Serological response of sheep to plasmid-encoded proteins of Yersinia species following natural infection with Y. enterocolitica and Y. pseudotuberculosis.

A prospective study of the serological response to natural infection with Yersinia enterocolitica and Y. pseudotuberculosis was performed in an experimental flock of sheep. A preliminary investigation with immunoblotting techniques showed that lambs infected with virulent Yersinia spp. produced antibodies to several yersinia outer-membrane proteins (yops) encoded by a virulence plasmid (pYV) of Y. enterocolitica or Y. pseudotuberculosis. Thereafter, an enzyme immunoassay (EIA) was developed to measure antibodies to yops. Criteria for interpreting the EIA were established by examining sera from a negative control population of lambs which had not been infected with Yersinia spp. since birth. Test samples comprised 25 pairs of pre- and post-infection sera from animals with bacteriologically proven infections with Yersinia spp. The results showed that infection of lambs with pYV-bearing strains of Y. enterocolitica or Y. pseudotuberculosis invariably evoked a significant antibody response to yops, even though all the infections were subclinical. No animal infected with so-called "environmental", pYV-negative Yersinia spp. seroconverted to yops. EIA with yops as antigen provided a sensitive and specific means to diagnose subclinical infection of lambs with virulent Yersinia spp.