Horse versus pony

To optimize wound healing in equids, the clinician should recognize that the horse’s response to injury is different to that seen in other species and even to that seen in ponies. The initial inflammatory response of horses to trauma is sluggish and less intense, which, in turn, delays the development of a healthy bed of granulation tissue, resulting in slower and sometimes problematic healing. ^1–5 This type of response is more commonly seen in wounds of the distal aspect of the limb (i.e., up to and including the carpus and tarsus) of horses compared to wounds of the body. One outcome is that wounds of horses, particularly those on the distal aspect of the limb, are more subject to infection, formation of exuberant granulation tissue (EGT), and delayed healing. This observation correlates with clinical studies that report that ponies are less susceptible to wound dehiscence, less likely to form a bony sequestrum, less likely to develop EGT, and less likely to suffer from delayed wound healing compared to horses. ¹ For more information on the differences in healing between horses and ponies, the reader is referred to Chapter 2.

Age of the patient

Advanced age of the patient is known to influence healing in humans, and this has also been documented in experimental animal models. ⁶ In laboratory animal models, old animals suffer from delayed formation of granulation tissue with reduced tensile strength of the wound early in the healing process, as well as delayed epithelialization and delayed wound closure. ⁷ Tissue ischemia has been implicated as the cause of these delays, but studies investigating the influence of age on wound healing vary widely, and the anatomic, physiologic, and biologic processes differ substantially between species, making translation of results from one species to another inaccurate. ⁷ The wound‐healing process in healthy, elderly humans appears to resemble that of younger patients, in terms of reaching a similar endpoint, and is usually protracted rather than impaired. ^6,8 In aged human skin, the function of macrophages is impaired, proliferation of keratinocytes is decreased, and the dermis is atrophied, all of which when combined, likely lead to a reduced ability of skin wounds of aged patients to progress promptly through all phases of healing. ⁷ Clinical features apparent during wound healing in human patients over 60 years old include a sluggish inflammatory reaction, a less effective immune response, and a reduction in the capacity of cellular replication, leading to delayed angiogenesis, collagen synthesis, and epithelialization. ^8,9

The effect of advanced age on healing has not been investigated in horses. The impact of aging on wound healing in animals is likely to be less important than in humans because of the relatively lower proportion of geriatric patients in veterinary practice. But, as the standard of veterinary care improves and the emotional value of pets increases, domestic animals are expected to live longer, such that an effect of advanced age on healing may become apparent. The appearance of age‐onset diseases probably bears a greater impact on wound healing in animals than does advanced age itself. ^7,10 Because equids seem to be more resistant to age‐related diseases than do smaller companion animals, advanced age is unlikely to be a significant detriment to wound healing in horses.

Nutritional status and disease

Suitable nutrition is required to sustain cellular repair during wound healing because nutrients are essential to the many biologic processes occurring in the skin. Suitable nutrition is particularly important when cellular division, movement, and differentiation are up‐regulated in response to injury. ¹¹ Consequently, in severely malnourished patients, the progression of healing is impeded, and the scar tissue that eventually fills the wound has a decreased tensile strength and is, therefore, susceptible to reinjury. ^11,12 The metabolic status (positive or negative) at the time of injury is also important because the energy requirements of healing represent an additional burden to the patient. The new demands imposed by healing cannot be met when the patient is in a negative metabolic state.

Macronutrients (i.e., proteins, carbohydrates, and fats) and micronutrients (e.g., vitamins A, B complex, C, E, and K, and minerals, such as copper, iron, and zinc) are known to play important roles in wound healing. ¹¹ Proteins provide major building blocks for cellular renewal and tissue growth after injury, and adequate protein is needed to maintain a positive nitrogen balance. Protein deficiency may affect hemostasis, inflammation, immunity, formation of granulation tissue, remodeling, and epthelialization. ^11,12 Deficiencies of the amino acids cysteine, proline, arginine, tyrosine, and histidine have been shown to adversely affect wound healing by negatively affecting angiogenesis, formation of collagen, and wound remodeling. ^11–13 Carbohydrates are the principal source of energy that sustains the high metabolic demands of tissue regeneration. ¹¹ Fats provide energy and contribute to inflammation and synthesis of cell membranes and intracellular matrix. ¹¹

Various human and animal studies have incriminated a vitamin or mineral deficiency in situations of impaired healing. ^11,14 A deficiency of vitamin A is associated with impaired synthesis and stability of collagen, impaired contraction and epithelialization of wounds, and an increase in the patient’s susceptibility to infection. ^11,15 Deficiency of vitamin B complex is associated with several skin disorders and anemia, each of which may affect wound healing. ¹¹ Deficiency of vitamin C has been implicated in impaired synthesis of collagen and scarring, as well as reduced responsiveness and function of the immune system. ¹¹ Vitamin E is important for stabilization of the cell membrane, and vitamin K plays a key role in hemostasis. ¹¹ Iron is essential for production of red blood cells and synthesis of collagen. ¹⁶ Copper deficiency is associated with formation of defective collagen and elastic tissue, leading to reduced tensile strength of the scar. ¹⁷ Zinc is a component of key enzyme systems needed for cellular replication and synthesis of protein (e.g., the metalloproteinases that play an important role in the proteolytic remodeling of extracellular matrix during tissue repair). ¹⁸ However, a recent study found that wounds of rats treated topically with zinc gluconate healed similarly and contained equivalent bacterial loads as wounds treated with the carrier solution (isotonic saline solution or chlorhexidine) alone. ¹⁹

Concurrent disease may affect wound healing. For example, Cushing’s disease (pars intermedia dysfunction), which affects mainly older horses, is characterized by high serum concentrations of endogenous cortisol that might suppress inflammation to such an extent that healing could be impaired. ²⁰ High serum concentrations of glucocorticoids may also be found in horses in response to stress. Protracted periods of hyperglucocorticoidemia may decrease the expression of pro‐inflammatory cytokines, including interleukin‐1, interleukin‐6, and tumor necrosis factor‐α, and some chemoattractants required for the inflammatory phase of wound healing and for supporting the migration of immune cells. ^21,22 Whereas Cushing’s disease and consistently elevated production of glucocorticoids are associated with an increased risk of infection and impaired healing of wounds of humans, their clinical impact on healing of wounds of horses is less clear. More importantly, the stress of a severe injury may decrease appetite, thereby leading to an intake of energy and nutrients insufficient to meet the demands of healing tissues.

Tissue perfusion/oxygen tension

Effective wound healing requires adequate circulation, itself dependent on sufficient hydration, to ensure optimal tissue perfusion and oxygenation. Oxygen is essential to several critical mechanisms underlying wound healing, including bacterial killing, collagen synthesis, and epithelialization. Oxygen plays a pivotal role in eliminating anaerobic bacteria and in reducing formation of biofilm and colonization by microorganisms resistant to antimicrobial drugs. The guideline of debriding a wound back to healthy bleeding tissue is based on the principle that healing progresses more quickly and efficiently under conditions where the granulation bed is well perfused and free from avascular and necrotic debris. ²³

Because most of the oxygen in blood is carried by hemoglobin, anemia might be expected to impair healing. Normovolemic anemia, with a hematocrit above 20%, however, in the presence of adequate blood flow and tissue perfusion, does not seem to be detrimental to wound healing. ^24,25 Conversely, hypovolemia can compromise tissue perfusion, thereby negatively impacting healing, mostly by a decrease in activity of leukocytes and production of collagen. ^26,27

Wounding disrupts the local blood vessels, leading to acute hypoxia in the wounded tissues. Hypoxia favors angiogenesis, and the newly formed, temporary capillaries carry much needed oxygen into the wound’s microenvironment. Oxygen tension of tissue near capillaries at a wound’s margin is between 60 and 90  mmHg compared to oxygen tensions of 30–50  mmHg in normal, uninjured tissue. Cells consume oxygen thereby creating an oxygen gradient between the edge of the wound and the center of the wound.

In humans, the concentration of oxygen in tissue is closely correlated to the progression of wound healing. ²⁵ Chronic, non‐healing wounds are hypoxic relative to normally healing acute wounds, with tissue oxygen tensions ranging from 5–20  mmHg. ²⁸ Studies in horses have shown that perfusion of chronic wounds of the limb is significantly inferior to perfusion of non‐chronic body wounds. ^29,30 Tissue perfusion is even worse in limb wounds that go on to form EGT. ^29,30 Metabolic disturbances suggestive of a deficient oxygen supply have been identified in equine limb wounds that have developed EGT, ³⁰ and a relative state of hypoxia during the inflammatory phase of repair of limb wounds, possibly due to occlusion of microvessels, secondary to endothelial hypertrophy, has also been identified. ^31,32

New blood vessels forming in an hypoxic environment are immature and bleed easily, ³³ which may explain the friable and hemorrhagic nature of equine EGT observed clinically. Because the phagocytic ability of leukocytes is oxygen dependent, ³⁴ the presence of hypoxia may explain the inefficient inflammatory response documented in limb wounds of horses thought to contribute to the development of EGT. ^4,5 Furthermore, low oxygen concentrations encourage angiogenesis, the growth of fibroblasts, and the synthesis by fibroblasts of components of the extracellular matrix, such as collagen, another mechanism whereby the hypoxia present in limb wounds might favor the development of unhealthy EGT.

Causes and types of wounds

Accidental wound

Accidental wounds may be categorized as closed (e.g., bruise, hematoma, contusion) or open (e.g., abrasion, puncture, laceration, avulsion, degloving, burn), the latter constituting the majority of accidental wounds encountered in equine practice. The greater the inciting trauma, the more severe is the soft‐tissue damage and the greater is the risk of subsequent infection. For example, sharp lacerations caused by metal or glass are unlikely to become infected because they are rarely associated with contamination, necrosis, or alteration of the skin’s vascular supply.

Conversely, crush injuries and injuries characterized by avulsion and tearing of tissue are at greatest risk for infection because of extensive thrombosis of associated vessels and subsequent loss of blood supply. These types of wounds are almost 100‐fold more likely to become infected than those caused by shearing forces. ⁴⁰ The blood supply to the distal extremity of a horse’s limb is commonly compromised, for instance, when wire encircles the limb, thereby creating a tourniquet‐like effect. Wounds caused by entrapment of the limb between fixed objects (e.g., a cattle guard or a stall door and wall) or by impact from collision with a solid object or from a kick, are especially susceptible to infection because of the magnitude of soft‐tissue injury (Figure 3.1).

Puncture wounds are susceptible to infection because the penetrating tract often closes, trapping bacteria deep within the tissues. Deep puncture wounds, where the point of perforation is small, may not allow sufficient drainage of contaminated fluids. Accumulation of these fluids within the wound provides an ideal environment for bacterial multiplication, particularly of anaerobic microorganisms. Opening the tract to ensure drainage, irrigating the tract, and packing it with saline‐soaked sterile gauze, reduce the risk of infection becoming established.

Age of the wound (time elapsed since injury)

A wound, either surgical or accidental, is a disruption in the integrity of the skin, which renders underlying tissues susceptible to contamination. A properly managed, clean or minimally contaminated wound should progress through the normal phases of repair. The probability that healing becomes delayed is related, at least in part, to the degree of contamination, the extent of tissue trauma, and the host’s response. These factors are particularly important in accidental wounds of horses, because accidental wounds are often contaminated with foreign material and large numbers of various microorganisms.

Historically, an accidental wound, if prepared properly, was considered suitable for primary closure, with little risk of infection, if it was less than 6–8 hours old. This time, referred to as the “golden period,” was based on research conducted in laboratory animals and was related to the time required for multiplying bacteria in a closed surgical wound to reach an infective concentration, considered to be more than 10 ⁵ organisms per gram of tissue or milliliter of exudate. ⁴¹ This concept of a golden period is largely outdated. ⁴² Although an open wound often can tolerate a greater bioburden (e.g., 10 ⁶ bacteria) without showing signs of deterioration, ⁴³ the wound’s outcome depends predominantly on the adequacy of the host’s immune response, the virulence of contaminating bacteria, and local environmental factors (e.g., the presence of devitalized tissue or foreign material) that can potentiate the virulence of bacteria. ⁴⁴

Although the duration since injury may be given some consideration, the approach to wound management should be selected in light of other factors, such as the degree and type of contamination, the location and type of wound, including the extent of the trauma to the blood supply and to other nearby or underlying structures, the management of the wound prior to presentation, and the patient’s overall physical condition and immune status. Many veterinarians become very astute at visually assessing wounds and at determining if the wound is suitable for primary closure or whether a different approach to wound management is more appropriate. The reader is referred to Chapters 4 and 8 for more information regarding the selection of appropriate methods to manage wounds.

Location of the wound

Specific healing limitations characterize different anatomic regions. For example, wounds on the distal aspect of the limb of horses, left to heal by second intention, expand during the first 2 weeks after trauma, in contrast to those on the body, which change little in size. ⁴⁵ The increase in size of wounds on the distal aspect of the limb can be substantial, with the surface area of the wound almost doubling over a 2‐week period. This time of expansion is referred to as the “lag phase” of healing. After a healthy bed of granulation tissue has formed, the wound starts to contract. The percentage decrease in the surface area of the wound attributable to contraction is greatest between the second and fourth weeks of healing. The capacity of body wounds to heal through contraction far exceeds that of wounds on the distal aspect of the limb. In a well‐designed study, the rates of contraction and epithelialization of experimental, full‐thickness wounds (7–9  cm ² ) on the body and limbs of horses and ponies, left to heal by second intention, were measured. The percentage decrease in wound surface area attributable to wound contraction between the second and the fourth weeks of healing was 47% for the body wounds of ponies vs. 38% for the body wounds of horses and 35% for the limb wounds of ponies vs. 0% for the limb wounds of horses. After week 4, the rate of wound contraction slowed to less than 5% per week for the wounds of ponies and the body wounds of horses, up to complete healing. The metatarsal wounds of horses showed a different pattern: the lag phase of healing lasted 4 weeks and this was followed by an average rate of contraction that did not exceed 2.5% per week. ⁴⁶ These differences in rates of contraction between wounds on the body and those on the limb have been attributed to the poorly oriented fibroblasts and myofibroblasts within the immature granulation tissue of limb wounds of horses. ⁴⁷ In these same experimental limb and body wounds, epithelialization progressed over the wound’s surface at a rate of 0.75  mm/week between weeks 3 and 7 of healing for body wounds of ponies, 0.62  mm/week for body wounds of horses, 0.63  mm/week for limb wounds of ponies, and 0.48  mm/week for limb wounds of horses. ⁴⁶

Moreover, wounds to the distal aspect of the limb of horses are often heavily contaminated with foreign material, including feces. Whereas short exposure to feces has been shown to improve healing of experimental wounds on the distal aspect of the limb of horses, perhaps by stimulating the acute inflammatory response, ⁴⁸ wounds contaminated by feces, which may contain up to 1  ×  10 ¹¹ organisms/gram, are likely to become infected if not treated properly. For more information regarding the management of wounds on the distal aspect of the limb, the reader is referred to Chapters 8 and 13.

Finally, wounds on the distal aspect of the limb of horses are more susceptible to the formation of EGT than are similar wounds located elsewhere on the body. ⁴⁹ The exact location of the wound on the limb further influences healing and formation of EGT; wounds over the dorsal surface of the metacarpo/metatarsophalangeal joint heal more slowly than similar wounds located over the dorsal surface of the metacarpus/metatarsus. Additionally, limb wounds located on the extensor and flexor surfaces of joints and the heel bulbs appear prone to the development of EGT. The reader is referred to Chapter 15 for more information regarding EGT.

Joints, tendons, and ligaments, all of which are vital to normal musculoskeletal function, are not protected by muscle at the distal aspect of the limb of horses. Consequently, a full‐thickness skin wound in this location is likely to damage one or all of these structures, causing a substantial increase in the cost of treatment and a guarded or poor prognosis for return to function. Large wounds over bony prominences, particularly those involving the distal aspect of the limb, heal more slowly than those in other regions because they are subjected to excessive movement and high tension. Movement disrupts capillary buds, collagen deposits, and fragile new epithelium, thereby perpetuating the inflammatory response, which, in turn, favors the production of EGT. Although primary closure or delayed closure (primary or secondary) of wounds in these regions may assist healing, it can also be problematic because tension on the suture line might compromise blood supply to the skin. For this reason, many wounds over bony prominences are allowed to heal by second intention.