A lthough Brucella abortus can infect other mammalian species, cattle are the preferred host for this species of Brucella. As humans are effectively dead-end hosts, in that brucellosis is essentially not transmitted between people, the maintenance of disease in animal hosts is the source of human infection. For that reason, many industrialized countries have invested heavily in brucellosis control programs in livestock due to public health benefits from the reductions in disease prevalence in animal reservoirs. In the United States, it was estimated in 2000 that approximately 11 billion dollars had been spent in efforts to eradicate brucellosis from cattle. This investment was rewarded in 2008 by the announcement from the U.S. Department of Agriculture that for the first time in the United States, all 50 states were simultaneously free of cattle brucellosis. Despite the eradication of B. abortus from domestic livestock, the persistence of B. abortus infection in freeranging bison and elk at Yellowstone National Park and the surrounding areas remains a concern for the reintroduction of brucellosis to cattle.Previous studies have demonstrated that bison are more susceptible to infection with B. abortus than are cattle, and a single vaccination with B. abortus strain RB51 is effective in reducing the incidence of abortion and infection in bison after experimental challenge (1, 2). In a previous study with a small number of bison, we (3) demonstrated that booster vaccination with RB51 at a 13-month interval increased protection against experimental challenge compared to that provided by a single RB51 vaccination administered during calfhood. In the study reported here, we expand on the previous booster vaccination study with greater experimental units and more extensive bacteriologic and immunologic characterization. MATERIALS AND METHODSB. abortus cultures. For the immunologic assays, RB51 suspensions (1 ϫ 10 12 CFU/ml) were inactivated by gamma irradiation (1.4 ϫ 10 6 rads), washed in 0.15 M sodium chloride (saline), and stored at Ϫ70°C.Animals and inoculation. Eight-to 11-month-old bison heifers were obtained from a brucellosis-free herd. After acclimation, the bison were randomly assigned to receive either saline (control; n ϭ 7) or a single intramuscular vaccination with RB51 (n ϭ 24). Some of the vaccinated bison (n ϭ 16) were randomly selected for booster vaccination with RB51 at 11 months after the initial vaccination.A commercial RB51 vaccine was obtained in lyophilized form (Colorado Serum Company, Denver, CO) and diluted in accordance with the manufacturer's recommendations. All hand inoculations were of 2 ml in volume and administered intramuscularly in the cervical region drained by the superficial cervical lymph node. Following vaccination, the concen-

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