Evaluation of vaccination with a commercial subunit vaccine on shedding of Salmonella enterica in subclinically infected dairy cows

Objective - To estimate the efficacy of a commercially available Salmonella enterica subunit vaccine on the subclinical shedding of S enterica in dairy cattle. Design - Randomized, controlled trial. Animals - 175 mature cows on 2 dairy farms with a history of S enterica infection. Procedures - 25% of the mature cows from each herd were systematically randomized to receive an S enterica subunit vaccine following label guidelines. The remaining 75% of cows in each herd served as nonvaccinated controls. Fecal samples were collected from all cows at the time of initial vaccination (day O), booster vaccination (day 14), 2 weeks following the booster vaccination (day 28), and 10 weeks following the start of the trial (day 70). All samples were processed on the day of collection and cultured for S enterica. Results - 651 fecal samples were obtained over the entire study period. Salmonella enterica was recovered from 46 (7.1 %) of the samples. Shedding of S enterica was similar for vaccinated and nonvaccinated control cows on each of the collection dates. Conclusions and Clinical Relevance - The study revealed no evidence that extralabel vaccination with a commercial subunit S enterica vaccine reduced shedding of S enterica in subclinically infected dairy cows in these herds.