Your horse has torn a major tendon in his leg, and the news from your veterinarian is not good. Even after months of rest and rehabilitation, she says, his tendon likely won't be as strong as it was before the injury. It may limit what you can do with him. But then she mentions a new treatment: stem-cell therapy. It's cutting edge—and costly. Could it help your horse?
Stem-cell therapy is one of several treatments that fall under the umbrella of regenerative medicine, a fast-growing field that's creating major buzz in veterinary and human medical circles. These treatments use the body's own repair tools with the goal of better healing. In this article you'll find what you need to know about two regenerative treatments being used in horses, stem-cell therapy and platelet-rich plasma. For guidance we turned to Alison Stewart, DVM, an equine surgeon and stem-cell researcher at the University of Illinois, and Jamie Textor, DVM, an equine surgeon who is researching PRP at the University of California at Davis.
These treatments are exciting, but they are very much works in progress. Researchers are searching for the most effective ways to use them, and there are many questions about how—and how well—they work in different situations. They require specialized facilities and know-how that may not be available in every private veterinary practice. But they're becoming more widely used, and if they live up to their promise, that trend will continue.
Stem cells are cells that have the potential to develop into some or many of the different specialized cells that make up body tissues, such as muscle, bone, blood and nerves. There are different kinds of these chameleonlike cells:
- Embryonic stem cells are derived from fertilized embryos. Taken early enough they are totipotent—they can develop into any tissue type, Dr. Stewart says. They go through several stages, becoming less able to differentiate as they mature.
- Induced pluripotent cells are adult cells that have been genetically reprogrammed in the lab so they behave like embryonic stem cells. Like embryonic stem cells, they proliferate readily and can differentiate into multiple tissue types. Researchers generated the first iPSCs less than five years ago and the first from horses only last year. They're still exploring the potential of these cells.
- Adult stem cells are found in many organs and tissues, typically in small numbers. They are multipotent—not as versatile as embryonic stem cells but able to differentiate into at least three specific cell types. These cells seem to remain at rest for many years until called on to generate replacements for cells that are lost through normal wear and tear, injury or disease. They also produce a range of bioactive proteins, including growth factors and other agents that help repair damaged tissue.
The commercial stem-cell treatments available for horses today make use of adult mesenchymal stem cells, or MSCs, which can differentiate into bone, cartilage, fat and fibrous connective tissue. The cells are autologous—derived from the horse being treated. (Experimental treatments have also been done using donor, or allogenic, stem cells.)
"The commercial treatments are mainly being used for soft-tissue injuries—tendons and ligaments," Dr. Stewart says. They are used less for joint problems generally, although some veterinarians think that they can be helpful for stifle injuries that involve a lot of soft-tissue damage.
Tendons and ligaments have different functions—tendons transfer the action of muscles to the skeleton, while ligaments lash bone to bone and keep joints from wobbling. But both are made up mostly of organized networks of dense, elastic connective tissue, rich in a tough protein called collagen. And they basically heal in the same way—that is, slowly and often poorly, with disorganized scar tissue in place of organized collagen fibers. This leaves your horse prone to reinjury.
MSC therapy may allow better repair, with more organized collagen and less scarring, Dr. Stewart says. Your horse will still need a lengthy layup and rehab period, though. The therapy seems to help most when it's given in the first few weeks after an injury, during the time when healing is getting underway. It's much less effective for longstanding or chronic conditions.
MSC Treatment. If you and your veterinarian decide to try stem-cell treatment for your horse's injured tendon, the first step will be to collect MSCs. The cells are harvested from bone marrow or fat, depending on the treatment method used. This is done with your horse standing and sedated, with local anesthesia.
- For marrow-derived stem cells, a sample of bone marrow is aspirated (drawn with a special needle) from the horse's sternum or hip. The marrow contains stem cells along with serum that's rich in growth factors. It's couriered to a laboratory where stem cells are extracted, cultured to expand the numbers, recombined with the serum and returned for injection. Usually 10 million cells are used to treat a tendon, Dr. Stewart says, and expansion can take two to three weeks. (VetCell, www.vetcell.com, a British company, developed the process and now offers it in North America through Equine Partners America, LLC, www.vetcellamericas.com.)
- For fat-derived stem cells, the veterinarian makes an incision, usually near your horse's tailhead, and collects a sample of fat tissue. The sample goes to a lab where it's treated with enzymes and spun in a centrifuge to separate nucleated cells from the fat that makes up most of the tissue. The nucleated cell fraction (which may include MSCs and some adult fat cells) can be sent back for use within three days or kept and cultured to get a higher concentration of MSCs. (Vet-Stem, www.vet-stem.com, in Poway, California, is a leading provider of this service in the United States.)
Does it matter where stem cells are collected and how they're processed? "Some would say yes, but there has been no head-to-head comparison of the long-term effects," Dr. Stewart says. The various stem-cell products are handled in much the same way when they come back from the lab:
In most cases the veterinarian injects the stem-cell product directly into the injured tissue, using ultrasound to guide the injection into the lesion. The treatment also can be delivered into the blood supply around the site (regional limb perfusion), but this may not be as effective. "A study comparing the two methods found many more MSCs at the lesion when they were injected directly at the site than when injected using regional limb perfusion," says Dr. Stewart.
The cost varies. "In most cases, from harvesting to reinjection, you're looking at $1,200 to $2,000 to treat a tendon injury," Dr. Stewart says.
Side effects are rare. As with any injection, there's a small risk of transient inflammation or infection. But with autologous cells there should be no immunogenic reaction—that is, your horse's immune system won't target the cells as foreign and try to destroy them. "Even with cells from another horse, MSCs should be less immunogenic than adult cells," Dr Stewart says. "But studies are still being done on this."
Will It Help? Stem-cell success stories abound, but there's not much peer-reviewed scientific research comparing outcomes with stem cells and traditional treatments. Researchers have done postmortem tissue studies that show good results in tendon and ligament injuries, including better alignment of fibers and less disorganized scar tissue—but benefits are hard to track in living horses because of individual differences in their injuries and care, Dr. Stewart says.
A recent follow-up study showed lower rates of injury after MSC treatment. Dr. Roger Smith, a professor of equine orthopedics at the Royal Veterinary College in Britain and developer of the VetCell technique, reviewed outcomes in 113 racing Thoroughbreds with core lesions in the superficial digital flexor tendons and found that treatment with marrow-derived MSCs cut reinjury rates by more than half. Horses who returned to training after stem-cell treatment and were followed up for three years had a reinjury rate of 27 percent, compared to 57 percent in conventionally treated horses. No comparable follow-up study has been done with fat-derived cells, Dr. Stewart says.
Stem cell therapy is evolving, and researchers are still trying to determine exactly what the cells do after they're injected. "Early research evaluated MSCs for their potential to replace damaged tissue," Dr. Stewart says. "More research now is directed at their ability to modulate inflammation and encourage healing."
How long they hang around the injection site varies with the cell type and the tissue. "In tendon, bone-marrow-derived MSCs seem to be present up to two weeks, and then the numbers drop off. We don't know if they've migrated somewhere else, died or divided so that the dye marking them no longer shows up," Dr. Stewart says. "Embryoniclike stem cells seem to persist longer at the site of injection and travel farther, but they're not yet commercially available."
The cells don't always behave as expected. Veterinarians at Cornell University tracked bone-marrow-derived MSCs injected into the fetlock or stifle joints of healthy and arthritic horses. They hoped the stem cells would migrate into damaged cartilage, where they might help repair it. Instead, the cells went to the synovial membrane, which surrounds the joint.