Merck Manual

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Toxic Myopathies in Horses


Stephanie J. Valberg

, DVM, PhD, DACVIM, ACVSMR, College of Veterinary Medicine, Michigan State University

Last full review/revision Feb 2014 | Content last modified Jun 2016


Ionophores are commonly added to ruminant feeds for their growth promotion and coccidiostat properties. Horses, however, are 10 times more sensitive to the toxic effects of ionophores in feed than cattle. When equine feeds are inadvertently contaminated with ionophores or horses eat cattle feed, some animals may die acutely with colic-like signs, myoglobinuria, hypokalemia, cardiac arrhythmia, and tachypnea. Cardiomyopathy is the most common chronic sequela.

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