Horses feed manufacturer ADM Alliance Nutrition, a subsidiary of the Archer Daniels Midland, Co., is responding to recent news reports and social media posts about its potential connection to an incident in which three South Carolina horses died and another was sickened.

Farm owner Anne Kennedy told TheHorse.com that in December, three horses residing at the farm were treated at the Edisto Equine Clinic in Yonges Island, South Carolina, for coliclike signs and later died. Another horse exhibiting similar clinical signs was also taken to the clinic for treatment, she said.

Kennedy subsequently sent a small sample of the ADM feed used at her farm to Michigan State University’s (MSU) Diagnostic Center for Population and Animal Health, in Lansing, for testing. She said the sample was “negative for ionophores (monensin),” but that a postscript included in the test results indicated the laboratory had identified monensin in trace quantities.

Ionophores are a type of antibiotic manufacturers sometimes add to cattle feed to promote weight gain. These substances, including monensin, can be toxic to horses. Monensin toxicity becomes apparent in horses in a number of ways, said Adrienne Bautista DVM, PhD, of the California Animal Heath and Food Safety Laboratory System at the University of California, Davis, School of Veterinary Medicine.

“Some horses go off their feed, some exhibit ataxia or weakness, some colic, and some simply die,” Bautista said. “When horses die, they generally die very quickly, and th