Development and evaluation of an enzyme-linked immunosorbent assay based on a recombinant SpaA protein (rSpaA415) for detection of anti-Erysipelothrix spp. IgG antibodies in pigs.

The aim of this study was to develop an enzyme-linked immunosorbent assay (ELISA) for detection of anti-Erysipelothrix spp. IgG in pig sera by utilizing recombinant polypeptide SpaA415 (rSpaA415) based on surface protective antigen (Spa) A (SpaA) of Erysipelothrix spp. The sensitivity of the rSpaA415 ELISA was evaluated on sera from pigs experimentally infected with E. rhusiopathiae serotype 1a (n=72), serotype 19 (n=12), or experimentally vaccinated with a commercial attenuated-live vaccine based on serotype 1a (n=12), a commercial bacterin based on serotype 2 (n=12), or an experimental bacterin based on serotype 2 (n=300). Specificity was tested using 221 negative control samples. The earliest antibody response was detected at 7 days post-inoculation (dpi) and 14 days post-vaccination (dpv). At the cutoff of 0.9 sample optical density, the sensitivity was 96.5% and the specificity was 100%. In experimentally infected pigs, the sensitivity of the rSpaA415 ELISA ranged from 5.5 to 100% which improved as dpi increased. Antimicrobial treatment, administered prior to appearance of clinical signs, decreased assay sensitivity. In vaccinated pigs, the rSpaA415 ELISA had a sensitivity of 48.3-100%. Serum samples from rabbits each hyperimmunized with one of the 28 Erysipelothrix spp. serotypes were used to determine cross-reactivity with strains expressing SpaB, SpaC or no currently recognized Spa protein and antibodies against E. tonsillarum were not detected. These data suggest that the novel rSpaA415 ELISA test is a useful tool to detect anti-IgG antibodies against different serotypes of E. rhusiopathiae in infected or vaccinated pigs without cross-reacting with the economically less important E. tonsillarum strains.