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

Abstract

Background: 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. Objectives: 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. Study design: Retrospective clinical study. Methods: 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. Results: 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. Main limitations: 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. Conclusions: 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.