Antibody titers to vaccination are not predictive of level of protection against a BVDV type 1b challenge in Bos indicus - Bos taurus steers.

Subclinical illness associated with infection is thought to reduce performance and increase production costs in feedlot cattle, but underlying components remain largely unidentified. Vaccination is frequently used in feedlot settings but producers lack metrics that evaluate the effectiveness of vaccination programs. The goal of this study was to determine if levels of serum neutralizing antibody titers were predictive of levels of vaccine protection in a commercial setting. During this four-year study, Angus-Nellore steers housed in a production feedlot setting were assigned to 1 of 3 vaccine treatments: killed vaccine (kV), modified live virus (MLV) vaccine, or no vaccine (control), and were challenged with a noncytopathic 1b field strain of bovine viral diarrhea virus. Rectal temperature and levels of circulating lymphocytes and platelets were monitored following challenge. While no animals were diagnosed as clinically ill with respiratory disease, indicators of disease (pyrexia, lymphopenia, and thrombocytopenia) were observed. The MLV treatment elicited higher antibody titers to the vaccination than the kV, and calves in the MLV treatment had higher mean titers at challenge. The year that elicited the highest antibody response to the vaccination and the year with the lowest frequency of phenotypic responses to the challenge were not concurrent. The MLV treatment had the highest proportion, 34.68%, of animals that were protected against the challenge regardless of the pre-challenge antibody titer and had the fewest number of lymphopenia cases in response to the challenge. Both vaccine treatments mitigated thrombocytopenia when compared to the control treatment, and the MLV treatment reduced lymphopenia; however, these symptoms were not completely eliminated in vaccinated animals. Pyrexia was present in 40.11% of the animals, but no difference in the frequency of cases between treatments was observed. Pre-challenge vaccination response was not indicative of the level of protection nor was anamnestic antibody response correlated with health status.