Introduction

Burns occur primarily from barn fires but may also result from contact with hot fluids; electrocution or lightning strike; friction, such as from a rope burn; radiation therapy; improper use of a cryogen; an electrosurgical accident; or a maliciously applied caustic agent, such as phenol or potassium hydroxide.^1,2 Horses are most commonly burned on the back and face.

Management of severe and extensive burns is difficult, expensive, and time consuming. The larger the surface area of the burn, the greater is the loss of fluids, electrolytes, and calories. Serious burns can rapidly result in severe shock, hypovolemia, and associated cardiovascular changes. The burned horse may also suffer from smoke inhalation or corneal ulceration.^1,2 Burns covering 50% or more of the body are usually fatal, although the depth of the burn also influences the horse’s likelihood of survival. Because burns are often pruritic, a burned horse requires long‐term close observation to ensure that it does not mutilate the wound. Burned horses are frequently disfigured or unable to return to full function. Prior to initiating treatment, therefore, the patient must be carefully examined, with particular attention paid to cardiovascular and pulmonary function, ocular structures, and the extent and depth of the burn. Cost of treatment and prognosis for survival and return to function should be discussed thoroughly with the owner before treatment is initiated.^1–4

Classification of burn injuries

Burns are classified according to the depth of the injury.^1–3 First‐degree burns involve only the most superficial layers of the epidermis. These burns are characterized by erythema, edema, and desquamation of the superficial layers of the skin. First‐degree burns are painful because nerve endings are exposed. The germinal layer of the epidermis is spared, and the burn heals without complication (Figure 20.1).⁴

Second‐degree burns involve the epidermis and may be superficial or deep. Superficial, second‐degree burns involve the stratum corneum, stratum granulosum, and a few cells of the stratum basale (see Figure 1.2). Second‐degree burns are painful because pain receptors remain intact. Because the basal layer of the epidermis (stratum basale) is largely uninjured, superficial second‐degree burns heal rapidly, within 14–17 days, with minimal scarring (Figure 20.2).^5–7 Deep second‐degree burns involve all layers of the epidermis, including the basal layer. These burns are characterized by erythema and edema at the epidermal–dermal junction, necrosis of the epidermis, accumulation of white blood cells at the base of the burn, and formation of eschar, a structure composed of exudate, collagen, and layers of dead skin resulting from destruction of the dermis. The only germinal cells spared are those within the ducts of sweat glands and hair follicles, which reside within the dermis. Deep, second‐degree burns may heal within 3–4 weeks if care is taken to prevent progression of dermal ischemia that may lead to full‐thickness necrosis, such as by reducing inflammation, removing tissue debris, and promoting a moist environment.^6,7 Deep second‐degree burn wounds, unless grafted, usually heal with extensive scarring.⁵

Third‐degree burns cause minimal or no pain because nerve fibers in the basal layer of skin are destroyed (Figure 20.3). Third‐degree burns are characterized by loss of the epidermal and dermal components, including the adnexa, and range in color from white to black (Figure 20.4, Figure 20.5). They are accompanied by fluid loss and a marked cellular response at the wound’s margin, formation of eschar, lack of pain, shock, local infection, and sometimes bacteremia and sepsis. Healing occurs by contraction and epithelialization from the wound’s margin or by autografting. Fourth‐degree burns involve all of the skin and underlying muscle, bone, ligaments, fat, and fascia (Figure 20.6).^6,7,9

Physical examination

The burned horse should receive a complete physical examination before the wound is evaluated. Only after the patient’s condition is stable should the burn be assessed. The extent and severity of the burn should be determined. The extent of the burn refers to the size of the area exposed, whereas the severity is related to the depth of the wound, which in turn is related to the maximum temperature the tissue attains and the duration of overheating. Heat is slow to dissipate from a burn, often complicating accurate evaluation, early after injury, of the amount of tissue damaged. This explains why injury often extends beyond the original boundaries of the burn.¹⁰

Physical criteria used to evaluate a burn include erythema, edema, pain, blistering, the presence of an eschar, body temperature, cardiovascular status, and the presence of infection.¹⁰ In general, erythema, edema, and pain are favorable signs because they indicate that at least some superficial tissue is viable, but pain is not a reliable indicator of the wound’s depth.¹⁰ Often, time must elapse to allow tissue changes to progress before the extent and severity of the burn can be ascertained.

The burn may be characterized by erythema and the presence of vesicles and singed hair, and the horse may show signs of pain. The burned horse may exhibit blepharospasm, epiphora, or both, signifying corneal damage. Coughing may indicate that the horse suffers from smoke inhalation, and a fever signals a systemic response to injury. Increased heart and respiratory rates are often accompanied by discoloration of mucous membranes, indicative of poor cardiovascular function.

Prognosis in relation to the percentage of body surface area burned

The percentage of total body surface area involved in the burn usually correlates directly with mortality, whereas the depth of the burn determines morbidity.^6,7,9,11 In human patients, the “rule of 9” is used to estimate the total surface area that has been burned. This method allows prognosis to be estimated according to the extent of the burn. Each arm represents 9% of the body’s surface area, each leg 18%, the head and neck 9%, and the thorax and abdomen each 18%.^4,6,7 Special care is taken to identify injury to major vessels of the legs and injury to the eyes, perineum, tendon sheaths, and/or joints.

Although specific guidelines do not exist for calculating the percentage of total surface area of the body of large animals that has been burned, euthanasia should be recommended for horses with deep second‐degree or third‐degree burns comprising 30% or more of the body’s total surface area.^12,13 The availability of adequate facilities for treatment, cost of treatment, and pain experienced by a burned horse should be considered when deciding whether or not to recommend euthanasia. Because convalescence may take up to 2 years, euthanasia is often more humane than treatment.¹⁴

Treatment

Wound care

The hair surrounding a burn wound should be clipped, and the wound debrided of devitalized tissue.⁹ The burned skin should be cooled using ice or a cold‐water bath. The wound should be copiously irrigated with a sterile solution of 0.05% chlorhexidine digluconate.¹¹ A water‐based cream or gel should be applied liberally to the burns to prevent loss of heat and moisture, to prevent bacterial invasion, and to loosen necrotic tissue and debris.

An effective and economical antibacterial water‐based medication for treating burns is silver sulfadiazine (Silvadene, Par Pharmaceutical, Inc.) in a 1% water‐miscible cream. This broad‐spectrum antibacterial cream is capable of penetrating an eschar and is active against Gram‐negative bacteria, especially Pseudomonas, and is effective against S. aureus, E. coli, Proteus, Enterobacteriaceae, and C. albicans.²⁹ It causes minimal pain when applied but must be applied twice daily because it is inactivated by tissue secretions.

Aloe vera, a gel derived from a yucca‐like plant, is often applied to burns. It has antithromboxane and antiprostaglandin activities and is reported to relieve pain, decrease inflammation, stimulate cellular growth, and kill bacteria and fungi.^30,31 Aloe vera and silver sulfadiazine are good first choices in therapy for burns and are used extensively in human medicine. One study examining the effect of platelet‐rich plasma (PRP) gel applied to deep second‐degree burns of horses found that PRP gel increased the speed of repair of the extracellular matrix and its components but had the potential to cause excess fibrosis. Treatment with two applications of the PRP gel accelerated formation of the extracellular matrix during the first half of wound healing. The PRP gel possibly provided antibacterial activity in burn wounds, due to its inflammatory cytokine content, thereby preventing complications due to contamination.³²

First‐degree burns are generally not life‐threatening, and thus, horses that have incurred a first‐degree burn are managed simply, using topical therapy in the form of cool compresses, cold‐water baths, and wound dressings to provide relief from pain. Pain can also be controlled by administering a non‐steroidal anti‐inflammatory drug (NSAID) or narcotic.

Second‐degree burns are characterized by vesicles and blisters and are not life‐threatening. Vesicles and blisters should be left intact for the first 24–36 hours after they form, because fluid within them provides protection from infection and because the presence of vesicles and blisters is less painful than the exposed burn. A topical wound‐care product, such as silver sulfadiazine, should be applied to the burns before and after the eschar forms.^1,2,11

Third‐degree (i.e., full‐thickness) burns can be difficult to manage. The horse’s condition should be stabilized as rapidly as possible prior to undertaking wound management. A third‐degree burn may be managed with occlusive dressings (closed technique), application and removal of wet dressings (semi‐open technique), allowing an eschar to form (exposed, or open technique), or excision and grafting.^6,7,10 The most effective and practical therapy for horses with a large burn is the open technique.^1,2,10

Using the open technique, the wound is left exposed to the air to form its own biologic barrier, the eschar. Leaving the burn wound uncovered (i.e., open) is a method of treatment based on the principle that bacteria do not thrive on a dry surface. The eschar should be covered with an antibacterial agent twice daily. The eschar does not prevent bacterial contamination of the burn, loss of heat, or evaporation of water, and the depth of tissue destruction may be marginally increased as the wound dries. The wound does not contract while covered by the eschar. The eschar is sloughed by the activity of bacterial collagenase within 4 weeks.³³ The exposed bed then contracts. If the wound is too large to heal by contraction and epithelialization, the wound should be grafted.

Excising the eschar that results when a burn is left uncovered is not practical for horses if the burn is extensive because removing the eschar leaves the wound exposed to environmental contamination and may result in massive loss of fluids and heat. The goals of open therapy are to keep the wound dry and protected from self‐mutilation.