An enhanced vaccination regime reduces the shedding of Salmonella Typhimurium from layer chickens

Salmonella in poultry production systems has been linked to the contamination of eggs and meat. In Australia, laying chickens are often vaccinated to provide immunological protection against wild-type Salmonella Typhimurium infection. In this study, layer chickens were vaccinated with a live, attenuated Salmonella Typhimurium vaccine (Vaxsafe® ST) at day 1 via oral gavage, at week 4 via drinking water and at week 12 via intramuscular injection, followed by an additional (4th) dose given in drinking water at week 15 of chicken age. Chickens were challenged with 10⁶ or 10⁸ CFU of wild-type Salmonella Typhimurium at week 17 using a contaminated dust model. The load of vaccine shedding in the faeces decreased with time post-vaccination, as significantly lower vaccine counts were detected at weeks 4, 6 and 8 compared with weeks 1 and 2 post vaccination. There was a significant increase in Salmonella-specific group B antibody following intramuscular vaccination at 12 weeks of age (3rd dose), and a further increase in mean antibody level after the additional vaccine dose (4th dose) was administered via drinking water at 15 weeks. All unvaccinated birds that were challenged with wild type Salmonella Typhimurium, shed bacteria in faeces; shedding of Salmonella Typhimurium was significantly lower in vaccinated chickens. Lower load of wild-type Salmonella Typhimurium in organs at weeks 1 and 2 post-challenge demonstrated that vaccination provided better protection to chickens that received 10⁶ CFU compared with 10⁸ CFU of wild-type Salmonella Typhimurium. Overall, the Salmonella Typhimurium shedding profile/load data showed that chickens vaccinated at various intervals with Vaxsafe ST and exposed to wild-type Salmonella Typhimurium via dust reduced bacterial shedding. Analysis of the gut microbiota revealed that Salmonella Typhimurium challenge resulted in changes (P < 0.05) in microbial community evenness but not richness. The dissimilarity in beta diversity was mainly caused by Salmonella Typhimurium at day 7 post-challenge in the unvaccinated chickens compared with the vaccinated chickens. Salmonella Typhimurium challenge resulted in lower (FDR < 0.05) abundance of Paludicola, Turicibacter, Alistipes, Megamonas and Psychrobacter. Vaccination reduced the shedding level of wild-type Salmonella Typhimurium without negatively impacting the caecal microbial ecosystem and an additional vaccine dose was beneficial in immune system modulation.