The operators of a New York boarding barn are blaming horse feed allegedly contaminated with monensin for the deaths of six horses there.

Monensin is an ionophore antibiotic sometimes included in ruminant, swine, and poultry feed, but which is toxic to horses. Clinical signs of ionophore poisoning in horses include poor appetite, diarrhea, weakness, rapid heart rate, labored breathing, exercise intolerance, depression, wobbly gait, colic, sweating, recumbency, and sudden death. Horses that survive can have long-term heart issues, resulting in diminished performance potential.

Andrew Yaffa, attorney for TMC Performance Horses, in Cuba, said 31 horses were boarded at the facility in 2016 when six animals allegedly began to display clinical signs consistent with ionophore poisoning shortly after ingesting feed manufactured by the North Java, New York-based company Reisdorf Brothers Inc. Three of the six horses died and three were euthanized due to their condition, Yaffa said.

“Veterinary records certify that the deaths were due to ionophore toxicity,” Yaffa said. “Also, the feed tested positive for ionophores by the state of New York