was reported by Flagg (1983). The strongest evidence of transmission between free-roaming bison and elk comes from Grand Teton National Park (GTNP) and National Elk Refuge (NER) in the Jackson area of Wyoming (Williams et al. 1993, 1997). A small herd of bison was established in the wildlife park at GTNP in 1948 and in 1963 was found to be infected by B. abortus. All adults were removed, and calves were vaccinated. Brucellosis-free bison were introduced from Theodore Roosevelt National Memorial Park in 1964. This population was tested thereafter; calves were vaccinated, and all seropositive animals were removed. The last identified reactor was removed in 1967, and all adult bison tested negative in 1968. Late in 1968 and in 1969, some bison escaped from the wildlife park, and attempts were made to return them to the park. By 1970, however, nine bison were free-roaming because they could not be recaptured. The herd subsequently grew in numbers (Peterson et al. 1991b). About 1980, the animals began to winter on the NER, where they came into contact with winter-fed elk that were known to be infected with brucellosis. Cattle were not present on NER. In 1989, 11 of 16 bison collected on NER tested seropositive for brucellosis. On the basis of their modeling results, Peterson et al. (1991b) believed that the bison became infected in about 1980, and they noted that the bison herd first wintered on the NER, a potential source of B. abortus from winter-fed elk, in 1979-1980. Because the GTNP bison herd is isolated from the YNP bison herd by the continental divide, infection in GTNP bison is assumed to have derived from their contact with infected elk on the wintering grounds. Although the possibility of brucellosis having survived in the bison at the time of their escape from the wildlife park cannot be ruled out, transmission from elk seems more probable.

Two horses contracted brucellosis in the Jackson, Wyoming, area, where the only known source of the disease was elk on the winter feeding grounds (see p. 35, ''True Prevalence").

One of the most contentious issues—because it is key to determining the need for control of the disease in GYA wildlife—is the probability of transmission of brucellosis between free-roaming bison and domestic livestock. Nearly all parties to the controversy agree that the risk of transmission of brucellosis from bison to cattle in the GYA is small, but not zero. Defining small depends on whether transmission has occurred in the past and, if so, how often. That is key to determining the need to control brucellosis in bison. Advocates of no control maintain adamantly that no case of transmission of brucellosis from bison to cattle in the free-roaming state in the GYA ever has been documented. Advocates of the need to control the disease in bison to protect livestock in the surrounding areas maintain equally stoutly

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