Effect of dose on the immune response and persistence of Salmonella choleraesuis infection in swine.

OBJECTIVE

To establish the effect of dose on persistence of and immune response to Salmonella choleraesuis in swine.

DESIGN

19 Salmonella-free pigs were allotted to 4 groups. Groups 1 (n = 5), 2 (n = 5), and 3 (n = 5) were inoculated intranasally with 10(9), 10(6), and 10(3) colony-forming units of S choleraesuis, respectively. Group 4 (n = 4) served as uninoculated controls.

PROCEDURE

Pigs were monitored for clinical signs of disease and bacterial shedding. Serum and lymphocytes were obtained to measure immune responses. Pigs from groups 1, 2, and 4 were necropsied at postinoculation (PI) weeks 6 and 15. Pigs from groups 3 and 4 were necropsied at PI weeks 6 and 10.

RESULTS

Pigs in group 1 shed S choleraesuis through PI week 15 and were tissue positive at PI weeks 6 and 15. Pigs in group 2 were tissue positive for S choleraesuis until PI week 6 and continued shedding through PI week 9. Salmonella choleraesuis was not recovered at any time from pigs in groups 3 or 4. Pigs in groups 1, 2, and 3 had serum IgG and IgM titers to S choleraesuis lipopolysaccharide and soluble antigens. Pigs in all groups had a lymphocyte response to concanavalin A, and pigs in groups 1 and 2 had a lymphocyte response to S choleraesuis endotoxin. Pigs in group 1 had a lower stimulation index in response to both antigens, indicating some form of lymphocyte immunosuppression.

CONCLUSIONS

Persistence of S choleraesuis in host tissues is dose dependent. Short-term persistence can occur after a dose as low as 10(6) colony-forming units of S choleraesuis. Higher doses result in development of long-term carrier status, which may be related to the observed lymphocyte immunosuppression.