California-based Western Milling LLC has agreed to pay more than $500,000 in fines in connection with its manufacture of monensin-contaminated horse feed.

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.

In 2015, Western Milling voluntarily recalled some batches of Western Blend horse feed after learning that several horses that reportedly consumed the feed either became ill or died.

Prior to the recall, Black Fence Farm in Clovis, California, purchased some of the feed and fed it to 51 horses at the farm, said Florida-based attorney Andrew Yaffa, who represents the Black Fence Farm horse owners. In 2016, the horse owners filed a complaint against Western Milling, alleging, among other claims, that the manufacturer was aware of monensin in its equine products