The effects of viral vaccination of dairy heifer calves on the incidence of respiratory disease, mortality, and growth.

Bovine respiratory disease (BRD) is one of the most common infectious causes of morbidity and mortality in young dairy cattle. The objective of this randomized clinical trial was to determine the effectiveness of 1 or 2 doses of a 5-way, modified-live viral vaccine, administered to heifer calves before weaning to aid in the prevention of BRD. The hypotheses were that vaccination would reduce the incidence of BRD and mortality, and that 2 doses would be more effective than 1. A total of 2,874 heifer calves from 19 commercial dairy farms in Minnesota and Ontario were enrolled at 1 to 7d of age and were followed until 3 mo of age. Calves were randomly assigned to receive a commercial, intramuscular, modified-live vaccine against bovine viral diarrhea virus types 1 and 2, bovine respiratory syncytial virus, bovine herpesvirus type 1, and parainfluenza virus type 3 at 15 to 21 d of age (2 wk only), 35 to 42 d (5 wk only), both 2 and 5 wk, or sterile saline at both times (unvaccinated controls). The incidence of failure of passive transfer was 11 or 32%, using cut-points of serum total protein of 5.2 and 5.7 g/dL, respectively. Overall, 22% of calves were treated at least once for BRD. The incidence risk of naturally occurring BRD was 7.7% before 2 wk of age, 8.0% between 2 and 5 wk, and 9.5% between 5 wk and 3 mo of age, and was not different between vaccination groups. Overall mortality throughout the 3-mo study period was 3.5%. Mortality was 1.6% before 2 wk of age, 0.5% between 2 and 5 wk, and 1.2% between 5 wk and 3 mo of age. The risk of mortality was not affected by vaccination. Mean average daily gain of 1.07 kg/d from 5 wk to 3 mo of age was not different between vaccine groups. In this population of commercial, home-raised calves, with an overall low incidence of failure of passive transfer, intramuscular vaccination with a multivalent, modified live viral vaccine at 2 or 5 wk of age or both was not associated with a decreased risk of BRD or mortality, or with growth until 3 mo of age. Reasons for these findings may include interference by maternal antibodies, unresponsiveness of the neonatal immune system, timing of immunity relative to pathogen exposure, disease caused by pathogens other than the viruses in the vaccine, or herd immunity. However, in populations with higher incidence of failure of passive transfer or risk of BRD, calves with low levels of specific antibodies may respond differently to vaccination.