All horses wear a history of their mishaps and calamities on their hides, though most repair work is hardly visible after a few months while a few injury sites remain glaringly obvious for life. Whether healing results in unblemished skin or an unsightly and disabling scar depends upon the wound type, its location on the body, any complicating factors during healing and the wounded individual's repair capabilities.
Only a never-injured horse is an unscarred horse, for the inalterable reality of mammalian life is that repaired tissue is inferior to the original, even when there's hardly a ripple in the hair overlying the healed spot.
"Scarring is an inevitable consequence of injury," says Derek Knottenbelt, MRCVS, head of equine studies at England's University of Liverpool and author of Handbook of Equine Wound Management. "It's a mechanism for limiting the amount of damage. Some reptiles and insects can 'remake' themselves [when injured], but as we've evolved, we've lost the ability to regenerate tissue. Even when you nick yourself and it eventually looks normal, there will always be a scar." Despite the inevitability of scarring, good nursing and appropriate veterinary care can encourage the least disfiguring outcome for wounds of all types.
Wounds under Repair
All wounds, whether inflicted by a protruding nail, battering horseshoe or surgical scalpel, go through the same stages of repair to return the tissues to wholeness. When the healing process is compromised and wound closure is hindered during one or more of these phases, scar tissue tends to proliferate. Thus, the best scar-minimizing strategy for horses is early and appropriate intervention that encourages rather than impedes the healing process.
Phase 1: Immediately after a horse is wounded, the blood vessels at the site constrict to taper blood flow and prevent hemorrhaging. Fibrin, a fibrous protein activated in the blood, forms a clot in and over each pinched vessel to help contain the damage and protect exposed tissue. Later, the fibrin will serve as the primary scaffolding for the regenerating tissue. On contact with air, the clot dries, forming a natural bandage, otherwise called a scab.
The blood vessels surrounding the injury have also swung into action, delivering inflammatory cells, called macrophages and neutrophils, that begin the cleanup effort. These cells migrate in from the edges of the wound to engulf and "digest" bacteria, forming pus and lifting out small foreign bodies, loose hair and dead tissue contaminating the wound.
Phase 2: Inflammation is simultaneously signaled by the first-response cells on the scene and typically lasts about six hours, barring complication such as infection. The effect is increased blood flow surrounding the wound to speed delivery of the materials necessary for the cleanup and repair activities. This localized flood of blood is responsible for inflammation's "cardinal signs": heat, swelling, redness and pain.
The common practice of using cold therapy or anti-inflammatory drugs to suppress the inflammatory reaction during wound treatment seems to fly in the face of all that's natural and necessary to healing, yet uncontrolled inflammation can become too much of a good thing.
"All wounds become inflamed, and the response corresponds to the degree of trauma," says Ted Stashak, DVM, professor of large-animal surgery at Colorado State University's College of Veterinary Medicine and author of Equine Wound Management. "We usually try to reduce the impact of inflammation because as long as the wound is inflamed, it delays the progress to the next phase."
Debridement is the removal of contaminants-continues in conjunction with inflammation. "White blood cells are brought in to engulf the bacteria and clean up dead tissue and foreign bodies," says Stashak. "This is the stage that [veterinarians] have the most effect on. We can lavage [wash] the wound and use a scalpel or scissors to remove dead tissue, allowing the wound to accelerate to the repair phase."
Phase 3: As decontamination proceeds, fibroblasts, cells that produce a raw, reparative matter, migrate into the area. Fibroblasts generate the mix of collagen fibers and ground substance, called granulation tissue, used to fill the wound site. "Fibroblasts put down an immature fiber called tropocollagen," says Stashak. "When it matures, it provides tensile strength."
During the granulation period, fibroblasts are converted into myofibroblasts that are capable of contraction and have the job of pulling the wound edges together to minimize the wound size. Granulation tissue also provides a surface for the next legion of cells-the initial skin cover-to enter the repair zone.
These epithelial cells are especially designed to migrate across the granulation bed filling a cavity or wound. "They 'walk' in over the surface of the wound, but they are very, very slow," says Knottenbelt. "They [move] two millimeters [one-twelfth of an inch] every 10 days. That's why we sometimes suture a wound: to minimize the distance the epithelial cells have to go. Between natural contraction and the suturing, the cells have to travel only one millimeter, meaning that most surgical wounds heal in about seven days."
Though they do not include normal skin structures, such as hair follicles and sweat glands, epithelial cells are relatively protective of the tissue under repair. This initial skin covering is strengthened by keratin, a horny protein found in hair and nails, and made thicker than normal skin by additional collagen and multiple layers. It is in this stage of healing that a scar is born.
Phase 4: Wound contraction may go on for weeks, and collagen maturation may continue for several months, until the scar tissue consists of dense fibrous tissue that is more or less arranged to withstand the lines of stress experienced by that area of skin. "The scar matures, and only the purposeful fibers are retained," says Stashak. "Less purposeful fibers dissolve."
Even after the horse's wound appears healed, the skin has a way to go before reaching its final form. "Myofibroblasts convert into fibrocytes, which are very small, dense, white cells that don't look like flesh," says Knottenbelt. "The blood supply is cut down so they take on a whiter appearance. Fibrocytes keep getting smaller and smaller so the scar tissue keeps remodeling and getting smaller as long as the animal lives." Though scar tissue strengthens over time, it regains only about 80 percent of the strength of undamaged tissue because it lacks the cross-links found in normal collagen.
Healed wounds may also leave permanent changes in the haircoat because of irreversible damage to production cells in the follicles. When dermal tissues lose their pigmenting capabilities, new hair growth is white. "With a freeze brand, for example, you're not damaging the ability to produce hair but damaging the cells that impart color to hair," says Knottenbelt. "Those cells are sensitive and easily damaged."
When scars remain hairless, the dermis was deeply wounded, but not all multiple-thickness wounds are doomed to baldness. "Hair follicles occur at different depths," says Steve Adair, DVM, associate professor of equine surgery at the University of Tennessee. "Sometimes the wound is not full thickness, and follicles in the deeper tissue are not damaged. When it heals the hair grows, just not as thick as before."
In general, wound size and duration are powerful influences on the amount of scarring that follows injury: The larger the wound and the longer the healing time, the greater the amount of scarring.
Wounds that are sutured and heal by what the medical world calls first intention produce smaller scars than those that heal without being stitched closed. Yet there's still a scar at the microscopic level. "If you take a cross-section of a sutured wound where the incision was made and put it under a microscope," says Adair, "you will see a difference in cell type."
The so-called second-intention healing of unsutured wounds requires a larger gap to be filled in with a granulation-tissue base before skin regrowth (epithelization) can cover the gap in the flesh. "Second-intention healing relies upon the inflammatory response," explains Knottenbelt. "The longer the wound takes to heal, the greater will be the scarring and the possibility of cosmetic and functional deficits."
But not all wounds can be sutured, either because the surrounding skin won't cover the injured area or because infection/excess fluid has to be eliminated before the wound can be closed. Some more complicated injuries may be candidates for delayed first-intention healing, in which suturing is done after the infection/inflammation is cleared up and surgical debridement "tidies" the site. Otherwise, the natural second-intention repair will have to proceed at its own slow, scarring pace.
Another major factor in scar formation is the wound's location. Torso injuries heal far more acceptably, both functionally and cosmetically, than those on the lower limbs. A deep, gaping wound on the neck may heal with only the faintest blemish, while a three-inch-long scrape over the cannon bone can leave an unsightly, hairless scar. "A general rule of thumb is that any wound that is larger than one-third the circumference of the distal limb will have difficulty healing on its own," says Adair. "It may need help to epithelize." The lower legs are, in fact, sitting ducks for all the impediments to quick, scar-free healing:
Blood supply: The horse's upper body is richly supplied with blood, carrying the full complement of cleansing and healing cells to the wounded tissues, while the lower leg receives a restricted supply.
Soft tissue: The upper-body fat and muscle masses allow some stretch and flexibility to the overlying skin, providing the "play" needed to bring the wound edges together for effective first-intention or prompt second-intention healing. With no muscle or fat below the knees and hocks, "there's more tension in the distal limbs, so there's not enough skin to stretch or pull together," says Anne Schwartz, DVM, a private practitioner in Ocala, Fla., who has researched wound management. You probably have visible proof of this principle on your own legs, where visible scars are much more likely to appear along your bony shin than on your muscular calf.
Movement: Repairing tissue keeps suffering setbacks when it's subject to movement, which happens a lot with wounds on or near lower-leg joints. Movement makes the biggest mess of healing when the wound is oriented so each step pulls the edges apart; the repair work is always being undone. "Constant motion tends to break the granulation bed," says Adair. "That's why high-motion areas tend to develop large scars."
Contamination: Contaminants are the enemy of all healing. The repair process stalls if debris, such as a splinter of wood, a metal shard or a bone fragment, remains imbedded in a wound. Bacteria discourage collagen development, and infection increases the inflammatory response, which retards granulation and epithelization. Because they're closer to dirt, debris and manure, wounds below the fetlocks are more likely to suffer the complications of contamination.
Excessive granulation: When wound contraction or epithelization occurs at a slower rate than the production of new tissue filling the wound, the granulation phase can run amok. Also called "proud flesh," this exuberant granulation typically occurs on the lower legs where the skin is more resistant to contraction.
When granulation tissue rises above the surrounding skin level and takes on a "cauliflower" appearance, healing comes to a halt and the wound actually increases in size. "Those little epithelial cells have to migrate," says Schwartz, "and they can't migrate over a huge mountain." If detected early, exuberant granulation tissue can be treated topically with a corticosteroid0-antibiotic mixture, but surgical removal may be the most effective treatment for protracted cases.
Wound healing is far from predictable, despite the size/duration/location rules of thumb. "A teacup-size wound may leave a scar the size of your little fingernail," says Knottenbelt. "You can't always tell at the time [of injury]." Systemic conditions, such as anemia, vitamin deficiencies and malnutrition, can impair healing, and, like people, some horses simply heal better and scar less than others.
Wound treatment and follow-up care have a significant effect on the degree of scarring that accompanies healing. Whether your horse suffers a minor scrape or extensive injury, he'll heal better if you heed the following guidelines:
When in doubt, consult your veterinarian. Not sure if a cut needs stitches or can heal properly on its own? Play it safe and contact your veterinarian early on. In most cases, suturing is best done within six hours of injury. Even if stitches are not necessary, your veterinarian may want to treat the wound to prevent infection.
Keep the wound clean and moist. Flushing the wound with plain water or saline solution is a safe way to clear dirt and debris from an open wound. Moisture also encourages granulation, epithelization and wound contraction.
Wounds that are left uncovered may benefit from daily cold hosing to prevent contamination, but avoid vigorous spraying of the site. High-pressure hosing may drive contaminants deeper into a fresh wound, and later it can harm the new tissues. "Water helps stimulate granulation tissue, and when you want it to appear, that's good," says Adair, "but epithelium is fairly fragile. If you pound it with water from a hose, you'll actually hinder it."
When possible, keep the wound covered. It's common sense that a bandaged wound remains cleaner, moister and better stabilized than an exposed wound, thus encouraging speedier healing.
Some studies show that wrapped injuries heal 30 percent faster than unbandaged wounds. But bandaging isn't ideal at every healing stage. Other studies suggest an association between bandaging and the development of proud flesh. "There's no clear answer to the question of bandaging," says Adair. "It depends on the timing."
In addition, bandaging effectively and safely demands a degree of practiced skill. A too-tight wrap can cause pressure sores, while a sloppily applied bandage can work loose. If you choose to bandage, use a nonstick pad against the wound as a barrier to the gauze, cotton or other absorbent materials that dry the tissues and can become embedded in the wound.
Restrict movement. Healing of lower-leg wounds may require limiting the recovering horse's activities or applying a bandage or cast to give epithelization a boost. Wounds elsewhere on the body may also need protection from rubbing, biting or other irritation that will disturb the fragile tissues.
Use topical treatments judiciously. There's been great debate over the benefits of topical agents in wound care. Some veterinarians support the use of certain topicals that may accelerate epithelization, while others reject "gunking up" a wound with ointments that disrupt natural healing and attract contaminants.
"The healing process can be viewed as an orchestra," says Knottenbelt. "Every instrument has a job to do. If you take one out, the orchestra is still recognizable, but it won't be as good as it can be. Anything you put on the wound can be harmful to the tissues."
Petroleum-based products, such as Vaseline, are generally cautioned against because they tend to promote proud flesh. Caustics, such as copper sulfate, are murder on granulation tissue and have no place in wound healing; even as supposed treatments for proud flesh, they do more harm than good.
Accept blemishes; revise significant scars. Even when you've done everything possible to facilitate healing, your horse may still end up with an ugly, hairless scar. In rare cases, scarring does affect function, as with facial scars that interfere with breathing or eyelid action. Surgery can be performed to remove or rearrange the scar tissue and rejoin the new wound edges so there is a likelihood of a more elastic and less disruptive union.
Revision can also improve the appearance of an ugly scar that has affected a valuable horse's show career or salability. But the results are not guaranteed. "The consequences may be even worse than the original scar," Knottenbelt cautions.
Skin from another part of the body is sometimes grafted to the scar-revision site to improve the final appearance, but, again, the results are variable. Skin grafts don't "take" to a fresh wound site as successfully as to granulating tissue in a healing wound. "In general, the thicker the graft, the less likely it is to survive, but the better the cosmetic effect will be if it takes," says Knottenbelt. "A grafted area will always be more fragile."
This article originally appeared in the April 2003 issue of EQUUS magazine.