Effect of calfhood vaccination on transmission of bovine viral diarrhea virus under typical drylot dairy conditions

OBJECTIVE: To estimate transmission of bovine viral diarrhea virus (BVDV) and crude morbidity and mortality ratios in BVDV-vaccinated and unvaccinated dairy heifer calves managed under typical dairy drylot conditions. DESIGN: Randomized clinical trial. ANIMALS: 106 female Holstein calves. PROCEDURE: Seroconversion rates for BVDV types I and II and proportional morbidity and mortality ratios were compared between calves given a killed BVDV type-I vaccine at 15 days of age and a modified-live BVDV type-I vaccine at 40 to 45 days of age (n = 53) and calves given no BVDV vaccines (53). Sera were collected at 45-day intervals as calves moved from individual hutches to corrals holding increasingly larger numbers of calves. Seroconversion was used as evidence of exposure to BVDV. RESULTS: Crude proportional morbidity (0.16) and mortality (0.17) ratios for control calves did not differ significantly from those of vaccinated calves (0.28 and 0.12, respectively). The proportion of control calves that seroconverted to BVDV type I through 9 months of age (0.629) was significantly higher than that of vaccinated calves that seroconverted, unrelated to vaccination, during the same period (0.536). Estimated overall protective effect of vaccination against BVDV type I through 4 to 9 months of age was 48%. The proportion of control calves that seroconverted to BVDV type II (0.356) was not different from that of vaccinated calves (0.470). CONCLUSIONS AND CLINICAL RELEVANCE: Findings suggest that calfhood vaccination may be an appropriate strategy to help reduce short-term transmission of some but not necessarily all strains of BVDV.