Commingled grazing as a risk factor for trichomonosis in beef herds.
OBJECTIVE: To evaluate commingled grazing on public lands as a risk factor for Tritrichomonas foetus infection in beef herd bulls. DESIGN: Case-control study. SAMPLE POPULATION: Based on 1994 data from the mandatory Idaho bull testing program, all 65 infected herds (case herds), 78 randomly selected test-negative herds that tested < 10 nonvirgin bulls, and 81 randomly selected test-negative herds that tested > or = 10 nonvirgin bulls (control herds). PROCEDURE: Managers of government-owned grazing lands in Idaho identified producers who had grazing permits for an allotment under their jurisdiction in 1993 and for that allotment recorded the number of animals the producer was permitted to graze on the allotment, the number of herds with grazing permits, the total number of animals permitted, and the dates on which grazing began and ended. The number of bulls tested, number of times tested, and test results were collated from the testing database. RESULTS: The relative sensitivity of bacterial culture of preputial smegma was 81%. The attributable fraction of T foetus infection associated with commingled grazing was 33%, and the odds ratio of infection was 9.0 for herds commingled with > or = 14 other herds. The total number of animals permitted on an allotment, the use of public lands, and type of public lands used were not significantly associated with infection status. CLINICAL IMPLICATIONS: These results suggest that to control the spread of trichomonosis, the number of herds commingling on a grazing allotment should be minimized and commingled herds should be managed collectively.