Repeated nasopharyngeal lavage predicts freedom from silent carriage of Streptococcus equi after a strangles outbreak.
Authors: Pringle John, Aspán Anna, Riihimäki Miia
Journal: Journal of veterinary internal medicine
Summary
Following a strangles outbreak affecting an entire 41-horse Icelandic herd, researchers tracked nasopharyngeal lavage (NPL) results at four time points (weeks 18, 28, 29, and 30 post-index case) to establish whether serial negative tests could reliably predict freedom from *Streptococcus equi* carriage, with final carrier status confirmed at week 45 using both NPL and guttural pouch lavage. The key finding was that horses with three or more negative NPL results across weeks 18–30 had a significantly higher probability of being carrier-free at week 45 (91% specificity, *P* = 0.03), whereas a single negative at weeks 28–30 alone provided poor predictive value, with 83% of non-carriers still testing positive at some point during earlier sampling. This work establishes practical guidance for biosecurity decisions on affected farms: clearing horses based on fewer than three negative NPL samples is unreliable, but achieving three negatives substantially increases confidence in true pathogen elimination. For equine practitioners managing strangles outbreaks, these findings support a protocol of serial NPL sampling with appropriately timed intervals rather than reliance on single or consecutive weekly tests, though guttural pouch involvement remains a consideration for horses with persistent nasopharyngeal carriage.
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Practical Takeaways
- •To confirm a horse is truly free of S. equi carriage after strangles, perform at least 3 separate NPL tests over multiple weeks rather than relying on a single negative result
- •If a horse tests negative on 3 or more NPL samples (obtained on separate occasions), there is a 91% probability it is not a silent carrier and can safely return to movement
- •Continue quarantine measures and serial testing until at least 3 negative NPL results are obtained; single negative tests are unreliable for clearance decisions
Key Findings
- •Of 24 non-carrier horses at week 45, only 4 were negative on all 3 consecutive weekly NPL samples at weeks 28-30
- •10 of 11 horses (91%) with at least 3 negative NPL results across weeks 18, 28, 29, and 30 were S. equi-free at week 45 (P = 0.03)
- •Repeated nasopharyngeal lavage on at least 3 separate occasions can predict carrier-free status after strangles outbreak
- •Serial testing with NPL is more predictive of true carrier status than single negative tests