By Ruth Ann Mosby

Wound size and duration are powerful influences on the amount of scarring that occurs.

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."