The 21 polo ponies that dropped dead at the U.S. Open Polo Championship in Wellington, Fla., eleven days ago most likely succumbed to an overdose of selenium, used to help muscles recover after strenuous exercise, the Florida Department of Agriculture and Consumer Services has reported.

The source of the toxic overdose appears to be supplement injections the horses received a few hours before they began falling to the ground. Franck's Pharmacy in Ocala, which filled the prescription for the supplement—a cocktail of selenium, vitamin B-12, potassium, and magnesium -- has owned up to the mistake: "The strength of an ingredient in a medication Franck’s Pharmacy prepared for the 21 horses on the Lechuza Polo team was incorrect," Jennifer Beckett, the pharmacy's chief operations officer, said in a statement. "We can confirm that the ingredient was selenium."

It is unclear whether the horses’ owners plan any action against the pharmacy.

According to John Harvey, a veterinary clinical pathologist at the University of Florida (UFL) College of Veterinary Medicine in Gainesville, where 15 of the horses were sent for autopsy, selenium levels were ten to 15 times higher than normal in the animals' blood and 15 to 20 times higher than normal in their livers.

Frederick Oehme, a veterinary toxicologist at Kansas State University in Manhattan, says that selenium, which is normally given to horses in their feed, takes a long time to reach toxic levels in the body. "If indeed, it is selenium, and if indeed it was injected, they must have gotten a whopping dose," Oehme says.

Excessive amounts of this mineral, which is essential for proper functioning of animal cells, could interfere with the ability of nerve cells to control muscles, including the heart, he notes. And although owners and trainers sometimes use selenium injections to alleviate muscle cramping in horses, Oehme says the safest and most natural way to administer the nutrient, which is found at various concentrations in a hay and grains, is to supplement their feed.

Image of a polo ponies (not the ones that died at the U.S. Open Polo Championship): ©