Use of quantitative real-time PCR to determine viability of Streptococcus equi subspecies equi in respiratory secretions from horses with strangles.
Authors: Pusterla N, Leutenegger C M, Barnum S M, Byrne B A
Journal: Equine veterinary journal
Summary
# Editorial Summary Streptococcus equi subspecies equi diagnosis in strangles cases traditionally relies on bacterial culture, but culture results can be compromised by sampling delays and field conditions. Pusterla and colleagues used quantitative real-time PCR targeting multiple virulence genes (SeM, SEQ2190, eqbE and szpSe) at both genomic DNA and messenger RNA levels to assess whether molecular markers could indicate viable, actively-shedding bacteria in respiratory secretions from affected horses. All 21 culture-positive samples were qPCR-positive at the genomic level, whilst up to 44% of culture-negative samples showed genomic DNA detection but only 19% showed active gene expression; critically, samples with detectable messenger RNA transcripts contained significantly higher bacterial gene copy numbers than those without expression. The authors demonstrate that measuring both bacterial DNA load and transcript presence provides a more nuanced picture of infection status than culture alone, potentially identifying viable organisms that conventional culture might miss. For equine practitioners, this work emphasises that any qPCR-positive result warrants immediate biosecurity implementation, as molecular detection is more sensitive than culture and suggests active infection risk even when culture returns negative—though field practitioners should remain aware that genomic detection alone cannot fully distinguish viable from non-viable organisms without supporting transcript data.
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Practical Takeaways
- •Any qPCR-positive result for S. equi should trigger biosecurity protocols to prevent spread, even if culture is negative, as the organism may still be viable
- •Combining gDNA quantitation with mRNA detection provides better assessment of S. equi viability than culture alone in respiratory secretions
- •Sample handling and processing delays can affect culture results; qPCR methods may be more reliable for confirming strangles in field-collected samples
Key Findings
- •All 21 culture-positive samples tested qPCR positive at gDNA level, while 43.7% of 64 culture-negative samples were gDNA-positive and 18.9% were cDNA-positive
- •Significant differences in absolute quantitation of S. equi gDNA existed between culture-positive and culture-negative samples
- •Samples with mRNA transcript expression had significantly higher S. equi target gene numbers compared to samples without transcript expression
- •qPCR can detect molecular viability of S. equi through absolute quantitation and mRNA detection, but positive results require cautious interpretation and biosecurity implementation