Connections of the 21 polo ponies that died at last year’s US Open are suing the pharmacy they claim is solely responsible. They are seeking damages of more than $4million.

Quantum Management Company and three owner/players last week began formal legal action against Franck’s Pharmacy in Ocala, Florida.

The lawsuit filed in Palm Beach County Circuit Court alleges that Franck’s Pharmacy was solely responsible for improperly mixing a vitamin compound, leading to the fatal selenium overdose.

On 19 April 2009, 21 Lechuza Caracas team ponies died beside the field at the US Open on 19 April after being given a pre-match vitamin compound.

A week later, Florida’s state vet Dr Thomas Holt said 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.

Shortly afterwards, Franck’s Pharmacy claimed responsibility for wrongly mixing a pre-match medication given to the ponies.

Franck’s Pharmacy has declined to comment on the lawsuit, and local, state and federal investigations in Florida are still ongoing.