Monensin and other ionophore antibiotics are used as growth promoters in livestock feeds and as coccidiostats in poultry. Problems usually occur when horse feed is inadvertently contaminated in a feed mill by being mixed with cattle feeds. This usually happens when the facilities are not cleaned properly between feed batches. In other cases, horses have gained access to cattle feeds. This summary refers to this situation.

Horses are very sensitive to the effects of these drugs. If they ingest large quantities of livestock feeds containing ionophores, they may show severe colic signs, collapse and may die within hours. Other lower grade and chronic injury can be caused by ingestion of smaller amounts over a longer period of time. The drugs cause destruction of heart, muscle, liver and kidney cells.