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The bizarre deaths of 21 polo ponies at last weekend's U.S. Open Polo Championship in Wellington, Fla., might have been caused by something in vitamin injections the horses received, the captain of the ponies' team has told reporters. Experts suspect that contamination or improper dosing of the cocktail are blame, but they say it's impossible to say just what killed the horses before the results of toxicology studies are released.

If you're just tuning in now, here is a recap of Sunday's events as told by the Argentine newspaper La Nacion: Venezuela's Lechuza Caracas team was getting ready to compete in the quarterfinals at the International Polo Club Palm Beach when spectators noticed a few of its ponies crumpling to the ground. As veterinarians rushed to care for them, other ponies on the team began collapsing. The animals "fell like domino pieces," a witness told the paper, and within a couple of hours, 21 of them were dead.

The captain of the team, Argentinian Juan Martin Nero, has since told La Nacion that all 21 horses that died had been injected with a supplement containing selenium, vitamin B-12, potassium, and magnesium. "There were five horses that were not given vitamin and [they are] well," Nero told the paper. But he did not indicate what, if any, substance might have contaminated the supplement.

This mix of vitamins and minerals is sold for use in animals under the brand name Biodyl in Europe, Asia, Latin America, and Africa. (The U.S. Food and Drug Administration has not approved the product for animals or humans.) But Duluth, Georgia-based Merial, the company that makes Biodyl, says it has nothing to do with the ponies' deaths. "The preparation that killed these horses is an illegally compounded copy of Biodyl," Merial spokesperson Steve Dickinson told ScientificAmerican.com.

According to the latest report by the Associated Press, a Florida veterinarian wrote a prescription for the 21 ponies for a Biodyl-like compound that was filled by Franck's Pharmacy in Ocala. Franck's is now cooperating with an investigation by the Food and Drug Administration and Florida state officials, according to the AP.

If the bootleg supplement is to blame, it could have been tainted with some lethal substance, says Frederick Oehme, a veterinary toxicologist at Kansas State University in Manhattan. It's highly unlikely vitamin B-12 or selenium by themselves killed the ponies. "B-12 doesn't cause death, even in an overdose," he says. And it takes a long time for dangerous levels of selenium to build up in the body, he notes.

"A potassium or magnesium overdose could have done this," says Pamela Wilkins, head of Equine Medicine and Surgery at the University of Illinois at Urbana-Champaign College of Veterinary Medicine. If the body is overloaded with potassium, nerve cells cannot fire properly and heart muscle cells lose their ability to keep the heart pumping regularly, causing overall weakness, and slowed heart rate, Wilkins explains; eventually the heart stops beating. A magnesium overdose could also cause fatal heart problems, muscle paralysis, and pulmonary edema (a buildup of fluid in the lungs that can eventually show up as froth in the nose), she adds. (The ponies did show signs of pulmonary edema, according to The Miami Herald.)

But until the toxicology study results are released, Oehme cautions, it is impossible to peg the cause of death.

Sarah Carey, a spokesperson for the University of Florida (UFL) College of Veterinary Medicine laboratories in Gainesville, where 15 of the horses were sent for autopsies, says that scientists have now completed most of the toxicology studies and turned them over to Florida's Department of Agriculture and Consumer Services. She said she could not reveal the results because the case is still under investigation. She wouldn't say whether those investigations were criminal.

The U.S. Department of Agriculture and Consumer Services will not reveal any findings right now but may provide more details next week, says department spokesperson Terence McElroy.

Image of a polo pony (not one of the ones that died last Sunday): nationalrural via Flickr