Humoral immune response to modified live bovine viral diarrhea virus vaccination in commercial Canadian dairy herds: A cross-sectional field study.

Vaccination is commonly used on Canadian dairy farms as a routine herd management practice to prevent infection with bovine viral diarrhea virus (BVDV) and to control its effects. This study aimed to determine the humoral immune response to vaccination with modified live BVDV vaccines administered on commercial dairy farms. A multisite observational cross-sectional study was conducted on 310 dairy herds across 3 Canadian provinces (Alberta, Ontario, and Québec), including 2,866 cattle aged 4 to 24 mo, either vaccinated or unvaccinated against BVDV. Vaccines used and vaccination protocols were farm specific. On a farm visit, up to 10 animals aged between 4 and 24 mo were selected, and their vaccination status against bovine viral diarrhea (BVD) was recorded (date of most recent BVD vaccination and brand of vaccine). Antibody ELISA were performed to assess the association between animal age at sampling and the time since vaccination with (1) the humoral immune response, measured by the sample-to-positive (S/P) ratio of a BVDV ELISA antibody test using negative binomial regression; and (2) the probability of testing positive on the same ELISA test (i.e., S/P ratio >0.199) using logistic regression. Time since vaccination and age at sampling were associated with the measured humoral immune response. The probability of a positive ELISA result increased significantly with time since vaccination, reaching 90% positivity in calves aged 4 to 10 mo and nearly 99% positivity in older young stock (>10 mo) by 28 d after vaccination. Animals aged 4 to 10 mo required a longer period of time before demonstrating a measurable humoral immune response. In conclusion, the results of this study demonstrate that vaccines administered under real farm conditions will lead to a measurable humoral response in the majority of calves and young stock. This suggests that modified live vaccines against BVDV are effective in inducing humoral immunity in young stock on commercial dairy farms.