A runner with a torn tendon has reason to envy a racehorse with the same affliction: horses have treatment options not available to human patients–most notably, injections of adult stem cells that appear to spur healing in these animals with shorter recovery time than surgical treatments. Now the same stem-cell therapies used routinely in competitive horses and increasingly in dogs are beginning to make their way into human testing.

Human stem-cell treatments are advancing quickly in many areas: therapies using adult stem cells derived from both fat and bone marrow are currently being tested for a variety of ailments, including Crohn’s disease, heart disease, and diabetes. (Bone-marrow-derived stem-cell transplants have been used for decades to treat blood diseases and some cancers.) But when it comes to orthopedic injuries, such as torn tendons, fractures, and degenerating cartilage, veterinary medicine has outpaced human care.

Veterinarians and private companies have aggressively tested new treatments for the most common injuries in racehorses, in large part because these animals are so valuable and can be so severely incapacitated by these wounds. “Soft-tissue injury is the number-one injury competitive horses will suffer and can end a thoroughbred horse’s career,” says Sean Owens, a veterinarian and director of the Regenerative Medicine Laboratory, at the University of California, Davis. Veterinary medicine also has much more lax regulations when it comes to treating animals with experimental therapies, allowing these treatments to move rapidly into routine clinical use without clinical trials. “Regulatory oversight of veterinary medicine is minimal,” says Owens. “For the most part, the USDA [U.S. Department of Agriculture] and the FDA [Food and Drug Administration] have not waded into the regulatory arena for us.”

Owens’s newly created research center aims to move both animal and human stem-cell medicine forward by conducting well-controlled trials not often performed elsewhere. “Part of our mission is to do basic science and clinical trials and also improve ways of processing cells,” says Owens. The center has a number of ongoing clinical trials in horses–one for tendon tears and one for fractured bone chips in the knee–that are run in a similar way to human clinical trials. The goal is to develop better treatments for horses, as well as to leverage the results to support human studies of the same treatments. Owens is partnering with Jan Nolta,director of the Stem Cell Program, at UC Davis, who will ultimately oversee human testing.

A handful of studies in animals have shown that these stem-cell therapies are effective, allowing more animals to return to racing, reducing reinjury rates, and cutting healing times. VetCell, a company based in the United Kingdom that derives stem cells from bone marrow, has used its therapy on approximately 1,700 horses to date. In a study of 170 jumping horses tracked through both treatment and rehabilitation, researchers found that nearly 80 percent of them could return to racing, compared with previously published data showing that about 30 percent of horses given traditional therapies could return to racing. After three years, the reinjury rate was much lower in stem-cell-treated animals–about 23 percent compared with the published average of 56 percent, says David Mountford, a veterinary surgeon and chief operating officer at VetCell.

While scientists still don’t know exactly how the cells aid repair of the different types of injuries, for tendon tears, initial studies show that stem cells appear to help the tissue regenerate without forming scar tissue.

Mountford says that the company chose to focus on tendon injuries in horses in part because they so closely resemble injuries in humans, such as damage to the Achilles tendon and rotator cuff. For both people and horses, tendon tears trigger the formation of scar tissue, which has much less tensile strength and elasticity than a healthy tendon. “It becomes a weak spot and prone to injury,” says Owens.

Next year, VetCell plans to start a human clinical trial of its stem-cell treatment for patients with degeneration or damage of the fibers of the Achilles tendon. As in the horse therapy, stem cells will be isolated from a sample of the patient’s bone marrow, then cultured and resuspended in a growth medium also derived from the patient. Surgeons will then inject the solution into the area of damage, using ultrasound imaging to guide the needle to the correct location. “Our long-term goal is to use it to treat a number of tendon injuries,” says Mountford.

Stem-cell therapies also show promise for arthritis. Vet-Stem, a California-based company that uses stem cells isolated from fat rather than bone marrow, has shown in a placebo-controlled trial that the treatment can help arthritic dogs. “About 200,000 hip replacements are done every year in humans,” says Robert Harman, a veterinarian and founder of the company. “That’s a very good target for someone to look at cell therapy.”

For osteoarthritis, the stem cells seem to work not by regenerating the joint, but by reducing inflammation. “But in the last couple of years, evidence has come out that the cells we use reduce inflammation and pain, and help lubricate the joint,” says Harman.

While Vet-Stem does not plan to move into human testing, Cytori, a company based in San Diego, has developed a device for isolating stem cells from fat in the operating room. (Vet-Stem does the procedure manually: veterinarians collect a fat sample from the animal and then send it to the company for processing.) Cytori’s device is currently approved for use for reconstructive surgeries in Japan but not yet in the United States.