Comparative Evaluation of the Immunogenicity and Protective Efficacy of Gamma-Irradiated Salmonella Gallinarum (γ-SG), SG ΔpurB Mutant, and their Combined Vaccine Formulations in Chicken.

Salmonella Gallinarum (SG), continues to impose economic losses in poultry, highlighting the need for better vaccination strategies to improve immunogenicity and protection. This study comparatively evaluated the immunogenicity and protective efficacy of a γ-SG vaccine, an SG ΔpurB live attenuated mutant, and their combined vaccine formulations in chickens. Five groups of chickens (A-E) were administered with SG ΔpurB IM (2.0 × 108 CFU), γ-SG Oral (2.0 × 108 CFU), SG ΔpurB + γ-SG Oral (2.0 × 108 CFU), SG ΔpurB + γ-SG IM (2.0 × 108 CFU), and an unvaccinated control (200 μL PBS), respectively. Combination vaccine doses were prepared using a 1:1 formulation, with each vaccine administered at 1.0 × 108 CFU per bird to achieve a total dose of 2.0 × 108 CFU. Comparative analysis of the vaccinated groups revealed that the SG ΔpurB + γ-SG Oral formulation induced the strongest early immune responses with significantly elevated CD4+ (31%) and CD8+ (19%) T cell populations at 1 WPV (week post vaccination) compared with other vaccinated and unvaccinated control groups (p < 0.0001). At 2 and 3 WPV, the highest levels of CD4+ and CD8+ T cells were observed in the γ-SG Oral group, whereas a transient decline in these T cell populations was observed in the other vaccinated groups compared to the γ-SG Oral group (p < 0.0001). Humoral IgG responses followed a similar trend, SG ΔpurB + γ-SG Oral group attaining the highest titers at 1 WPV (p < 0.0001). At 2 and 3 WPV, antibody titers were highest in the γ-SG Oral group, while a significant decline was observed in all other vaccinated groups compared to the γ-SG Oral group (p < 0.0001). Although the other vaccinated groups also demonstrated protection against virulent SG challenge, but the highest protective efficacy was achieved by the γ-SG Oral group, which provided 100% survival. However further dose optimization of combined vaccine formulation is required to obtain sustained immunity.