A Florida vet is blaming the deaths of 21 polo ponies on the mineral supplement Selenium which is often given to horses to help them recover from muscle fatigue.

Vet Thomas J. Holt said that tests on the ponies showed that they had above normal levels of the mineral in their bodies. He also said that the ponies had shown signs of toxic poisoning when they were dying.

Dr Garland, head of toxicology at Texas A&M’s Veterinary Medical Diagnostic Laboratory stated that the horses would have suffered a painful death after the overdose.

It is not yet known if the overdose was a vet or pharmacy error.