A gelding that collapsed and died moments after winning a $US150,000 jumps race in Tennessee on Saturday had suffered a ruptured pulmonary aneurysm.
Experts said the condition that killed Arcadius could not have been prevented and was impossible to detect in pre-race veterinary checks.
An aneurysm is a weak point in the wall of a blood vessel. They balloon out and, if they rupture in a main vessel, the consequences can be fatal.
Arcadius, aged 8, died as he was being cooled down after his win in the 71st running of the 3-mile Iroquois Steeplechase at Percy Warner Park.
It was the horse’s second start of the season in a 21-race career that began in 2008, with his final race win taking his total race earnings over $US300,000.
His last win was in the feature race of the day, before 25,000 spectators.
The horse showed no signs of distress during the race, which was run in good conditions.
A veterinarian was there to help Arcadius moments after he went down, but efforts to save him were in the vain.