Fecal concentration of Rhodococcus equi determined by quantitative polymerase chain reaction of rectal swab samples to differentiate foals with pneumonia from healthy foals.
Authors: Cohen Noah D, Flores-Ahlschewde Patricia, Gonzales Giana M, Kahn Susanne K, da Silveira Bibiana Petri, Bray Jocelyne M, King Emily E, Blair Caroline C, Bordin Angela I
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary Rhodococcus equi pneumonia remains a significant challenge in foal medicine, prompting researchers to investigate whether quantitative PCR analysis of rectal swab samples could offer a non-invasive diagnostic tool to identify infected foals before clinical signs become severe. Cohen and colleagues conducted a case-control study on two New York breeding farms, collecting rectal swabs from 39 foals diagnosed with R. equi pneumonia and 53 matched healthy controls, then quantifying virulent R. equi DNA (specifically the vapA gene associated with pathogenicity) in fecal samples using real-time qPCR. Whilst pneumonic foals demonstrated significantly higher fecal concentrations of virulent R. equi than healthy age-matched herd mates, the assay's diagnostic performance fell short of clinical utility: at the optimal threshold of 14,883 vapA gene copies per 100 ng fecal DNA, sensitivity reached only 79.5% and specificity 83.0%, with an area under the ROC curve of 83.7%. These findings indicate that although qPCR-based quantification shows promise in differentiating infected from non-infected foals at a population level, the diagnostic accuracy is insufficient for reliable clinical decision-making in individual cases—meaning rectal swab qPCR cannot yet replace traditional diagnostic approaches (clinical evaluation, thoracic ultrasound, culture) for identifying foals requiring treatment. Further refinement of the assay methodology, exploration of alternative sample types, or investigation of longitudinal qPCR trends may enhance its clinical applicability for early detection and management strategies on high-risk farms.
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Practical Takeaways
- •Rectal swab qPCR cannot yet be reliably used as a standalone diagnostic tool for R. equi pneumonia in foals; traditional clinical and diagnostic methods remain necessary
- •While fecal qPCR shows promise as a research and screening tool, the 79-83% accuracy rates mean approximately 1 in 5 results may be misleading in clinical practice
- •This test may have potential value as part of a diagnostic algorithm alongside clinical signs, imaging, and culture, but further development is needed before clinical adoption
Key Findings
- •qPCR of rectal swabs showed 79.5% sensitivity and 83.0% specificity for detecting R. equi pneumonia at a threshold of 14,883 vapA copies per 100 ng fecal DNA
- •Area under the ROC curve was 83.7% (95% CI, 74.9-92.6), indicating moderate diagnostic accuracy
- •Fecal concentrations of virulent R. equi were significantly higher in pneumonic foals than healthy foals from the same environment
- •Despite significant differences between groups, qPCR of rectal swabs lacks adequate diagnostic accuracy for clinical use