Florida State Veterinarian Thomas J. Holt, DVM, reported today that an overdose of selenium was the probable cause of death of the 21 polo horses that collapsed prior to a competition in Wellington on April 19.

In a memorandum to Florida Agriculture and Consumer Services Commissioner Charles H. Bronson’s office, Holt stated that the animals had “significantly increased selenium levels” in samples tested. He reported that the findings obtained at the department’s Animal Disease Diagnostic Laboratory in Kissimmee were confirmed by independent testing conducted at the University of Florida’s College of Veterinary Medicine in Gainesville, the University of California, Davis, Animal Health and Food Safety lab, and at testing facilities at Cornell University in Ithaca, N.Y.

“Signs exhibited by the horses and their rapid deaths were consistent with toxic doses of selenium,” Holt said.

Selenium is a trace mineral that is essential for normal cell function and health. It is often included in small quantities in supplements and feed for horses. Large doses, however, can be fatal to animals