Effect of infective dose on humoral immune responses and colonization in chickens experimentally infected with Salmonella typhimurium.

The influence of infective dose on chicken immunogenicity was examined in 1-week-old chickens. Chickens were infected orally with various doses of chi 3761 or chi 3985. Fecal shedding, colonization of the cecum, and induction of Salmonella-specific serum immunoglobulin isotypes were analyzed over a 5-week period. The delta cya delta crp Salmonella typhimurium vaccine strain chi 3985 was used to assess the effect of vaccination dose on protection after oral vaccination of chickens at 1 day and 2 weeks of age. Wild-type S. typhimurium strain chi 3761 was used to challenge vaccinated and unvaccinated chickens at 6 weeks of age, and the recovery of Salmonella from the cecum was used as a measure of protection. Infection of 1-week-old chickens with chi 3985 was more effective in reducing fecal excretion and cecal colonization than was infection with chi 3761. Double vaccination with 10(8) or 10(7) CFU of chi 3985 at 1 day and 2 weeks of age protected vaccinated chickens against cecal colonization by the challenge strain chi 3761. Immunogenicity of Salmonella is dose and genotype-dependent.