Efficacy of a Salmonella Siderophore Receptor Protein Vaccine on Fecal Shedding and Lymph Node Carriage of Salmonella in Commercial Feedlot Cattle.

The efficacy of a Salmonella vaccine for reducing fecal shedding of Salmonella during the finishing period and lymph node (LN) carriage at harvest was investigated in commercial feedlot cattle. The study was designed as a pen-level randomized complete block with two treatment groups, a Salmonella Newport siderophore receptor and porin proteins-based vaccine (VAC) and a nonvaccinated control (CON). Cattle were randomly allocated into 24 pens within 12 blocks based on the time of allocation. Twenty to 25 fecal pats were collected from each of the study pen floors once a month from June to August 2013. During harvest, a minimum of 25 sub-iliac LN were collected from carcasses within each study pen. Fecal and pulverized LN samples were cultured for Salmonella quantification and detection. Mixed models were used to analyze the effect of vaccination on fecal shedding and LN carriage of Salmonella. Montevideo and Anatum were the predominant Salmonella serotypes among fecal samples and LNs; no Newport isolates were recovered. Vaccination was not significantly associated (p = 0.57) with the prevalence of Salmonella in feces over time; the mean within-pen prevalence was 62.3% and 66.0% among VAC and CON, respectively. Sampling month was significantly associated (p < 0.01) with fecal prevalence; mean prevalence was 71.4% for June, 48.6% for July, and 70.8% for August. Across all pens, the cumulative prevalence of Salmonella in LN was 86.4%. Vaccination resulted in no significant reduction in LN prevalence (p = 0.52); mean prevalence was 85.7% for VAC and 87.4% for CON groups. Although vaccinated cattle had numerically fewer Salmonella LN and fecal positives, there were no statistically significant vaccine effects. Potential reasons for the lack of vaccine efficacy could include an overwhelming Salmonella exposure, a lack of cross-protection against non-Newport serotypes, and insufficient duration of immunity relative to harvest.