The Skyhorse Post

WELLINGTON, Fla. (AP) — Seven more Venezuelan polo horses sickened just before a Florida tournament died overnight, raising the death toll to 21, and officials said Monday they may have been killed by some type of poison.

Link:
AP: 21 Polo Horses Dead

Another report from Australia suggests injections given to the horses may be responsible:

Suspicion falls on vet’s shots as 14 ponies die
April 21, 2009 – 12:00AM
FLORIDA: Before a hushed crowd at International Polo Club Palm Beach on Sunday, veterinarians feverishly tried to save 14 expensive polo ponies from dying before a US Open Championship match. Fourteen horses of the Venezuelan-based Lechuza Caracas team, worth as much as $US1 million ($1.4m), died before their scheduled game against Florida’s BlackWatch team. Two other horses are being treated at an equine clinic. Another is being treated at Lechuza’s complex near the stadium. When the horses started getting sick and collapsing about 45 minutes before the match, stadium officials announced it was cancelled and that an exhibition game would be played instead. A spokesman for the club, Tim O’Connor, said two horses collapsed at first, and as vets and team officials scrambled to revive them, five others became dizzy. “A total of seven died on our property,” O’Connor told CNN, adding that seven other horses died en route to a farm and a veterinary hospital.
. . .
According to several sources, the horses had a reaction to a steroid derivative that might have been tainted with a cleaning solution. The shots apparently were administered by an Argentine vet, not licensed in the US.

Link to source:
Brisbane Times