Horse feed manufacturer Cargill says an investigation has revealed that a minuscule amount of monensin was detected in a specific bag of its Nutrena SafeChoice Senior feed, but declined to speculate on what caused the deaths of two California horses.

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 a written statement, Nutrena parent Cargill said two horses boarded at the La Tuna Stables in Sun Valley were found dead in their stalls in April, three days apart. Both the animals had consumed feed from the same bag of SafeChoice Senior.

“When the first horse died, there was speculation that it was a heart attack,” the statement said. “After the death of the second horse, there were questions about whether the two events were related.”

Samples of the feed, as well as ingesta from the second horse, were sent to the University of California, Davis, for testing. The statement said laboratory testing revealed monesin levels “well below 1 ppm (part per million), and indicated that the actual amount was likely less than that.

“The report also states that monensin at levels in the 1 ppm range is common in commercial feed, and did not speculate as to the actual cause of death of each of the horses,” the statement said.

No one from La Tuna Stables was available for comment.

Cargill continues to investigate the incidents, the statement said.