Forte had been the early 3-1 favorite; his absence reduces the field to 18 horses for the 1 1/4-mile race.
Mike Repole, co-owner of Forte, said veterinarians from the Kentucky Horse Racing Commission had concerns about a bruised right front foot.
Forte stumbled during a workout on Thursday, although trainer Todd Pletcher had downplayed it. He still has two horses in the race: Tapit Trice and Kingsbarns.
A crowd of about 150,000 is expected to jam Churchill Downs to wager and watch the Derby. Post time is 6:57 p.m. EDT.
The horse deaths included Derby contender Wild On Ice. Two of the horses were trained by Saffie Joseph Jr. He was indefinitely suspended by the track, although investigators have yet to determine a cause for the deaths of his horses.
Four horses were scratched — Practical Move, Lord Miles, Continuar and Skinner — in recent days. Practical Move and Skinner had fevers, while Continuar wasn’t in peak condition, according to his Japanese trainer. Lord Miles was Joseph’s Derby horse.
Forte was last year’s 2-year-old champion and has a five-race winning streak.
New antidoping and medication rules to be enforced by the sport’s new central governing body won’t take effect until May 22, after the Derby and the Preakness.
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