A ruptured aneurysm is being blamed for a horse's death during a training session for the chuckwagon races at the Calgary Stampede currently underway in Alberta, Canada.

The Calgary Stampede's GMC Rangeland Derby chuckwagon race pits four teams of horse-drawn wagons against each other in a series of elimination races over several days; traditionally, each team consists of four wagon horses and four outrider horses.

In recent years, several horses have died in the chuckwagon races. In 2011, stampede management announced new rules intended to increase safety for both equine and human chuckwagon race participants, including veterinary inspections to determine whether horses are fit to compete. Those deemed unfit are scratched from the event, while those deemed fit to compete also undergo post-race veterinary inspections. The rules also mandate that no horse can run in the event more than three consecutive days, and all horses used in the chuckwagon races must have at least one rest day in every four.

A necropsy and pathology tests are also performed on every animal that dies at the stampede, said Calgary Stampede spokeswoman Bonni Clark