The owners of horses kept at a California boarding barn have filed a lawsuit against the manufacturer of a horse feed that allegedly caused the illness and death of their animals because it was contaminated with monensin.

Monensin is an ionophore antibiotic sometimes included in ruminant, swine, and poultry feed that can be toxic to horses. Robert J. MacKay, BVSc, PhD, Dipl. ACVIM, professor of large animal medicine at the University of Florida, in Gainesville, said horses that ingest monensin can show clinical signs of ionophore poisoning including poor appetite, refusal of the grain product, diarrhea, weakness, rapid heart rate, labored breathing, exercise intolerance, depression, wobbly gait, colic, sweating, recumbency, and sudden death.

“If a horse ingests a high amount you are likely to see neurologic symptoms and death within three days,” MacKay said. “Horses that survive can exhibit long term heart issues including diminished performance potential.”

Last September, Western Milling LLC voluntarily recalled some batches of its Western Blend horse feed that were distributed in California and Arizona after learning that several horses reported to have consumed the feed either became ill or died