Monensin Poisoning in Horses: Clinical Signs and Outcomes

Some horses that consume monensin eventually recover—a few even return to work—but it hasn’t been clear how toxicity impacted their hearts and subsequent athletic performance. Here’s what researchers learned.
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monensin poisoning in horses
Sixteen months post-monensin-exposure, 34 of the 64 horses (53%) for which this information was available had return to their previous function. | Photo: iStock

It’s a terrible feeling to know something you’ve fed your horse has inadvertently made him sick. In some instances, however, it happens.

Monensin is an antibiotic feed additive mainly used to promote growth in cattle and poultry. While it can have positive effects on those and other livestock species, it’s highly toxic to horses. Although monensin can affect all cell types, the toxic effect is primarily observed in muscle cells. Clinical signs of consumption can include a loss of appetite, rapid or irregular heart rate, exercise intolerance, sweating, colic, and unexpected death.

Sporadic intoxications are usually caused by accidental feeding of commercial monensin-containing feed intended for cattle or poultry, or from contamination of horse feed with monensin during production

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Written by:

Erica Larson, former news editor for The Horse, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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