Investigation of a 24-Hour Culture Step to Determine the Viability of Streptococcus equi Subspecies equi Via Quantitative Polymerase Chain Reaction in Nasal Secretions From Horses With Suspected Strangles.
Authors: Pusterla Nicola, Barnum Samantha M, Byrne Barbara A
Journal: Journal of equine veterinary science
Summary
# Editorial Summary: Improving Strangles Diagnosis Through Molecular Viability Testing Traditional PCR testing for *Streptococcus equi* subspecies *equi* (the causative agent of strangles) frequently produces false positives by detecting bacterial DNA from non-viable organisms, potentially leading to unnecessary treatment and isolation protocols. Researchers at UC Davis evaluated whether a 24-hour culture step combined with quantitative PCR analysis of *SeM* gene messenger RNA transcripts and genomic DNA quantitation could reliably distinguish viable from non-viable bacteria in nasal secretions from 42 horses with suspected strangles. Standard culture identified viable *S. equi* in only 11 samples, yet mRNA detection found viability in 25 samples and genomic quantitation showed increased bacterial load in 17 samples; critically, using both mRNA transcript detection and absolute gene quantitation together achieved 100% agreement with culture-positive results and successfully determined viability status across all 42 samples. This molecular approach offers substantially improved diagnostic accuracy compared to PCR alone, with the mRNA transcript method showing 88% overall agreement with conventional culture—a meaningful advance given that culture itself is time-consuming and culture-negative samples have historically created diagnostic uncertainty. For practitioners managing suspected strangles cases, these findings suggest that a 24-hour culture enrichment step paired with molecular viability markers could refine case identification and justify clinical intervention more reliably than current single-method approaches, particularly valuable when culture yields negative results but clinical suspicion remains high.
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Practical Takeaways
- •Culture-negative samples for strangles may still contain viable S. equi; combining a 24-hour culture step with molecular detection (mRNA transcripts) improves diagnostic accuracy and reduces false-negatives
- •qPCR-based detection without viability assessment overestimates disease prevalence; implementing the 24-hour culture protocol followed by mRNA analysis provides more reliable epidemiological data for herd management decisions
- •For horses with suspected strangles where culture is negative but clinical signs persist, molecular viability testing may support continued isolation and treatment protocols
Key Findings
- •Culture alone detected S. equi in only 11/42 samples (26%), while mRNA transcripts for SeM gene were detected in 25/42 samples (60%)
- •A 24-hour culture step combined with qPCR for mRNA transcripts and absolute quantitation achieved 100% viability determination across all 42 samples
- •Overall agreement between culture alone and three viability criteria was only 59%, demonstrating that culture significantly underestimates viable S. equi prevalence
- •mRNA transcript detection achieved 88% agreement with viability criteria, while absolute target gene quantitation alone achieved only 74% agreement