Evaluation of the Efficacy of Different Vaccination Programs Using Two Live Salmonella Vaccines Against Salmonella Enteritidis, Salmonella Typhimurium, and Salmonella Gallinarum in Brown Layer Hens.

Attenuated live Salmonella vaccines are available for controlling this pathogen in poultry. Due to the wide variety of serovars, cross-protection is a matter of debate. This study evaluated five different vaccination programs combining two live vaccines, with focus on cross-protection against challenges with Salmonella enterica serovar Enteritidis (SE), Salmonella enterica serovar Gallinarum biovar Gallinarum (SG), and Salmonella enterica serovar Typhimurium (ST) strains. Vaccination with a SG9R live vaccine in combination with an attenuated SE strain showed the highest levels of protection against all three serovars. Vaccination with the attenuated SE vaccine alone significantly reduced the systemic infection, as evidenced by the complete absence of SE in spleen and liver samples at 7 and 10 days post challenge, and by up to a 2-log reduction of ST in spleen. Unvaccinated chickens exhibited higher fecal shedding of SE and ST. Challenge with pathogenic SG resulted in up to 87.5% mortality in unvaccinated chickens and 42% mortality in birds vaccinated only with the SE vaccine. In contrast, no mortality occurred in groups vaccinated with SG9R, whether administered alone or in combination with SE vaccine. All vaccinated birds showed reduced degeneration of internal organs and were able to mount a timely cellular and humoral immune response, mitigating the impact of fowl typhoid. The use of two different vaccine strains in the same program had no deleterious effects; on the contrary, it contributed to complementary immune responses, enhancing overall protection.