Long-term assessment of horses and ponies post exposure to monensin sodium in commercial feed.
Authors: Hughes K J, Hoffmann K L, Hodgson D R
Journal: Equine veterinary journal
Summary
# Editorial Summary Monensin sodium contamination of equine feed causes well-documented acute toxicity, yet the prognosis for animals surviving sublethal exposure remains unclear. Hughes and colleagues examined 37 horses and ponies (aged ≥6 weeks post-exposure) using serum biochemistry, exercise stress testing, electrocardiography, and resting/post-exercise echocardiography, with long-term follow-up via owner interview at ≥52 months. Whilst 11 animals initially demonstrated reduced or low-normal left ventricular fractional shortening (FS) at rest, eight of these showed significant improvement at re-examination ≥11 months later; overall, mean FS increased significantly (P <0.001) in 15 animals undergoing serial assessment, suggesting myocardial recovery is possible rather than inevitable. Of 35 animals with follow-up data, 21 successfully returned to athletic or reproductive use, whilst 13 were retired immediately and one died, indicating that sublethal monensin exposure does not necessarily result in permanent career-ending cardiac damage. Farriers, veterinarians and coaches should recognise that serial cardiac evaluation—particularly echocardiography combined with exercise stress testing—offers valuable prognostic information in contamination cases, allowing evidence-based counselling regarding return to work rather than premature retirement decisions.
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Practical Takeaways
- •Horses and ponies exposed to sublethal monensin doses may recover cardiac function with time and rest; prognosis is not always poor for return to work
- •Implement systematic cardiac monitoring (echocardiography, ECG, exercise stress testing) in suspected monensin cases to objectively assess severity and guide return-to-use decisions
- •Allow extended recovery periods (≥11 months rest minimum) before reassessing animals with evidence of monensin-induced cardiomyopathy, as improvement over time is possible
Key Findings
- •11 of 37 animals (30%) showed reduced or low-normal left ventricular fractional shortening at rest, with 8 improving by ≥11 months later
- •21 of 35 animals (60%) with follow-up data returned to athletic or reproductive use ≥52 months post-exposure
- •Mean fractional shortening increased significantly (P<0.001) between initial and second examination in 15 animals
- •Exercise stress testing, electrocardiography and echocardiography were useful for detecting cardiac dysfunction and predicting return-to-use capability