recurrent infections and remain carriers and a source of exposure and possible infection for other animals. The first known case of brucellosis in Yellowstone bison was reported in 1917 and is believed to have been transmitted from domestic cattle. Transmission occurs primarily through ingestion of infected products of birth or abortion. As a result, the risk of transmission is greatest if cattle and bison are in each other’s presence immediately after birthing. Although vaccinating cattle provides some protection, it does not eliminate the infection risk, as evidenced since 2004 by instances of brucellosis transmission from infected wildlife to vaccinated cattle in Idaho and Wyoming. Within the scientific community and among people interested in bison management, opinions differ about whether it is most appropriate to control or to eradicate the disease. Within the regulatory community, however, APHIS officials remain committed to establishing the means to suppress and eliminate contagious livestock diseases, including brucellosis. Although both bison and elk carry brucellosis, the two species are managed differently by federal and state agencies. Yellowstone bison are subject to strict disease-control measures, ranging from capture and vaccination to slaughter, whereas elk herd movements are not restricted. Agency officials indicate that this difference in management approach stems largely from the lower prevalence of brucellosis in elk than in bison, combined with the tendency of elk to feed at higher elevations than bison and to calve in isolation, thereby lessening the risk of transmission. This difference in management remains even though there have been multiple suspected elk-to-cattle transmissions in recent years in Idaho and Wyoming, some of which have been detected through DNA testing; the National Research Council reported in 1998 that the risk of transmission from bison to cattle was low; and there have been no known cases of brucellosis transmitted from bison to cattle in a wild, uncontrolled setting. APHIS scientists note, however, that although transmission in the wild is difficult to document—particularly given the decades-long effort to maintain separation between bison and cattle—controlled tests have proven that it is possible for bison to transmit the disease to cattle. We reported in 1997 that the extent to which domestic cattle risk infection through exposure to diseased bison and elk—either from mingling directly with infected wild animals or from using rangeland where infected wild animals had previously grazed—was the subject of the bison management controversy between the federal and state agencies because the risk of such transmission had not been quantified. Scientists and researchers disagreed on even the most basic factors influencing the risk of transmission, such as whether studies on cattle are applicable to bison. Ten years later, such debates continue. To keep bison away from cattle and mitigate the risk of brucellosis transmission, the interagency bison management plan describes agency activities to restrict bison movement to or through bison management zones within or just beyond the park’s boundaries near Gardiner, Montana, on the north, and near West Yellowstone, Montana, on the west (see fig. 3). During step one of the three-step plan, bison attempting to leave the park on the north side are to be hazed, captured, or killed, and a limited number of bison on the west side of the park—as many as 100 animals— that test negative for brucellosis exposure are allowed to roam in bison management zones outside the park. Once this limit on the west side is reached, additional bison crossing beyond the western boundary are also to be hazed, captured, or killed. The plan states that all captured bison are to be tested for brucellosis exposure at capture facilities located on the park’s northern and western boundaries. Partner agency officials may take a variety of actions with captured bison testing negative for exposure to the disease, including vaccinating and temporarily holding them in the northern capture facility for release back into the park or removing them for research. Bison that test positive for brucellosis exposure are generally to be sent to slaughter. Under both steps two and three of the plan, the majority of bison must be prevented from leaving the park, and the few bison that are allowed to leave are generally to be restricted and monitored within the bison management zones and other management areas. Step two on the park’s north side will allow up to 100 bison testing negative for brucellosis to be released into these bison management zones; pregnant females are to be monitored using radio telemetry, and all bison are to be hazed back into the park or otherwise removed by April 15 of each year. Under step two, on the park’s west side, up to 100 bison that have not been tested will be allowed to exit the park, and officials are required to vaccinate these bison remotely; May 15 is the deadline for all bison to be back in the park. Similarly, in step three, an effective remote vaccination program must be operating in the park before up to 100 untested bison will be allowed out of the park into the bison management zones on both the north and west sides. However, except in a few locations, if more than 100 bison attempt to migrate beyond the park’s northern and western boundaries, under step three they are to be hazed, captured, or killed. Many of the management actions specified in step one are to continue indefinitely, even when step three is fully implemented. A brucellosis outbreak among domestic cattle in Montana would likely have the following direct effects: (1) abortion of calves, (2) decreased weight gain by calves, (3) delays in calf production, (4) increased rates of culling and replacement, and (5) increased testing and vaccination costs.