A recent article was published that was titled, “Failure of serological testing for antigens A and C of Streptococcus equi subspecies equi to identify guttural pouch carriers” and authored by A.E. Durham and J. Kemp-Symonds.

This retrospective study investigated the sensitivity of serological testing for antibodies against Streptococcus equi subspecies equi antigens A and C to detect guttural pouch carriers.

Retrospective data was reviewed from 287 horses arriving at a welfare charity quarantine unit that underwent microbiological sampling of guttural pouches and serological testing. 

In total, nine (3.1%) horses were found to be guttural pouch carriers on microbiology, and 35 (12.2%) horses were seropositive (had at least one ELISA OD value ≥0.5). Only one (11%) of the nine horses found to be guttural pouch carriers was seropositive at a cut‐off of OD ≥ 0.5, but lowering the cut-off to OD ≥ 0.3 identified two additional guttural pouch carriers.

Of the 35 seropositive horses, one (2.9%) had evidence of S. equi in a guttural pouch lavage. 

Of the 252 horses with OD values <0.5, eight (3.2%) had evidence of S. equi in guttural pouch samples. 

Bottom line: There was no association between serological status and guttural pouch carriage of S. equi. Therefore seronegativity to antigens A and C should not be relied upon when determining if a horse is a chronic carrier of S. equi.