Use of survival analysis to compare cull rates between bovine leukemia virus seropositive and seronegative dairy cows

Franklin L. Pollari From the Department of Epidemiology, School of Public Health and Community Medicine, University of Washington, Seattle, WA 98195 (Pollari, DiGiacomo), and the Department of Veterinary Clinical Medicine and Surgery and Washington Animal Disease Diagnostic Laboratory, College of Veterinary Medicine, Washington State University, Pullman, WA 99164 (Evermann).

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Ronald F. DiGiacomo From the Department of Epidemiology, School of Public Health and Community Medicine, University of Washington, Seattle, WA 98195 (Pollari, DiGiacomo), and the Department of Veterinary Clinical Medicine and Surgery and Washington Animal Disease Diagnostic Laboratory, College of Veterinary Medicine, Washington State University, Pullman, WA 99164 (Evermann).

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James F. Evermann From the Department of Epidemiology, School of Public Health and Community Medicine, University of Washington, Seattle, WA 98195 (Pollari, DiGiacomo), and the Department of Veterinary Clinical Medicine and Surgery and Washington Animal Disease Diagnostic Laboratory, College of Veterinary Medicine, Washington State University, Pullman, WA 99164 (Evermann).

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Summary

Bovine leukemia virus (blv) infection and culling of cows in a commercial dairy herd were evaluated to determine whether a relation existed between the 2 factors. Cattle from the study population, a Holstein dairy herd consisting of approximately 400 milking cows, were tested for antibodies to blv, using the agar gel immunodiffusion test, semiannually for 2 years, annually for 2 years, and when cattle were culled. Complete records of blv test results were available for 849 (79%) of the 1, 078 cattle that had at least 1 test during the study period. Using the Cox hazard model, the cull hazard rates (culls/cow-months) were greater for blv seropositive cows than for seronegative cows > 36 months old. Hence, among older dairy cows, blv-infected cows were culled prematurely, compared with uninfected cows.

Summary

Bovine leukemia virus (blv) infection and culling of cows in a commercial dairy herd were evaluated to determine whether a relation existed between the 2 factors. Cattle from the study population, a Holstein dairy herd consisting of approximately 400 milking cows, were tested for antibodies to blv, using the agar gel immunodiffusion test, semiannually for 2 years, annually for 2 years, and when cattle were culled. Complete records of blv test results were available for 849 (79%) of the 1, 078 cattle that had at least 1 test during the study period. Using the Cox hazard model, the cull hazard rates (culls/cow-months) were greater for blv seropositive cows than for seronegative cows > 36 months old. Hence, among older dairy cows, blv-infected cows were culled prematurely, compared with uninfected cows.

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