For generations, horsepeople have sought better ways to prevent and treat proud flesh, scientifically referred to as “exuberant granulation tissue.” This disorder, which is more common in horses than in other animal species, derails the normal healing process of wounded skin, prolonging it for weeks, months and—in the worst cases—years. It can create unsightly scars and cost horse owners excessive amounts of time, money and energy. Even horses cared for by the most skilled and conscientious people may develop proud flesh.
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The frustrating, somewhat mysterious nature of this condition makes it a fascinating topic for veterinary researchers to study. “We have a great deal more to learn, but what has been discovered so far should help all horsepeople tackle this problem with confidence,” says Christine Theoret, DVM, PhD and Diplomate of the American College of Veterinary Surgeons. A leading expert in wound management in horses, Dr. Theoret is a professor of equine surgical anatomy and the director of the Comparative Veterinary Tissue Healing Laboratory at the University of Montreal. She and her research team have conducted several studies specifically designed to advance our understanding of the physiology, immunology and local environmental impact on aberrant wound healing, including proud flesh.
What is Proud Flesh?
Your horse’s skin consists of two major layers: the deeper, “dermal,” layer, which consists mostly of connective tissue to cushion the body, and the outer, “epidermal,” layer, which is made of cells called keratinocytes and serves as a barrier against the inhospitable environment. Any wounds more serious than minor surface abrasions usually tear through both of these layers.
During the early stage of wound healing, the body fills the gap with granulation tissue. As its name implies, this tissue appears very granular (lumpy) because it contains many blood vessels, which bring oxygen and nutrients to the newly forming skin cells. This tissue creates a base layer over which new keratinocytes, produced by the intact skin surrounding the wound, will migrate. Known as “epithelialization,” this process enables the keratinocytes to work their way from the edges toward the center of the wound, thereby forming a new epidermal layer. First visible as a pale pink border at the wound’s periphery, the new epidermis forms into scar tissue. It is not as strong as the original tissue and is devoid of hair, pigment and various other normal skin components, such as sweat glands.
Meanwhile, cells within the granulation tissue, called “myofibroblasts,” pull the wound edges closer together in a process known as “wound contraction.” This process complements the epithelialization, mainly by decreasing the surface area of the wound that the migrating keratinocytes must cover. In wounds located on the leg of a horse, about 30 percent of healing is accomplished by contraction while 70 percent relies on epithelialization.
Depending on the size and depth of the wound, granulation tissue will take days to weeks to fill the gap until it is level with the surrounding wound edges. In a normally healing wound, the granulation tissue stops growing once it has closed the gap and the keratinocytes start building the new scar tissue over it. Sometimes, however, the granulation tissue continues to grow, mushrooming over the skin surrounding the wound. This is what we call exuberant granulation tissue, or proud flesh. Proud flesh almost exclusively occurs in wounds of the lower leg and is rarely found in wounds on the rest of the body.
The mushroom shape of proud flesh hinders the contractile activity of the myofibroblasts and makes the keratinocytes’ job much harder. Instead of traveling across a flat surface, the keratinocytes now must move up and over the edge of this lump of granulation tissue, much the way a rock climber would have to maneuver to get up and over an overhanging ledge. This slows down both wound contraction and epithelialization, sometimes stopping them altogether.
Left untreated, the mushroom crown of granulation tissue can continue to grow many inches beyond the horse’s normal skin surface. The protruding lump of tissue is susceptible to re-injury, which leads to more irritation and inflammation, thereby prolonging the healing process even further. In most cases, proud flesh will not resolve on its own.
What Causes Proud Flesh?
So why does such a seemingly unhealthy condition occur in otherwise healthy animals? No one is entirely sure yet but, according to Dr. Theoret, possibly the most significant factor underlying the development of proud flesh in horses is the presence of a prolonged inflammatory response to injury, especially in wounds occurring on the leg. An essential component of the initial phase of normal wound healing, inflammation is a localized protective mechanism that serves to defend against environmental contamination and to rid the wound of damaged tissue. The early inflammatory response to wounding is not very robust in horses, and the weaker it is initially, the more likely it seems to linger beyond that necessary, first phase. Prolonged inflammation becomes counterproductive, fueling the unchecked growth of granulation tissue and the production of proud flesh.
Although more research is required to fully understand why the inflammatory response to injury is weaker in wounds located on the legs of horses, Dr. Theoret’s team has published evidence incriminating low oxygen levels resulting from the occlusion (blockage) of the blood vessels within the granulation tissue of wounds on the leg. She explains, “Oxygen is required for the proper function of inflammatory cells, which clear bacteria and debris from wounds. Because wounds on the lower leg are closer to the ground, they are naturally more likely to come into contact with bacteria, dirt and other environmental contaminants, which will trigger more inflammation if they are not promptly cleared away.”
Another reason wounds on the lower legs may heal more slowly and therefore be susceptible to the formation of proud flesh is that skin in this location is devoid of a muscle called “panniculus carnosus,” which exists elsewhere in the body. This is the muscle you see horses use when they twitch their skin to shake off a fly. “In body wounds, the panniculus carnosus is thought to contribute to wound contraction,” says Dr. Theoret. “The absence of this muscle on the leg no doubt hinders the contractile force of the skin surrounding wounds in this location.”
The body also has looser skin than the lower legs do, which enables wound edges to be pulled together more easily. The tighter skin in the lower legs creates more tension around the outside of a wound, thus resisting the little bit of contractile activity provided by the myofibroblasts in the granulation tissue. Wounds located on the lower legs also experience more motion, particularly in areas over joints such as the knee, hock or fetlock, where there’s a lot of movement, and along the cannon bone where tendons run under the skin to extend and flex the foot. Motion tears apart the new granulation tissue and keratinocyte layer, slowing healing and inciting yet more inflammation.
Discouraging Proud Flesh
Although we don’t yet have a sure-proof way to prevent proud flesh, one of the best defenses against it is to be proactive during the early phase of the wound- healing process. Minimize contamination by immediately hosing the wound with plenty of clean, lukewarm tap water. As you run water over the wound, gently rub it with clean fingers to remove dirt and debris. Dr. Theoret advises, “The skin surrounding the wound can be cleaned by scrubbing it with an antiseptic detergent such as povidone-iodine or 4 percent chlorhexidine gluconate, but these detergents must not contact the wound itself.”
If you decide to clip the hair around the wound—for example, if the hair ends are touching it, threatening continued contamination and irritation—smear a water-soluble hydrogel, recommended by your veterinarian or something like Intrasite Gel or K-Y® Jelly, over the wound beforehand. Dr. Theoret explains, “This will prevent any clipped hairs from falling into and sticking to the wound. Afterward, rinse off the gel with tap water. Do not use petroleum-based ointments on the wound; these can slow the healing process.” (If the wound is oozing quite a bit, it’s OK to smear a petroleum product on the area beneath it to protect the skin from being scalded by the discharge.)
If the wound looks more serious than a small cut or abrasion—or if you’re not sure how serious it is—call your veterinarian immediately. This is always the best course of action says Dr. Theoret. “Don’t be tempted to ‘wait and see.’ Even wounds that don’t look like a big concern at the onset may be deeper than you think and may have contaminated structures underlying the skin, such as joints or tendon sheaths. If these are not promptly and correctly treated by your veterinarian, they can cost you a great deal of time and money in the end.”
Unfortunately, closing skin wounds with sutures is not always completely successful in horses. Studies have shown that only about 25 percent of stitched wounds stay closed until they are fully healed. As discouraging as this sounds, even if the stitches hold only a small portion of the wound closed, they will speed healing overall and improve the cosmetic outcome, so in most cases it is worth a try.
During the initial visit, your veterinarian may debride the wound. This means that he or she will trim away any dead, contaminated skin and underlying tissues, using a scalpel, to leave a clean, fresh surface to promote rapid and successful healing.
A good cleaning and debridement are usually more effective than topical medications, says Dr. Theoret. Using topical or oral antimicrobial medications might contribute to the growing antimicrobial resistance that has become a serious concern in both veterinary and human medicine. Also, topical products are not subjected to as stringent Food and Drug Administration testing and approval as medications administered systemically, so first check with your veterinarian before use. Dr. Theoret also suggests, “If your veterinarian thinks that your horse might be at risk of infection, ask him or her about treating the wound with medical-grade honey. It has natural antimicrobial properties that don’t add to the resistance problems caused by antimicrobial drugs. Honey is also very gentle and helps to keep the wound moist, which enables the keratinocytes to migrate faster across the surface of the granulation tissue during epithelialization. Many edible honeys contain bacteria, though, so ask your veterinarian to help you find a honey product that is safe to use on wounds.
“One thing you don’t want to do early in the healing process,” she adds, “is interfere with the horse’s natural inflammatory response. The stronger that response is, the less likely it will become chronic and lead to the development of proud flesh. So avoid administering an anti-inflammatory medication, such as phenylbutazone, unless your veterinarian says it is absolutely necessary for pain relief.”
For your bookshelf: Equine Wound Management
The Pros and Cons of Bandaging
Whether or not horses’ wounds should be bandaged is still somewhat controversial. During the initial stage of healing, bandaging can help to reduce the risk of contamination, particularly when the wound is located close to the ground. Bandages also help to keep wounds moist, which promotes epithelialization and thus speeds healing. However, if discharge accumulates under the bandage and is held against the wound, this may prove irritating to the newly forming tissue and it may create a barrier that prevents much-needed oxygen from reaching the wound cells.
“Traditional nonabsorptive pads tend to contribute to this problem,” says Dr. Theoret, “but newer foam and alginate wound dressings are better at drawing discharge away from the wound.” If your horse’s wound is producing large amounts of discharge, ask your veterinarian to help you find one of the latter products and be sure to change the dressings as frequently as instructed to do so. For wounds that are not producing much discharge but still require protection, a silicone-gel sheet dressing was shown by Dr. Theoret’s team to prevent the development of proud flesh in leg wounds.
Bandaging can also cause even further damage to the skin if you do it on areas with a lot of movement, such as the knee or the heel bulbs. The motion can make the bandage bunch up and cause pressure sores. If you’re not sure if the area is safe to bandage—or if your bandaging experience is limited—consult your veterinarian.
Unbandaged wounds usually develop a scab in the early days, which helps to hold in moisture and protect against contamination. Bandaged wounds, on the other hand, don’t usually develop scabs. The bandage essentially serves the same purposes as a scab.
After the first week or two, the scab on an unbandaged wound can interfere with the wound’s ability to contract. So, if it has not fallen off naturally, it may be advisable to remove it. At this point, enough granulation tissue should have filled in the gap to protect against contamination. For the same reason, if you’ve been bandaging a wound, says Dr. Theoret, “once the wound has filled with healthy granulation tissue, bandaging is no longer required to protect from contamination, though it may speed epithelialization by maintaining a moist surface for keratinocyte migration. If, however, the wound is at risk of developing proud flesh, it may be a good idea to stop bandaging at this point. Studies have shown that horses are more likely to develop exuberant granulation tissue if the wound is bandaged for a long period. So if you decide to continue bandaging, watch the wound closely for signs of proud flesh.”
Once a wound has healed completely without developing proud flesh, you’re still not necessarily out of the woods. The resulting, weaker scar tissue will be more vulnerable to reinjury and, if it reopens, the same factors—contamination, movement and chronic inflammation—can lead to proud flesh. “For this reason,” says Dr. Theoret, “it may be wise to protect scar tissue, using a leg wrap, for example, when your use of the horse—exercise, shipping, etc.—may predispose it to injury.”
Treating Proud Flesh
You’ll know that proud flesh is developing if the granulation tissue begins to protrude beyond the level of the wound edges. Once again, this is not a time to procrastinate. Call your veterinarian immediately.
The best treatment at this stage is to resect—cut away—the excess tissue with a scalpel. There are no nerves in young granulation tissue, so this process is not painful. Your veterinarian should be able to perform it with your horse standing up. If your horse has grown sensitive to having the area around the wound handled, a sedative may be necessary to help him stand still for the procedure.
Don’t be surprised if this procedure produces a lot of blood. Because granulation tissue is so infused with blood vessels, this is normal. The bleeding will stop quickly, usually within 30 minutes. Your veterinarian may advise you to bandage the wound for 24 hours following removal of the proud flesh but to leave it unbandaged thereafter.
Most cases of proud flesh need to be resected more than once. Again, a good debridement and resection are the best way to treat this condition, so be proactive and follow your veterinarian’s advice. Also consult him or her before administering any medications or applying any topical treatments to the wound. Dr. Theoret advises, “Now may be an appropriate time to treat your horse with orally administered anti-inflammatory drugs to reverse the chronic, unnecessary inflammatory response that’s promoting the growth of proud flesh. Some topical products, on the other hand, are so caustic and irritating that their use over time not only slows the healing process but can cause the development of cancerous cells.”
In some cases of proud flesh, a skin graft may be appropriate to encourage healing. After the proud flesh has been resected surgically and inflammation has been eliminated from the wound bed, the veterinarian takes small pieces of healthy skin from the horse and implants them into holes created in the healthy granulation tissue of the wound. The skin donor sites are in unobtrusive locations, such as under the mane or trail, so the resulting small scars are hardly noticeable. The procedure is done under sedation with local anesthesia with the horse standing. It is time-consuming, but requires no costly or specialized equipment. “Although, to some horse owners, it may seem like an expensive treatment,” says Dr. Theoret, “very often the results are good and since skin grafts substantially speed healing, the owner can save time and money, for example, on bandage materials.”
If your horse does develop proud flesh, don’t despair. With proactive effort on your part, the wound will heal eventually. In the meantime, scientists will continue to study this challenging skin disorder and work toward developing even more-advanced treatments.
Studies have found that ponies are far less predisposed to developing proud flesh than are horses, says Dr. Christine Theoret, editor of a 700-page textbook, Equine Wound Management, the third edition of which will be published by Wiley in 2016. “They also have a better success rate when their wounds are stitched than do horses. We don’t know exactly why. Some have theorized that ponies have better blood flow to their extremities because of their smaller size, while others have found that ponies have a superior acute inflammatory response to wounding. The best we can guess at present is that ponies have some sort of genetic predisposition for better wound healing.”
This article originally appeared in the March 2016 issue of Practical Horseman.