Is Serology the Right Test for Asymptomatic S. equi Carriers? Veterinarians might not be able to rely on serological testing for Streptococcus equi, which causes strangles, as a way to identify asymptomatic carriers. Asymptomatic carriage within the guttural pouch is considered a major factor in the persistence of this highly infectious disease. Researchers performed a retrospective study to investigate the sensitivity of serological testing for antibodies against S. equi subspecies equi antigens A and C in detecting carriers from 287 horses arriving at a welfare charity quarantine unit that underwent microbiological sampling of guttural pouches. They reviewed the serological testing results and found 9 (3.1%) horses were found to be guttural pouch carriers on microbiology, and 35 (12.2%) horses were sero-
positive (had at least one ELISA OD value ≥0.5). Of the 35 seropositive horses, 1 (2.9%) had evidence of S. equi in a guttural pouch lavage. Of the 252 horses with OD values <0.5, 8 (3.2%) had evidence of S. equi in guttural pouch samples. Only 1 (11%) of the 9 horses found to be a carriers was seropositive at a cut-off of OD ≥ 0.5, but lowering the cut-off to OD ≥ 0.3 identified 2 additional carriers. Serology is a common way of identifying asymptomatic carriers of S. equi, but this study means veterinarians cannot rely on seronegativity to antigens A and C when determining if a horse is a chronic carrier of S. equi. The researchers said the most reliable method for determining carriage is microbiological sampling. MeV
For more information: Durham AE, Kemp-Symonds J. Failure of serological testing for antigens A and C of Streptococcus equi subspecies equi to identify guttural pouch carriers. Equine Vet J. 2020 May 6 (Epub ahead of print). https://beva.onlinelibrary.wiley.com/doi/10.1111/evj.13276
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