Summary

Monensin toxicity in horses is a serious and often fatal condition caused by accidental exposure to monensin‑containing cattle or poultry feed. Monensin is an ionophore used to control coccidia and improve feed efficiency in livestock, but horses are extremely sensitive to it. The toxin causes oxidative damage to mitochondria, most severely affecting cardiac muscle and also damaging skeletal muscle and nervous tissue. Clinical signs may begin with reduced appetite, colic, or choke and progress to weakness, ataxia, muscle tremors, recumbency, and death. Some horses show minimal signs but have elevated muscle and cardiac enzymes, while others may develop delayed heart failure weeks to months after exposure. Recommended management includes immediate veterinary evaluation, diagnostic blood tests, feed sample preservation, and supportive treatment. Outcomes range from full recovery to euthanasia due to cardiac failure.