Monensin and Lasalocid Toxicity in Horses
Important Concepts
Ionophore Antibiotics: Monensin and lasalocid are used in poultry and ruminant feeds but are highly toxic to horses.
Clinical Signs
Monensin:
Dose-dependent toxicity.
Depression, incoordination, weakness, paresis, paralysis, poor performance, heart failure, unthriftiness.
Cardiac arrhythmias, high heart rates, prominent jugular vein distension.
Acute ingestion can cause death within hours; chronic exposure results in prolonged symptoms like colic, diarrhea, stiffness, and muscle weakness.
Pathological Changes
Cardiac Damage: Permanent damage to the heart muscles leading to heart failure and arrhythmias.
Diagnosis
Definitive Diagnosis: Requires feed analysis or post-mortem examination.
Clinical Evaluation: Symptoms suggestive of toxicity, changes in blood work reflecting dehydration and electrolyte imbalances.
Treatment
Supportive Care: IV fluids, oral laxatives, anti-inflammatory, and pain medication.
No Specific Antidote: Immediate removal of suspect feed sources is crucial.
Prevention
Education: Ensuring feed companies prevent cross-contamination of equine feed with ionophore antibiotics.
For BCSE preparation, it is essential to recognize that horses are the most sensitive domestic species to ionophore toxicity (lethal dose ~2–3 mg/kg), understand the typical exposure scenarios (feed mill errors, contaminated mixed rations), and link the characteristic clinical picture—depression, weakness, ataxia, colic-like signs, tachycardia, jugular distension, myoglobinuria, and poor performance—with irreversible myocardial damage and a guarded to poor prognosis, even in survivors.
