Kentucky Derby winner Sunny's Halo has a form of ringworm and his trainer will decide by Thursday whether he will run in the Preakness Stakes.
"We have a lot of concern about it," trainer David Cross Jr. said today.
Thursday is the day the maximum 14 entries are decided for the $300,000-added race.
Before winning the Derby, Sunny's Halo came down with a small rash on his back about three to four inches behind the saddle, but the rash now covers most of his body, Cross said.
"It's a rash, it's a ringworm," he said. "If he was just another horse in the barn, you wouldn't think about it" because the ailment isn't dangerous.
"It has nothing to do with his running ability. "But it's his appearance; he's all blotched. It's just unfortunate it came up at this time."
Cross, who consulted with veterinarians in Canada, New York and here, said the ringworm diagnosis was made late last week.
He said the 3-year-old colt was being treated with antibiotics and would work out Monday at Pimlico Race Course, where the 108th running of the Preakness will take place Saturday.
"So far, he's acting like 100 percent; he's training good," Cross said.
"I'm going to be a very, very disappointed person if I can't run this horse for (Pimlico General Manager) Chick Lang and the other people at Pimlico. The Preakness means as much to me as the Derby."
In other racing news, trainer Bill Boniface said he would run Parfaitement in the Preakness, after his horse sped six furlongs in a smart 1:12 in a workout this morning.
In a less impressive workout, Flag Admiral, a Preakness hopeful partly owned by former president Jimmy Carter, worked a mile in 1:39 1/5.
Parfaitement rushed into third position in the Derby approaching the stretch, only to fade out of the money.
He finished second in the faster section of the divided Wood Memorial at Aqueduct, despite going wide on the turn with jockey Herb McCauley.
Parfaitement will join stablemate Deputed Testamony, another disappointment in Kentucky.
Both appear to have more than sufficient lifetime earnings to qualify, making Flag Admiral's chances of starting in the classic more doubtful.
The Preakness is restricted to the 14 entries with the greatest earnings. A total of 16 horses will be under consideration.
Flag Admiral drew attention at Churchill Downs on Derby Day when he won a mile allowance race in a sparkling 1:35 3/5.