Selenium has been named as the likely cause of the deaths of 21 polo ponies in Florida last week.

Florida’s state vet Dr Thomas Holt said last night the signs exhibited by the Lechuza Caracas team ponies and their rapid deaths were “consistent with toxic doses of selenium”.

The findings were obtained by a Florida laboratory, and confirmed in independent tests in New York and California.

Selenium is a trace mineral often included horse feed and supplements in small quantities. While it is essential for normal cell function and health of animals, is can be fatal in large doses.

Florida’s Agricultural Commissioner Charles Bronson said the horses that died had “significantly increased selenium levels” in samples tested.

He declined to comment any further, since inquiries are still ongoing into the deaths of the 21 Lechuza ponies at the US Open on 19 April.

A Florida pharmacy has already claimed responsibility for wrongly mixing a medication given to the ponies before the match.

The substance has been likened to Biodyl, a drug commonly used to treat muscle disorders. Manufactured by Merial, Biodyl is not licenced in the US.