Horse feed milled at plants that also produce other livestock feeds with ionophores could contain a substance which can be toxic to horses.

Ionophore antibiotics are added to feed to improve weight gain and control protozoal and bacterial infections in ruminants, swine and poultry. Several ionophores are approved for use in the United States with the most common being monensin, salinomycin, lasalocid, laidlomycin, and narasin.

Horses are much more sensitive to ionophore poisoning than other species. For example, the safety zone for monensin in horses is 2 to 3 mg, whereas cattle can tolerate 20 to 34 mg and poultry 90 to 200 mg. When higher-than-acceptable concentrations of ionophore are found in equine feed, a horse could be dead in less than 24 hours after ingestion.

Ionophore toxicity inhibits sodium and potassium ion transport across cell membranes, which can kill cells—especially muscle cells—leading eventually to total system failure and death. Signs of ionophore poisoning include poor appetite, diarrhea, muscle weakness, depression, wobbling, colic, excessive urination, sweating, lying down, and sudden death. There is no specific treatment for a poisoned horse and those that survive can have permanent heart damage