Long term silent carriers of Streptococcus equi ssp. equi following strangles; carrier detection related to sampling site of collection and culture versus qPCR.
Authors: Pringle J, Venner M, Tscheschlok L, Bächi L, Riihimäki M
Journal: Veterinary journal (London, England : 1997)
Summary
# Streptococcus equi Silent Carriers: Detection Methods Matter Silent carrier horses pose a significant biosecurity challenge following strangles outbreaks, potentially harbouring live *Streptococcus equi* ssp. equi for extended periods despite clinical recovery. This longitudinal study tracked two naturally occurring outbreaks (98 yearling warmbloods with 100% morbidity and 38 mature Icelandic horses with 53% morbidity) over 6+ months, systematically comparing culture and qPCR detection methods across multiple sampling sites including nasal swabs, nasopharyngeal lavage, and direct guttural pouch visualisation with lavage. Carrier prevalence varied dramatically by detection method: culture identified 3–13% of previously infected horses, whilst qPCR detected 15–37%, with all culture-positive horses also testing qPCR-positive. Critically, one horse that initially tested qPCR-positive but culture-negative later yielded a positive culture eight months later, confirming that qPCR-negative results do not exclude carrier status. For practitioners managing biosecurity post-outbreak, these findings underscore that relying solely on guttural pouch appearance or single nasal swabs will miss carriers; qPCR-positive horses should be treated as infectious regardless of culture status, and multiple sampling sites over time may be necessary to confidently rule out carrier status before reintroduction to naive populations.
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Practical Takeaways
- •After strangles outbreaks, relying on culture alone will miss silent carriers; use qPCR for more accurate identification of carrier status in recovered horses
- •Sample from multiple sites (nasal, nasopharyngeal, and guttural pouch) rather than guttural pouch alone to increase detection sensitivity of persistent S. equi carriers
- •Horses testing qPCR-positive but culture-negative should be treated as suspected live carriers and managed with appropriate biosecurity protocols to prevent transmission
Key Findings
- •qPCR detected significantly higher carrier rates than culture alone: 15% vs 3% in outbreak A and 37% vs 13% in outbreak B
- •All culture-positive samples were also qPCR-positive, but qPCR identified additional carriers that culture missed
- •Guttural pouch sampling alone may not capture all silent carriers; multiple sampling sites (nasal swabs, nasopharyngeal lavage, guttural pouch lavage) improved detection
- •One qPCR-positive, culture-negative horse later tested culture-positive 8 months later, suggesting qPCR detects live bacteria before culture becomes positive