In October 2006 the Jockey Club convened the first Welfare and Safety of the Racehorse Summit. The group set up its Durability Index, which shows that the number of starts made by the average U.S. racehorse declined from 11.3 per year in the 1960s to 6.4 in 2006. The goal, Bowen says, is to allow breeders to identify thoroughbreds possessing above-average durability, or ability to race frequently over multiple years.
There is strong evidence that horses inherit racing prowess, if imperfectly, says Ernest Bailey, professor of genetics at the Gluck Equine Research Center at the University of Kentucky in Lexington and coordinator of the Horse Genome Project. But he says a decline in durability would likely require changes in multiple genes.
"I don't understand how such a change could have come about within 50 years on a genetic basis for a population that may be one million worldwide," he says. More plausible, he adds, is that horses might inherit particular types of fragility. Researchers are mining the horse genome, assembled last year, looking for genes that might contribute to disorders such as osteoarthritis or respiratory ailments.
"There are horses that are notorious for having certain types of problems," Arthur says. He cites a "prominent stallion" that had congenitally constricted airways but who "could have had three ears and they would have bred him … his horses were so great."
Bailey, however, notes that four generations of selection for a gene for HYPP (hyperkalemic periodic paralysis), which bestowed quarter horses with large, well-defined buttocks but led to injuries, spread the gene to only two percent of horses—descendents of Impressive (1968 to 1995), the stallion that carried the gene.
Up in the air, as well, is whether the Durability Index would reflect limits imposed by genetic defects or by changes in the way animals are regarded and cared for. "In the old days it was not uncommon to run-run-run-run the horses," Arthur says. "Maybe they were having the same problems but they ran them anyway."
Modern horses receive exceptional care and treatment, Scollay says, but by the same token, their owners demand wins. "One needs to be very careful when you increase the physical demands on a person or an animal … to keep them performing at that level," she says. Horses in top condition, such as Eight Belles, are at particular risk, she says, for microscopic tears in their bone and cartilage that may build up over months of hard racing without causing the horse any discomfort.
"We've got very good evidence that bone disease and microdamage cause all fractures," says Wayne McIlwraith, an equine orthopedic surgeon and director of the Equine Orthopaedic Research Center at Colorado State University in Fort Collins. Eight Belles suffered fractures in the sesamoid and metacarpal condyle bones of the fetlock, or ankle joint.
"If you look at a horse finishing a race, they're usually very hyperextended in that joint," which is the site of almost all fatal fractures, McIlwraith says. Although the autopsy uncovered no preexisting problems, Scollay says that a microscopic analysis of the bone tissue was apparently not done.
McIlwraith says he and his colleagues are currently working on a blood test to screen for microdamage. They enrolled 200 thoroughbred racehorses in training and sampled their blood every month to check for seven biological markers—portions of collagen and sugar-flecked protein molecules—exposed when bone and collagen start to degrade.