Failure of serological testing for Strangles to identify guttural pouch carriers

Respiratory disease is a real problem when introducing new horses to a stable population. This study suggests that about 3% of horses may be chronic carriers of Strangles without symptoms. They be infectious and spread Strangles to a susceptible population. Unfortunately testing for the presence of the organism is not found to be effective at finding these guys. The first thing that should be done is a thorough investigation of the health history of the horse and the barn the horses are coming from. There are barns that are known for new cases of Strangles particularly in the newly arriving individuals. Did the horse come from one of these barns? The second step should be consideration of a good vaccination program using the intranasal Strangles vaccine at least while horses are coming in and going out of the barn.
DrO

Equine Vet J. 2020 May 6.
Failure of serological testing for antigens A and C of Streptococcus equi subspecies equi to identify guttural pouch carriers.
Durham AE1, Kemp Symonds J2.

Author information:
1. The Liphook Equine Hospital, Liphook, Hampshire, GU30 7JG, UK.
2. Bransby Horses, Bransby, Lincoln, LN1 2PH, UK.
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 Strep. equi. Only one of the 9 carriers (11%) was seropositive using a cut-off of OD ≥0.5, and only 3 of 9 (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.