Safety of bivalent live attenuated Salmonella vaccine and its protection against bacterial shedding and tissue invasion in layers challenged with Salmonella

Nontyphoidal Salmonella infection was one of the predominant foodborne illnesses in humans. The medical burden and antimicrobial resistance of salmonellosis gained importance in public health and requested the poultry industry to seek effective measures to control the disease. The objective of this study was to evaluate the safety and effectiveness of a commercial bivalent live attenuated vaccine (AviPro Salmonella DUO) in specific-pathogen-free (SPF) chickens and field layers. It explored its safety and efficacy against medically important strains, Salmonella Enteritidis (SE) and S. Typhimurium (ST). The results demonstrated that ten vaccine doses in SPF chickens and regular doses in commercial layers showed desirable safety without affecting chicken health. Vaccinated layers demonstrated lower flock mortality and higher egg production performance than the unvaccinated layers during the raising and egg production periods. Additionally, no visceral colonization and egg contaminations were detected. Cloacal shedding of vaccine strains was noted, but the colonization of Salmonella disappeared within four weeks of the last vaccination. Regarding vaccine efficacy, one dose significantly reduced Salmonella cloacal shedding (P = 0.037 for SE and P = 0.027 for ST) and viable cell counts (P = 0.003 for SE and ST) on day 7 post the challenges. Significantly low Salmonella loads of cloacal samples on day 14 after the challenges were also determined in the vaccinated group (P = 0.006 for SE; P = 0.041 for ST). Triple immunizations effectively prevented layers from the cloacal shedding on either day 7 or day 14 post Salmonella challenges. Total viable counts of SE and ST in tissues of vaccinated layers were also reduced on day 14 after the challenges (P = 0.026 for SE; P = 0.002 for ST). To conclude, one dose of vaccine exhibited inhibitory effects on Salmonella shedding and tissue invasions in young layers. Following the regimen of triple vaccinations, Salmonella shedding was completely inhibited, and tissue invasions were significantly reduced. Incorporating this vaccine into a comprehensive Salmonella control program is promising to protect layers from the risks of contaminating the flocks and egg products.