Failure of serological testing for antigens A and C of Streptococcus equi subspecies equi to identify guttural pouch carriers.

Abstract

Serology is commonly used as a means of identifying horses that might be chronic and silent carriers of S.equi but its sensitivity is rarely examined. The study was designed to investigate the sensitivity of serological testing for antibodies against S.equi antigens A and C to detect guttural pouch carriers of S. equi. Retrospective clinical study. As part of routine surveillance and quarantine procedures horses arriving at a welfare charity quarantine unit were subject to both microbiological sampling of guttural pouches and also serological testing for antibodies directed at S.equi antigens A and C. Laboratory results and endoscopic findings were examined to determine associations between serological results and guttural pouch carriage of S.equi. Of 287 included horses, 9 (3.1%) were found to be guttural pouch carriers. There was no significant association between serological status and guttural pouch carriage of S.equi Only one of the nine carriers (11%) was seropositive using a cut-off of OD≥0.5, and only three of nine (33%) using a cut-off of OD≥0.3. Horses examined in this study were new arrivals at a welfare centre rather than from a general, well-managed, equid population. As a retrospective clinical study, the laboratory test results could not be repeated for further confirmation. Caution is advised when relying on seronegativity to antigens A and C in order to discount the possibility of chronic carriage of S.equi in guttural pouches.