Development of a novel fluorescent microbead-based immunoassay and comparison with three enzyme-linked immunoassays for detection of anti-Erysipelothrix spp. IgG antibodies in pigs with known and unknown exposure.

A novel fluorescent microbead immunoassay (FMIA) using the recombinant polypeptide SpaA415 was developed for detection of anti-Erysipelothrix spp. IgG in pig sera. The diagnostic performance of the FMIA was evaluated on samples from pigs with known and unknown Erysipelothrix spp. exposure and compared to an in-house enzyme-linked immunosorbent assay (ELISA-1) based on the same capture antigen, and two commercially available ELISAs (ELISA-2 and ELISA-3). Sera from pigs experimentally infected with Erysipelothrix rhusiopathiae serotype 1a (n=60) or 19 (n=12), sera from pigs vaccinated with a commercial attenuated-live vaccine based on serotype 1a (n=12) or a commercial bacterin based on serotype 2 (n=12), and 90 field samples were utilized. The sensitivity on 22 true positive samples collected in the later stages of infection/post-vaccination was 100% for the FMIA and ELISA-1, 63.6% for ELISA-2 and 81.8% for ELISA-3. The earliest antibody response was detected 7days post inoculation with the FMIA (77.8%) and ELISA-1 (11.1%), and at 14days post-vaccination (dpv) with FMIA (50%) and ELISA-1 (50%). On field samples, a higher seroprevalence was found in pigs older than 21days with all four assays. Kappa analysis indicated that the FMIA and ELISA-1 had almost complete agreement whereas the agreement was slight with ELISA-2 and fair with ELISA-3. The sensitivity of both immunoassays based on the rSpaA415 antigen was higher compared to that of the two commercial ELISAs. The rSpaA415 FMIA has great potential as an inexpensive ELISA alternative for detection of antibodies against E. rhusiopathiae in the future.