The effect of Salmonella vaccination on Salmonella Dublin blood enzyme-linked immunosorbent assay results

The Salmonella Dublin (S. Dublin) antibody enzyme-linked immunosorbent assay (ELISA) is used to estimate the preva­lence in dairy herds and as the primary diagnostic method in national control plans. Limited evidence is available regard­ing how U.S. currently licensed Salmonella vaccines produce Salmonella Dublin antibodies that are detectable by a commer­cial ELISA. The objective of this study was to determine if any of the commercial Salmonella vaccines produce detectable an­tibodies using this test. Fifty Holstein heifer calves were allo­cated into 5 groups of 10 and randomly assigned into 1 control and 4 vaccine groups. Calves were vaccinat­ed or received a sham injection twice 2 weeks apart. All were tested using the S. Dublin serum ELISA prior to vaccination and again 2 and 5 weeks following the second vaccination. Individual calf ELISA results were reported as percent positiv­ity (%PP) with ≥ 35% considered positive. Percent positivity results were analyzed using a mixed linear regression model in Stata, where %PP was the outcome. Fixed ef­fects included the vaccine group, day post-vaccination, and their interaction. Based on the cutoff of ≥ 35%, 5 weeks fol­lowing second vaccination, the number of positive calves were 1, 4 and 5 for calves administered ENDOVAC-Dairy®, a Salmonella Dublin-Typhimurium bacterin (Colorado Serum), and EnterVene®-d, respectively. Calves administered the SRP® vaccine or saline did not have positive ELISA results at any point in the study. This study demonstrates that certain commercial Salmonella vaccines complicate the interpre­tation of S. Dublin ELISA results, questioning the use in prevalence studies and control programs.