Descriptive epidemiology of postnatal bovine viral diarrhea virus infection in intensively managed dairy heifers.

OBJECTIVE: To evaluate risk of bovine viral diarrhea virus (BVDV) infection between birth and 9 months of age for dairy replacement heifers raised under typical dry-lot management conditions. DESIGN: Longitudinal observational study. ANIMALS: 446 calves. PROCEDURE: Calves were randomly selected from 2 dairies that used killed and modified-live BVDV vaccines. Repeated serologic and BVDV polymerase chain reaction assays were used to estimate risk of BVDV infection in calves of various ages (1 to 60 days; 61 to 100 days; 101 days to 9 months) and to estimate overall infection rate by 9 months of age. RESULTS: Risk of BVDV infection increased with age (maximum risk, 150 to 260 days). Proportion of calves infected with BVDV by 9 months of age was higher for dairy A (0.665), compared with dairy B (0.357). Percentage infected with BVDV type I did not differ between dairy A (18.2%) and dairy B (15.2%), whereas percentage infected with BVDV type II for dairy A (50%) was twice that for dairy B (21%). Between 210 and 220 days of age, infection with BVDV regardless of type was > 1.3%/d on dairy A and 0.5%/d on dairy B. CONCLUSIONS AND CLINICAL RELEVANCE: Under dry-lot conditions, a considerable amount of BVDV infection may occur before 9 months of age. Risk of infection increases with age. Although dairies may appear to have similar management practices, there can be considerably different risks of BVDV infection among dairies.