Virulent Salmonella typhimurium-induced lymphocyte depletion and immunosuppression in chickens

The effects of experimental Salmonella infection on chicken lymphoid organs, immune responses, and fecal shedding of salmonellae were assessed following oral inoculation of 1-day-old chicks or intra-air-sac infection of 4-week-old chickens with virulent S. typhimurium wild-type χ3761 or avirulent S. typhimurium Δcya Δcrp vaccine strain χ3985. Some 4-week-old chickens infected intra-air-sac with χ3761 or χ3985 were challenged with Bordetella avium to determine the effect of Salmonella infection on secondary infection by B. avium, S. typhimurium χ3761 caused lymphocyte depletion, atrophy of lymphoid organs, and immunosuppression 2 days after infection in 1-day-old chicks and 4-week-old chickens. The observed lymphocyte depletion or atrophy of lymphoid organs was transient and dose dependent. Lymphocyte depletion and immunosuppression were associated with prolonged fecal shedding of S. typhimurium χ3761. No lymphocyte depletion, immunosuppression, or prolonged Salmonella shedding was observed in groups of chickens infected orally or intra-air-sac with χ3985. Infection of chickens with salmonellae before challenge with B. avium did not suppress the specific antibody response to B. avium. However, B. avium isolation was higher in visceral organs of chickens infected with χ3761 and challenged with B. avium than in chickens infected with B. avium only. Infection of chickens with χ3985 reduced B. avium colonization. We report a new factor in Salmonella pathogenesis and reveal a phenomenon which may play a critical role in the development of Salmonella carrier status in chickens. We also showed that 10⁸ CFU of χ3985, which is our established oral vaccination dose for chickens, did not cause immunosuppression or enhance the development of Salmonella carrier status in chickens.