The U.S. Food and Drug Administration (FDA) is investigating the deaths of six horses in Minnesota allegedly due to horse feed contaminated with monensin.

Monensin is an ionophore antibiotic sometimes included in ruminant, swine, and poultry feed that is toxic to horses. 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, and recumbency. Most die within three days of ingesting it. Horses that survive can exhibit long-term heart issues, resulting in diminished performance potential.

On June 9 a farm owner in Gilman, Minnesota, reported that a horse became ill and unable to stand after consuming feed mixed by Gilman Co-Op Creamery, the FDA said in a written statement. Two days later the horse was euthanized.

On July 12 two other horses were discovered laying down in their pasture, unable to stand. One horse died that day and the other was discovered dead the following day. Three more horses died over the next month, the FDA said.

The agency connected the deaths to horse feed containing monensin, which was manufactured by Gilman Co-Op Creamery.

“The firm did not perform adequate cleanout between batches to remove monensin from the equipment before mixing the horse feed,” the FDA said.

No one from the Gilman Co-Op Creamery was available for comment.

The FDA investigation into the deaths continues.