Evaluation of the efficacy of intraperitoneal immunization in reducing Salmonella typhimurium infection in chickens

Conventional methods of parenteral immunization with killed bacterin vaccines have met with limited success in protecting the avian intestinal mucosa from pathogens such as Salmonella typhimurium. For mucosal vaccines to be successful they must be evaluated for their ability to stimulate local secretory immunoglobulin (SIgA) at the mucosal surface, which acts as the first line of defense against invading pathogens. Previously we have demonstrated the ability of i.p. immunization with nonreplicating antigen in an appropriate adjuvant to induce a primary immune response, which, after an oral booster immunization, stimulates enhanced intestinal IgA responses in chickens. In the experiments reported here we have applied this immunization protocol to vaccinate against S. typhimurium in chickens, and examined the protection provided against subsequent S. typhimurium challenge by placing vaccinated birds on seeded litter with cohabitant infected birds. Immunized+challenged birds displayed delayed onset of S. typhimurium infection, both at the mucosal surface and within the reticuloendothelial system. Elevated anti-S. typhimurium IgG and IgA titers were detected in serum after vaccination, which markedly increased after challenge, to levels higher than in control+challenged chickens. Anti-S. typhimurium IgA in bile and intestinal scrapings supernatant was also higher in the immunized+challenged birds than in the control+challenged birds 15 d after challenge. This study illustrates the potential for i.p. vaccination to induce a mucosal immune response to S. typhimurium in chickens, which, in the challenge model employed here, provided partial protection against intestinal challenge with the same pathogen and was reflected in deferred onset of bacterial infection and shedding.