Neospora caninum serostatus and culling of Holstein cattle

Objective - To determine whether time until culling or risk of culling was associated with Neospora caninum serostatus among Holstein cattle in dairy herds in Ontario. Design - Retrospective cohort study. Animals - 3,412 cows in 56 herds. Procedure - Blood samples were collected, and serum was tested for antibodies against N caninum. Information on cows that were culled was collected during the 1-to 2-year period that producers were unaware of serostatus of individual cows in their herds. Results - Herd prevalence of N can/num-seropositive cows ranged from O to 68.3% (median, 7.0%). During the time of the study, 184 of 359 (51.3%) N caninum-seropositive cows were culled, compared with 1,388 of 3,053 (45.5%) seronegative cows. Mean time from blood sample collection to culling for seronegative cows (289 days; 95% confidence interval, 280 to 299 days) was not significantly different from mean time for seropositive cows (296 days; 95% confidence interval, 269 to 323 days). Survival analysis indicated that N caninum serostatus was not associated with time to culling or risk of culling. Conclusions and Clinical Relevance - Results suggest that N caninum serostatus of Holstein cows in Ontario was not significantly associated with either time to culling or risk of culling. Thus, N caninum serostatus alone should not be used to determine whether cows should be culled.