How Horse Wounds Heal

Whether large or small, serious or innocuous, all wounds follow a distinct and complex healing process. During the 2013 Western Veterinary Conference, held Feb. 17-21 in Las Vegas, Nev., one veterinarian reviewed how wounds heal and how owners can help facilitate healing.

"(Wounds are) a fascinating topic; you never know what you're going to come across," said Bimbo Welker, DVM, MS, a clinical associate professor in the Ohio State University (OSU) College of Veterinary Medicine Department of Veterinary Preventive Medicine and a practitioner at OSU's Large Animal Services, in Marysville, Ohio.

Welker first reviewed some basic wound management steps. Although there's been "a tremendous amount of research on wound healing, we still can't speed wound healing up," he explained. We can, however, ensure wounds have an optimum environment in which to heal.

He also reminded veterinarians that skin is a complex organ that can't regenerate. Instead, wound defects are replaced with fibrous tissue covered by surface epithelium, which reestablishes continuity, he explained.

"Healing always progresses in the same way," he said. "Wounds heal in stages, and each is dependent on the stage before it."

Welker then discussed the stages of wound healing, noting that while each of these stages must occur, many overlap and take place at the same time.

The Inflammatory Phase

The horse's body begins reacting as soon as an injury occurs with the inflammatory phase, Welker said. The skin around the wound begins retracting due to tension; immobilizing the wound can help reduce this effect, he said. Skin retraction can continue for up to 15 days post-injury, he said.

Within five to 10 minutes after the horse sustains a wound, Welker said, a vessel response occurs. During this response, "intense vasoconstriction" (narrowing of the blood vessels) occurs at the wound site, followed by vasodilation. It's during this response that fibrin—an insoluble protein that forms the nucleus of a blood clot—arrives at the wound site.

Within 30 minutes of injury, the body's cellular response kicks in, Welker said. Blood platelets and leukocytes (white blood cells) "line up" at the wound site to begin cleaning it. These cells are required for healing, he said, and their presence activates the fibrin, allowing clotting to begin.

And finally, within an hour of injury, the localization response takes place. At this point, a fibrin clot has localized damage to just the affected area; the clot also prevents contaminants from getting into the horse's bloodstream or surrounding undamaged tissues, Welker said, and forms the framework needed to repair the defect.

Unfortunately, the localization response comes with a downside, Welker said. Because the contaminants have been localized to one central area, inflammation (including swelling, redness, heat, and pain) develops. Excessive inflammation delays healing, Welker said, and can lead to pressure necrosis, pain, scarring, and bacteria development.

The Debridement Phase

The next phase is debridement, which is critical for all wounds and injury healing, Welker said: "Healing cannot proceed without the completion of this stage," he said.

The debridement phase takes place when neutrophils (a type of white blood cell capable of engulfing and destroying bacteria and other disease agents, immune complexes, and cell debris) enter the wound defect and kill bacteria, break down debris, and enhance the inflammatory response; unfortunately, when too many neutrophils enter the wound, the healing process slows, Welker said. At that point, pus develops, which further slows the healing process by breaking down fibrin working to fill the defect. To prevent excessive neutrophils from inhibiting healing, he said, keep the wound clean and administer antibiotics.

Some inflammation is good, Welker added, but excessive inflammation slows healing.

Epithelialization—which Welker described as the first sign of defect repair—begins between eight and 10 hours after a wound occurs. During this stage epithelial cells "migrate" under the scab at a rate of 0.2 millimeters per day on the horse's upper body and 0.09 millimeters per day on the animal's limbs or lower body, Welker said. Factors that inhibit or delay epithelialization, he said, include infection, excessive granulation tissue (commonly known as proud flesh), repeated bandage changes, extreme hypothermia, and dessication (the wound drying out).

The Repair Phase

By the fourth or fifth day after a wound occur, fibroblasts (cells responsible for forming connective tissues) move into the area and begin tying the wound edges together to fill the defect. The fibroblasts will continue moving over the defect until they contact other fibroblasts. Welker explained that fibroblasts produce a substance that enhances the fibrin matrix before producing collagen, which essentially serves as a glue holding the layers of skin (or in this case, new epithelial tissue) together.

In the third to sixth day post-injury, Welker said, granulation tissue begins to form and subsequently allows wound contraction to occur (more on that in a moment). Welker said granulation tissue is an important part of wound healing: It provides a surface for the epithelial cells to migrate over, it's resistant to infection, wound contraction centers around it, and it carries the fibroblasts responsible for collagen formation. But granulation tissue can cause problems in some cases.

"Horses are overachievers and can keep producing excessive granulation tissue," Welker said. "This is when it becomes a problem."

He said he considers granulation tissue that rises above the skin level to be excessive.

Next, Welker discussed wound contraction, the process by which open skin wounds reduce in size due to the movement of surrounding full-thickness skin. Welker said special cells on the surface of the granulation tissue bed--modified fibroblasts called myofibroblasts--draw the full-thickness skin toward the center of the wound.

"Wound contraction works best in areas where the skin is relatively loose (upper body), but where skin is relatively tight (lower limb) contraction is much less efficient and will result in wider scars," Welker said.

The Maturation Stage

The final stage in wound healing is maturation, Welker said, and it can last for months to a year or more, depending on wound severity. In this phase, the number of fibroblasts in the area decreases while collagen production and lysis (decomposition) continue, he said. Also in this phase, the wound's tensile strength increases. Welker cautioned that once a wound heals fully, the defect's tensile strength will always be 15-20% weaker than the surrounding areas.

Keys to Consider

As Welker mentioned, we can't speed wound healing, but we can provide an environment to help facilitate it. Some factors that negatively impact healing include age, disease status, malnutrition, and multiple trauma sites. However, there are options veterinarians and owners can employ to help facilitate healing including non-steroidal anti-inflammatory drug administration, steroid administration, wound lavage, wound debridement, and bandaging.

Welker stressed that a veterinarian should examine any full-thickness wound and treat each wound individually.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.