More from the doctor performing the surgery, according to a Reuters report:

The surgery was being performed by Dr. Dean Richardson at the University of Pennsylvania's New Bolton Center for Large Animals. He said the operation could take several hours.

Richardson, the chief of surgery for the center, said the injuries were "very, very serious," and that he's never worked on so many catastrophic injuries to one horse.

"You do not see this severe injury frequently because the fact is most horses that suffer this typically are put down on the race track," Richardson said. "This is rare."

"It's about as bad as it could be," he added. "The main thing going for the horse is a report that his skin was not broken at the time of injury. It's a testament to the care given to the team of doctors on the track and (jockey) Mr. Prado on the racetrack."