Cell Therapy in Horses: Stem Cells for Musculoskeletal Disorders

The use of stem cells in regenerative medicine for horses is a relatively recent development; it first emerged in 1995 as a treatment for mechanical injuries. Since then, technologies have evolved rapidly and have been refined to target lesions more precisely, treat them more effectively, and reduce associated risks.   Discovered in 1981 in mice and in 1998 in humans, stem cells possess unique properties that make them unparalleled tools in both human medicine and veterinary medicine. As immature cells, they not only have the ability to differentiate into many cell types, but they can also self-renew indefinitely, providing an unlimited supply of material. When applied to medicine, they can replace failing cells in a patient and regenerate impaired functions. Their utility is therefore vast. They are used to treat conditions ranging from corneal lesions to connective tissue in tendons. Currently, more than 350 clinical trials of cell therapy using mesenchymal stem cells in humans are underway worldwide. In horses, for more than ten years, cell therapies have been used to treat connective tissue disorders such as tendinitis and joint problems.  

Not Just Any Stem Cell

As in humans, simply injecting a stem cell is not enough for its regenerative potential to take effect. Several factors come into play and interact with the properties of these cells. As with any introduction of foreign tissue (blood, graft), stem cells can trigger an immune response. They must therefore be carefully selected. The risk of a reaction increases when the stem cells are allogeneic—that is, genetically different—because the donor is another horse. Consequently, adult stem cells are frequently harvested from bone marrow or adipose (fat) tissue directly from living animals to avoid the risk of an immune reaction. This approach is still regularly used because it offers a clear logistical advantage in emergency situations, but the process has been refined through the purification of stem cells obtained from the fat or bone marrow of the injured horse. The goal today is to create a bank of allogeneic cells that are readily available for immediate treatment without the need for harvesting. This is because genetic background can affect the proliferation and differentiation capacity of stem cells. Furthermore, as the donor ages, the quantity and quality of the harvest also decline. Having high-quality, ready-to-use stem cells from young horses would greatly improve the care of recipient animals. The advancement of cell therapy depends largely on the practicality, safety, and efficacy of the techniques used. Ideally, neonatal stem cells, harvested from umbilical cords, should be used to maximize clinical outcomes. Their use in humans has been validated, and since 2006, the number of transplants using neonatal stem cells from umbilical cord blood has exceeded that of transplants using adult bone marrow. Neonatal stem cells are believed to have greater anti-inflammatory, proliferative, and immunoregulatory potential than adult cells. Some equine veterinary clinics are already offering breeders the option to preserve the umbilical cord at foaling, recognizing that it could one day be useful in a horse’s athletic career. This personalized therapeutic tool can complement the treatment of foal injuries sustained during training, even up to ten years later. However, it is not enough to simply choose the age of the stem cell; its origin is also important, as it determines its medical utility. In horses, the most commonly used type is the mesenchymal stem cell (MSC). They originate from the mesoderm layer during embryonic development, which gives rise to muscle and connective tissue. MSCs can therefore differentiate into cells such as tenocytes—a fundamental component of tendons—and be used to treat musculoskeletal conditions in horses.  

Stem Cells for Treating Mechanical Disorders in Horses

Most of the studies published and evaluated to date concern the use of MSCs to treat tendon injuries. Whether acute or chronic, these injuries are often debilitating for horses and progress slowly. Existing therapeutic solutions are less than satisfactory. Faced with these challenges, veterinarians seek a treatment capable of combining minimal recovery time with a reduced risk of recurrence and a rapid return to optimal athletic performance. By forming a natural biological reservoir mobilized during the healing process, cell therapy helps move toward this goal. It targets tissue lesions, delivers a supply of healthy cells specific to the injured tissue that can replace damaged cells, and secretes growth factors that promote local vascularization, stimulate the body’s own healing capabilities, and create a lasting anti-inflammatory environment. Regenerative medicine thus strengthens the body’s own defenses by locally supplementing stem cells, which encourages the restoration of tissue function and the horse’s return to its original athletic level. It is not so much that the horse will heal more quickly, but rather that the tendon, after healing, will be closer to normal and less fragile—which is particularly important for sport horses.

The results are promising, particularly for superficial injuries to the digital flexor tendons, reducing the risk of recurrent injury from 55% to 25%.

Research on the treatment of equine meniscal tears (in the cartilage between the stifle joints) has also shown promise, resulting in a higher rate of return to exercise. However, further studies are still needed. The technique is currently being refined. Questions remain, particularly regarding the optimal timing, cell type, or number of cells to maximize therapeutic effects, as well as regarding combination therapies or adjuvants. These are areas that remain largely unexplored. But trials on the subject are proliferating. It will take a little more patience to learn the results and put them into practice in real-world settings.