Ask the Vet, Sallie S. Hyman VMD, DACVIM: Burn Injuries

Fire Related Injuries in Horses

The recent high profile barn fire at True Prospect Farm in Pennsylvania that killed six horses and injured Boyd Martin’s Olympic alternate Neville Bardos and the terrible wild fires plaguing Colorado that have displaced hundreds of horses remind us of the dangers of fire and the severe and often fatal injuries that it can cause. It takes only 2-3 minutes for straw in a stall to ignite and burn a 10x10 foot area, burning at temperatures of 300 degrees. Thermal injury to a horse will occur when they are within 6 feet of that fire in their stall.

Horses exposed to fire usually sustain two types of related, yet separate injuries. The first is burns. The second is smoke inhalation. The extent of injury dictates treatment and prognosis. Superficial burns are easily treated and not very expensive to manage. Serious burns can result in burn shock and hypovolemia, require aggressive and extensive treatment, and can be very expensive to manage. Burns of 50% or more of the body are almost always fatal, although burn depth can affect the prognosis. Horses that survive and heal from extensive burn wounds may have scarring that prevents them from returning to normal function. Inhalation injury can cause damage to the lungs that affects performance.

Burns

Burns are classified by depth. 

First-degree burns affect the superficial layers of the epidermis. These burns are painful, red, and edematous. The germinal layer of the epidermis is spared so the skin heals without complications.

Second-degree burns involve the epidermis and can be superficial or deep. Superficial second-degree burns are painful, minimally affect the basal layer of the epidermis, and usually heal with minimal scarring in 14-17 days. Deep second-degree burns involve all layers of the epidermis. There is minimal pain, as nerve endings are also involved. An eschar (slough produced by thermal injury) forms over the wound. These burns can heal spontaneously, but usually with a lot of scarring. Skin grafts are recommended for optimal results.

Third-degree burns involve the epidermal and dermal layers of the skin as well as the adnexal structures (hair follicles, sweat glands, sebaceous glands, nerve endings). The wound can appear from black to white in color. There is a noticeable lack of pain. These burns can result in fluid losses, shock, wound infections, and possible septicemia. They heal by contraction from the wound margin (like other full thickness wounds) and usually require skin grafts for optimal results.

Fourth-degree burns involve all of the skin, underlying muscle, bone, fat, and fascia. The prognosis for fourth-degree wounds is poor.

Horses can also sustain corneal burns and damage particulates in the air. Loss of vision is possible.

Inhalation Injury

The respiratory system is particularly susceptible to injury in fire situations. The delicate tissues of the entire respiratory tract from the nostrils to the lungs can be affected. Inhalation consists of three components: direct thermal injury; carbon monoxide poisoning; and chemical insult.

Direct thermal insult causes edema and upper airway obstruction. The lungs are usually spared from thermal injury, as the superheated air is cooled due to the normal heat exchange mechanisms in the oropharynx and pharynx. The air is cooled before it reaches the lungs. Thermal injury can also cause damage to tissues of the trachea that can later slough and cause a tissue “plug” that obstructs the airway.  Carbon monoxide poisoning decreases oxygen levels available to tissues. It has a 230-times greater binding affinity for hemoglobin (the molecule that delivers oxygen to tissues) than does oxygen. This carboxyhemoglobin is unable to deliver oxygen to the tissues. Carbon monoxide also binds myoglobin, thus preventing oxygen delivery to muscle tissue. 

Chemical insult depends on the materials that are burning. Pressure treated lumber contains hazardous chemicals, cleaning products, paints, pesticides, herbicides and many other substances can become dangerous when burned. Many of these products can turn into cyanide gasses, aldehydes, hypochloric acid or sulfuric acid. All of these are incredibly irritating to the respiratory tract. Some can even cause sloughing, edema, bronchoconstriction, decreased bacterial clearance, and decreased mucociliary clearance.

Secondary pulmonary infections are very frequent after inhalation injuries due to the compromise of the clearance mechanisms of the respiratory tract. 

Burn Shock

After severe burns there is a dramatic cardiovascular effect called burn shock. It is similar to hypovolemic shock and can actually progress to hypovolemia. Heat causes capillaries to leak fluid. Tissue injury also causes chemical factors such as cytokines, nitric oxide, and oxygen radicals that also cause the capillaries to leak. The cycle is perpetuated when damaged vessels exacerbate tissue injury when they become clogged with cellular debris and leak fluid.

Treatment of Burn Wounds and Inhalation Injury

Treatment depends on the severity of the injuries. Initial examination should focus on stabilizing and maintaining a patent airway and treating/preventing hypovolemic shock. If severe upper airway edema is present, a tracheostomy may be necessary. Rapid fluid replacement should be initiated, but caution is necessary to prevent pulmonary edema. Massive amounts of protein can be lost through burn wounds and can result in fluid shifts into the lungs when large amounts of crystalloid (water-like) fluids are administered. Plasma and other colloidal fluids such as hypertonic saline or hetastarch may be necessary. Electrolytes should be monitored and replaced as necessary. Initially there is usually a hyperkalemia (high potassium) due to release from damaged cells, but hypokalemia (low potassium) usually ensues in 2-3 days. Intranasal oxygen may be needed to combat carbon monoxide poisoning or low oxygen levels due to anemia from burn induced intravascular hemolysis.

Flunixin meglumine (Banamine) will help with discomfort and inflammation, but some horses will require srtonger pain relief with opiates. Free radical scavengers can be administered to combat oxygen radicals released from damaged tissues. Superficial wounds are cleaned and debrided and covered with a topical antimicrobial ointment such as silver sulfadiazine. Deeper wounds with blisters or eschar are left for 2-3 days to allow for any continued tissue death and are then treated. Systemic antibiotics are not usually effective for burn wounds and are used for inhalation injury only when there is clear evidence of infection. Corneal damage is treated with topical antimicrobials if not too severe, but may require corneal grafts if severe.

Adequate nutrition is an essential part of healing from burn wounds. Burn victims become hypermetabolic to produce heat. A lot of heat is lost through evaporation of fluids lost through wounds. Protein is also lost in these fluids. Caloric expenditure and protein catabolism are greater in burn injuries than in any other injury. Accurate weight measurements should be made on a daily basis and nutrition adjusted if losses are noted. If a horse has severe burns to the muzzle, oral cavity,

 
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