Attenuation and immunogenicity of Delta cya Delta crp derivatives of Salmonella choleraesuis in pigs

Six different isogenic Delta cya Delta crp derivatives of a strain of Salmo nella choleraesuis var. kunzendorf-chi 3246 virulent for pigs were construc ted by transposon-mediated deletion mutagenesis. These strains were evaluat ed for virulence and ability to elicit a protective immune response in youn g weaned pigs after oral administration and were compared to a commercially available vaccine which lacks the 50-kb virulence plasmid (vpl(-)). These derivatives were Delta cya Delta crp vpl(+), Delta cya Delta crp vpl(-), De lta cya Delta(crp-cdt) vpl(+), Delta cya Delta(crp-cdt) vpl(-), Delta cya D elta crp pmi-3834 vpl(+), and Delta cya Delta(crp-cdt) pmi-3834. In experim ents to evaluate safety, no significant adverse effects of any of the vacci ne constructs were observed, except that two of the strains which carried t he virulence plasmid (vpl(+)) caused a small, short-term elevation in maxim um temperature compared to pretreatment temperature values. Orally immunize d animals, except for those vaccinated with the Delta cya Delta crp pmi-383 4 vpl(+) strain or SC-54, developed significant serum antibody responses 21 days postvaccination as measured by enzyme-linked immunosorbent assay. No cell-mediated immune responses to heat-killed S. choleraesuis were noted at the same time point as measured with heat-killed bacteria as antigen in a lymphocyte proliferation assay. In an oral challenge exposure model with a highly virulent heterologous strain of S. choleraesuis, the Delta cya Delta crp strains with deletions in pmi were not protective. As measured by morb idity scores, the responses to challenge of the pigs vaccinated with the ot her four Delta cya Delta crp derivatives were significantly better than tho se of the nonvaccinated, challenged group. With the exception of temperatur e elevation and slight differences in diarrhea scores postchallenge, none o f these strains differed significantly from each other in the other clinica l parameters analyzed. While the commercial vaccine was protective by most of the parameters measured, it was not fully protective against challenge w ith virulent S. choleraesuis as judged by diarrhea scores and temperature e levation. Collectively, these data demonstrate that Delta cya Delta crp der ivatives, with or without the virulence plasmid but not with deletions in t he pmi gene, are candidates for vaccines for protection against salmonellos is in pigs.