Samples and data from a serum bank involving 998 cows from 268 dairy herds in Ontario were used to estimate the prevalence of dairy cows with anti-bovine leukemia virus (BLV) antibodies, and to investigate associations between BLV serological test results and farm management practices, selected production indices, and culling. Sera were tested for anti-BLV antibodies by the BLV agar-gel immunodiffusion test. Data matched with the serum bank consisted of production records from the Ontario Dairy Herd Improvement Corporation, including breed-class average for milk, calving interval and somatic cell counts. Also matched with the serum bank were farm management data that had been collected by a mail survey during a previous cross-sectional study. The individual-cow prevalence of BLV seropositivity was 24.2%; herd prevalence was 47%. All analyses were adjusted for herd size. The odds of a herd having at least one seropositive cow were increased (relative to other categories) if cows calved in separate calving pens and if calves were raised in calf hutches in the winter months, and were decreased for farms on which cows calved in separate pens in the summer. No significant associations were detected between BLV seropositivity and milk production, somatic cell counts, age at disposal and culling. BLV-seropositive cows had a slight, statistically significant, increase in calving interval, after accounting for age and milk production in the most recent lactation.
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