Evaluation of a nested PCR test and bacterial culture of swabs from the nasal passages and from abscesses in relation to diagnosis of Streptococcus equi infection (strangles).
Authors: Grønbaek L Møller, Angen O, Vigre H, Olsen S Nautrup
Journal: Equine veterinary journal
Summary
# Diagnostic Methods for Strrangles: PCR Outperforms Culture Nested PCR testing offers substantially improved sensitivity over bacterial culture for detecting *Streptococcus equi* in clinical samples from horses with strangles, with diagnostic sensitivity reaching 80% for abscess swabs compared to just 20% via traditional cultivation methods. Grønbaek Møller and colleagues evaluated this DNA-based approach across two naturally infected herds over 15 weeks, testing 323 samples from 35 horses and confirming species specificity using 165 non-*S. equi* Lancefield group C streptococci isolates; notably, all 45 *S. equi* isolates tested positive whilst no false positives occurred. The research revealed intermittent shedding patterns persisting up to 15 weeks post-exposure, with bacterial excretion occurring from both clinically affected and asymptomatic horses—a finding with significant biosecurity implications for herd management. For equine practitioners, these findings support using PCR as a more reliable diagnostic tool than culture, particularly when investigating suspected cases or conducting contact screening; however, the authors acknowledge that single-technique reliance carries inherent limitations, suggesting that future protocols might benefit from integrating multiple detection methods rather than relying solely on nested PCR.
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Practical Takeaways
- •Nested PCR is significantly more sensitive than bacterial culture for diagnosing strangles, particularly for abscess samples (80% vs 20%), improving diagnostic accuracy for clinical decision-making
- •Abscess samples are more reliable than nasal swabs for PCR testing, but both sample types may miss infection; negative results don't exclude disease, especially early in outbreak
- •Expect prolonged and intermittent shedding of S. equi for up to 15 weeks even after clinical recovery, and monitor both symptomatic and apparently healthy horses during outbreaks to control transmission
Key Findings
- •Nested PCR test achieved 100% species specificity (45/45 S. equi isolates positive, 0/120 other Lancefield group C streptococci positive)
- •PCR diagnostic sensitivity was 45% for nasal swabs and 80% for abscess samples, significantly higher than bacterial culture at 18% and 20% respectively
- •S. equi shedding persisted intermittently for up to 15 weeks in infected horse populations
- •Bacterial shedding occurred in both clinically affected and subclinical horses