Acute, subacute and chronic sequelae of horses accidentally exposed to monensin-contaminated feed.
Authors: Gy Clémentine, Leclere Mathilde, Bélanger Marie-Claude, Allano Marion, Beauchamp Guy, Lavoie Jean-Pierre
Journal: Equine veterinary journal
Summary
# Monensin toxicity in horses: cardiac damage and long-term prognosis Accidental exposure to monensin-contaminated feed presents a significant clinical challenge in equine practice, with the ionophore causing potentially irreversible cardiac injury. This observational study examined 76 horses exposed to contaminated feed across three timepoints—within 14 days (acute), 15–45 days (subacute) and 4–10 months (chronic) post-exposure—using physical examination, electrocardiography and echocardiography to identify which cardiac changes predict outcome. Cardiac abnormalities were widespread, detected in all four acutely affected horses, 66% of subacute cases and 44% of chronically affected animals; at 16-month follow-up, only 53% had returned to previous work, whilst 22% were dead or euthanised and 20% remained exercise intolerant. Myocardial thinning at any stage emerged as the most concerning prognostic indicator, alongside myocardial heterogeneity in the early post-exposure period and features including ventricular dyssynchrony, chamber dilation and reduced fractional shortening during the chronic phase. Clinicians should recognise that monensin toxicity produces highly variable outcomes even among sublethal exposures, and early echocardiographic assessment—particularly identification of heterogeneous or thin myocardium—offers valuable prognostic information to guide realistic owner discussions regarding recovery and athletic function.
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Practical Takeaways
- •If monensin contamination is suspected in feed, immediate cardiac evaluation (ECG, echocardiography) should be performed as abnormalities appear within the acute period and guide prognosis
- •Myocardial thinning and early heterogeneity are red flags for poor long-term outcomes; affected horses should be monitored closely and may require permanent work restrictions or retirement
- •Feed quality control and strict separation of cattle and equine feed is critical, as inadvertent monensin exposure carries high morbidity (22% mortality/euthanasia at 16 months) despite sublethal initial doses
Key Findings
- •Cardiac abnormalities were detected in 100% of acutely exposed horses (4/4), 66% in subacute period (19/29), and 44% in chronic period (31/70)
- •At 16 months post-exposure, 53% of horses returned to previous use, 20% were exercise intolerant, 5% retired, and 22% were dead or euthanized
- •Myocardial thinning at any timepoint was associated with negative outcome; heterogeneity in acute/subacute period predicted poor prognosis
- •Some horses with significant chronic-phase cardiac changes still returned to previous work, indicating variable clinical outcomes despite objective findings