Control of strangles outbreaks by isolation of guttural pouch carriers identified using PCR and culture of Streptococcus equi.
Authors: Newton J R, Verheyen K, Talbot N C, Timoney J F, Wood J L, Lakhani K H, Chanter N
Journal: Equine veterinary journal
Summary
# Editorial Summary: Detecting Strangles Carriers Using PCR Streptococcus equi carriage in the guttural pouches presents a significant challenge to outbreak control, as over 50% of strangles cases result in clinically healthy horses harbouring the organism long after signs resolve. Newton and colleagues investigated three separate strangles outbreaks across different premises using endoscopy and polymerase chain reaction (PCR) testing alongside conventional culture to develop a more sensitive detection method, finding that between 29–52% of sampled horses carried infection, with 9–44% identified as asymptomatic carriers whose guttural pouch colonisation persisted for up to 8 months without spontaneous clearance. PCR proved substantially more sensitive than culture, detecting S. equi in 56% of nasopharyngeal swabs compared to 30% by culture alone, and in 76% versus 59% of guttural pouch samples from established carriers. Identifying and isolating these carriers through PCR screening, combined with rigorous hygiene protocols, effectively controlled all three outbreaks and restored normal yard operations, though the authors acknowledge that PCR's capacity to detect non-viable organisms warrants caution against false positives. For equine practitioners managing strangles incidents, this work supports using PCR as a complementary diagnostic tool to culture when screening for asymptomatic carriers—particularly when conventional swabbing has failed to identify a persistent source—rather than relying on nasopharyngeal culture alone, which may produce false-negative results in guttural pouch carriers.
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Practical Takeaways
- •Use PCR testing of nasopharyngeal and guttural pouch swabs to identify asymptomatic S. equi carriers after strangles outbreaks, as culture alone will miss many infected horses
- •Implement isolation and treatment protocols for identified carriers rather than relying on natural clearance, as prolonged carriage (up to 8 months) will not resolve without intervention
- •Combine carrier detection with strict hygiene measures to effectively control strangles outbreaks and prevent recurrence on affected premises
Key Findings
- •PCR detected S. equi in 56% of nasopharyngeal swabs compared to 30% by culture, and in 76% of guttural pouch samples versus 59% by culture
- •Between 9-44% of horses in strangles outbreaks became asymptomatic carriers after clinical signs resolved, with guttural pouches as the predominant site of colonisation
- •Prolonged carriage lasted up to 8 months and did not cease spontaneously without treatment
- •Detection and isolation of PCR-identified carriers combined with strict hygiene measures controlled outbreaks and allowed premises to return to normal activity