The horse that suffered a heart attack belonged to the team of Reg Johnstone, seen in the yellow shirt and black cowboy hat. (CBC)

A horse has died of a heart attack after completing a race at the Calgary Stampede.

Stampede officials said the horse died on Sunday night after the first heat of the GMC Rangeland Derby chuckwagon race, an event where teams of horses pull wagons around a dirt track.

The animal was returning to the barn when it collapsed.

Doug Fraser, the media relations manager with the Calgary Stampede, called the death "a tragedy."

"To lose an animal, regardless of the circumstances, is always difficult for the participants in the GMC Rangeland Derby, because these horses are treated like family," he said on Monday.

"The horse died relatively quickly so there was little the veterinarians could do to save it."

Humane society saddened by news

Fraser said the horse completed the race and the post-race parade and that the horse showed no signs of stress during the race. The animal belonged to the team of driver Reg Johnstone, of Bashaw, Alta.

The death of a horse as a result of a heart attack is considered a rare occurrence, Fraser said.

But Peter Fricker with the Vancouver Humane Society said chuckwagon races are the biggest cause of death among animals at the Calgary Stampede. His organization tried to run advertisements in Calgary questioning the treatment of animals in some rodeo events, but two major newspapers refused to run the ads.

"This will be the 22nd chuckwagon horse to die [at the Calgary Stampede] since 1999," he said. '"Our understanding is that it's directly related to the stress the animal is under during the race.

"It's very sad news to hear another animal has died at the Calgary Stampede."