Why are horses particularly sensitive to monensin, and what clinical signs indicate monensin toxicity?

• A. Monensin is safe in horses
• B. Monensin is a known hazard
• C. Colic signs
• D. Monensin is highly toxic to horses; signs include colic, sweating, tachycardia, weakness, and possible death

Answer: (D)

Monensin is highly toxic to horses because ionophore antibiotics disrupt normal ion transport across cell membranes, hitting cardiac and skeletal muscle especially hard. In horses, this interference leads to energy failure in these muscles, causing rapid and serious damage that can be deadly. The resulting clinical picture combines gastrointestinal distress with systemic cardiovascular effects. Expect colic symptoms from abdominal discomfort, paired with sweating and a fast heart rate as the sympathetic response and heart strain develop, followed by weakness and the potential for death if the toxicity progresses. This description best fits the observed danger and the full range of signs, whereas saying it’s simply colic or that it’s safe ignores the serious cardiotoxic and muscular injury horses suffer.