High specificity of the Salmonella Pullorum/Gallinarum rapid plate agglutination test despite vaccinations against Salmonella Enteritidis and Salmonella Typhimurium

In Europe, monitoring of breeding stock for Salmonella Pullorum (SP) or Salmonella Gallinarum (SG) infections is compulsory at the point of lay. Vaccinations against Salmonella Enteritidis (SE) and Salmonella Typhimurium (ST) are increasingly administered in Europe. These vaccines might induce cross-reactions in the rapid plate agglutination (RPA) SP/SG test due to shared O-antigens, possibly resulting in a lower test specificity. The extent to which the specificity of SP/SG serological tests is influenced by SE and/or ST vaccinations in the field has not been reported. In this paper, we report the diagnostic and flock specificity of the commercially available RPA SP/SG test using 1:2-1:16 serum dilutions on four panels of sera: SPF sera, field sera from flocks of varying age and SE/ST vaccination status, and reference sera from an international proficiency testing scheme. The results showed that the use of live SE/ST vaccines did not influence the specificity of the RPA SP/SG test. Inactivated vaccines showed a drop of the diagnostic specificity to 96.54% and a flock specificity of 34.1% when the 1:2 serum dilution was used. The 1:8 serum dilution showed a diagnostic specificity of 99.41% and a flock specificity of 86.4%. In conclusion, the use of SE/ST vaccines has either no effect or a modest effect on the specificity of the RPA SP/SG test used to monitor flocks. The main factors are the type of vaccine, and the serum dilution used for testing and a cut-off.