A prospective study of the serological response to natural infection with and was performed in an experimental flock of sheep. A preliminary investigation with immunoblotting techniques showed that lambs infected with virulent spp. produced antibodies to several yersinia outer-membrane proteins (yops) encoded by a virulence plasmid (pYV) of or . 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 spp. since birth. Test samples comprised 25 pairs of pre- and post-infection sera from animals with bacteriologically proven infections with spp. The results showed that infection of lambs with pYV-bearing strains of or invariably evoked a significant antibody response to yops, even though all the infections were subclinical. No animal infected with so-called “environmental”, pYV-negative spp. seroconverted to yops. EIA with yops as antigen provided a sensitive and specific means to diagnose subclinical infection of lambs with virulent spp.


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