Vaccination against Salmonella enteritidis in Dutch commercial layer flocks with a vaccine based. On a live Salmonella gallinarum 9R strain: Evaluation of efficacy, safety, and performance of serologic salmonella tests

This study describes a field trial in which 80 commercial layer flocks, wit h an increased risk of Salmonella enteritidis (SE) infection and placed on farms with a certified Standardized Biosecurity Programme (SBP) or a reques t for a SEP certificate, were vaccinated with a vaccine based on a live att enuated Salmonella gallinarum (SG) 9R strain. An evaluation is presented of the efficacy of the vaccine against SE infections, the effect on the perfo rmance of serologic Salmonella tests, and the spread of the vaccine strain to the egg content. For the efficacy study, assessment of the flock level occurrence of SE infe ctions in the vaccinated group of 80 flocks was compared with that of a non vaccinated group of 1854 flocks hatched in the same period. This control gr oup was examined according to the compulsory control programme in The Nethe rlands. An evaluation was done of the performance of serologic Salmonella t ests and the spread of the vaccine strain to the inner egg content of five of the vaccinated flocks. Findings demonstrated the flock level occurrence of SE infections in the va ccinated group (2/80 = 2.5%) to be significantly (P = 0.01) lower than that of the nonvaccinated group (214/1854 = 11.5%). Vaccination resulted in 59. 0% positive test results in lipopolysaccharide ED enzyme-linked immunosorbe nt assay (ELISA) for detecting antibodies against Salmonella serogroups B a nd D and 0% positive test results in the rapid plate agglutination test for detecting antibodies against S. pullorum (SP)/SG. The mean specificities o f two blocking ELISAs (gm- and i-double antibody sandwich ELISAs) based on the flagellar antigen of SE and Salmonella typhimurium (ST) on the same ser a were 99.6% and 96.1%, respectively. The vaccine strain could not be isola ted from any of the 450 pools of 10 eggs. On the basis of these results, we concluded that vaccination with a vaccine based on an attenuated SG 9R str ain contributes to the reduction of SE infections in commercial layer flock s. Furthermore, serologic monitoring of SE, ST, and SP/SG can still be carr ied out on flocks vaccinated with an attenuated SG 9R strain. Additionally, we found no indication of the spread of the vaccine strain to the egg cont ent.