* 21 polo horses died last year due to wrong mixture

* Company defends its operations as legal and vital (Adds byline, company statement)

WASHINGTON, April 19 (Reuters) - U.S. authorities on Monday said they were seeking an injunction against a Florida pharmacy that incorrectly mixed medicine that killed 21 polo horses on a Venezuelan team a year ago.

Franck’s Compounding Lab Inc and its chief executive Paul Franck were accused of improperly mixing active pharmaceutical ingredients into animal drugs, a practice the Justice Department and Food and Drug Administration said was illegal in veterinary medicine.

The FDA does not test or review such medicines and the agency said many of the products that Franck’s Compounding Lab made were illegal copies of similar drugs that had been approved by the FDA.

“We allege that the practices at issue in this case contributed to deadly results,” said Tony West, assistant attorney general for the Justice Department’s civil division, referring to the deaths last year.

Last year the firm admitted it had prepared the mixture that killed the horses on the order of a veterinarian.

The horses belonged to the Lechuza Caracas polo team and died at the April 2009 U.S. Open Polo Championship hours after receiving a fatal dose of the mineral selenium in a vitamin supplement, according to the state veterinarian.

The Justice Department and FDA asked a court to issue a permanent injunction against the Florida firm and the CEO to halt the practice of using active ingredients in mixing medicines for animals, known as compounding.

Compounding is a practice in which pharmacists combine, mix or alter drug ingredients to create a medication tailored to a specific patient. The FDA said traditional compounding provided a service to meet patient needs.

“But when compounders like Franck’s circumvent, and thus undermine, the statutory drug approval process by manufacturing drugs under the guise of pharmacy compounding, we are concerned that poorly compounded drugs can jeopardize the health of animals,” said Bernadette Dunham, director of the FDA’s Center for Veterinary Medicine.

The company said it would fight the legal action and defended its operations, saying they were “both legal and medically vital.”

“Our quality control procedures are strong and fully comply with regulatory requirements,” the company said.

The horses that died were valued at up to $100,000 each and belonged to Lechuza Caracas owner Victor Vargas, a millionaire businessman and president of the Venezuelan Banking Association.

The case is USA v. Franck’s Lab Inc. and Paul Franck in the U.S. District Court for the Middle District of Florida, 10-cv-00147.