Serologic studies of experimentally induced Salmonella choleraesuis var kunzendorf infection in pigs.

Two indirect ELISA containing outer membrane protein (OMP) and lipopolysaccharide (LPS) antigens from a field isolate of Salmonella choleraesuis var kunzendorf were developed and evaluated in experimentally infected and uninfected control pigs. Experimentally induced infection with S choleraesuis was successfully established in 10 pigs by oral inoculation with 10(8) organisms, and 3 pigs died of clinical salmonellosis at postinoculation (PI) weeks 1, 2, and 4. Swab specimens from tonsils, nostrils, and rectum of pigs were obtained for culture, and sera were evaluated at weekly intervals for 9 weeks after inoculation. The ELISA containing OMP and LPS antigens with either anti-swine IgG or protein albumin-to-globulin ratio (antiglobulin) conjugates were standardized for serologic evaluation. All 4 ELISA (2 OMP and 2 LPS) detected seroconversion by PI week 3 and had sensitivities and specificities of 97.8 and 88.8, 100 and 100, 95.6 and 88.8, and 93.3 and 72.5%, at their ideal cutoff points (negative mean optical density + 2 SD). There was excellent agreement between all 4 ELISA systems as determined by kappa values. Cultures of fecal, tonsil, and nasal swab specimens were positive for S choleraesuis until the fourth week of infection. Fecal swab specimens from 1 pig were positive for S choleraesuis until PI week 7. Persistent infection after antemortem culture results were negative was detected by all 4 ELISA, which indicated consistently high titers until the end of PI week 9. Conventional bacteriologic examination of intestines, mesenteric lymph nodes, bone marrow, lung, liver, spleen, and bile yielded positive results for S choleraesuis in the 3 pigs that died of clinical infection, whereas results were negative in the other 7 pigs infected by the end of PI week 9. Histologic examination of lung, liver, spleen, intestines, and mesenteric lymph nodes from the 3 pigs that died of S choleraesuis infection revealed severe ulceration and inflammatory cell infiltration.