Factors that Delay Healing

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Factors that disturb normal corrective processes inevitably complicate wound healing. Early recognition of healing difficulties allows prompt correction. Delayed healing inevitably results in development of chronic inflammation, and although transition through the chronic inflammatory stage is almost inevitable in naturally occurring wounds, it is the most undesirable event in the healing cascade.

Prolonged chronic inflammation causes progressive production of exuberant granulation tissue, or alternatively a reduction in the production of granulation tissue; in either case, an inhibited epithelial cell replication results.

The longer a wound takes to heal the larger will be the scar and the longer will be the recovery period. The more extensive the scar the greater may be the limitations to function. Most non-healing wounds are preventable by suitable management in the early stages after injury, and others are understandable or predictable. Failure to recognize potential reasons for failure of healing means that the wound will become chronically inflamed and so the healing process will be unnecessarily prolonged. Healing failure mediated through chronic inflammation can be instigated by several factors described below.

Infection/Infestation

Infected wounds heal slower than uninfected ones. Mixed infections are relatively common (Figure 27), and tissue bacteria numbers above 1 × 10⁶ organisms delays healing⁶. Bacterial species that produce collagenase or other destructive enzymes have a profound effect on healing (Figure 28).

Figure 27 An infected granulating wound on the distal cannon. A mixed growth of bacteria was cultured.

Figure 28 A severely infected non-healing wound from which a pure growth of Pseudomonas aeruginosa was cultured.

Infection with Staphylococcus aureus can cause pyogranuloma within the wound site. Clinically this resembles both granulation tissue and sarcoid, but histologically diffuse microabscessation is present (Figure 29).

Figure 29 This wound failed to heal because of staphylococcal microabscessation (pseudomycetoma/botriomycosis).

Fungal infections of superficial wounds is relatively common. For example, Pythius spp., or Basidiobolus haptosporus infection (deep or superficial mycosis or hyphomycosis) can be catastrophic complications of relatively trivial wounds. Parasitic infestation, e.g. with Habronema musca or the larvae of certain flies (myiasis), also retards healing (Figure 30). The larvae of Lucilla sericata has been found to have a beneficial debriding effect in some wounds under controlled conditions.

Figure 30 Habronema musca infestation of wound on the ventral abdomen, illustrating the role of parasitic infestation in inhibition of wound healing. (Courtesy of J Marais.)

Movement at the site or in the attached tissues delays healing (Figure 31). Excessive mobility disrupts capillary buds and increases collagen deposition, directing the healing process towards chronic inflammatory status. Anatomical knowledge may establish the likelihood of deep tissues that are moving significantly relative to the wound itself. Wounds on the body may fail to heal because of movement of the underlying muscle, but this is less significant in horses.