A study evaluated the occurrence of carrier state of horses that were involved in two naturally occurring outbreaks of strangles (Streptococcus equi.) Outbreak A involved 98 yearling warmbloods, and outbreak B involved 38 mature Icelandic horses. Fully recovered horses were sampled at least 6 months after index cases using nasal swabs, nasopharyngeal lavage, and guttural pouch visualization and lavages for culture and qPCR. Any horse with at least a single sample positive was deemed a carrier. Carrier rates in outbreak A were 3% based on culture and 15% based on qPCR and for outbreak B 13% based on culture and 37% based on qPCR. All culture positives were also qPCR positive. One carrier culture negative sampled after an additional 8 months was culture positive to S. equi, indicating that qPCR positives should be suspected to carry live bacteria. Findings indicate that reliance on guttural pouch sampling and appearance does not capture all silent carriers. The study suggests that Streptococcus equi can persist in clinically normal silent carriers for months to years.

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Long term silent carriers of Streptococcus equi ssp. equi following strangles; carrier detection related to sampling site of collection and culture versus qPCR.