Often beginning as heat stress or dehydration, the progression to heat exhaustion and ultimately heat stroke can occur with blinding speed. Horses’ large muscle mass allows them to generate a tremendous amount of heat, which makes them susceptible to a loss of water and electrolytes through sweat. As the amount of sweat increases, so does the imbalance of body fluids and electrolytes.
Heat exhaustion is characterized by hyperthermia (temperature > 41oC/106oF), tachycardia (heart rate > 60 beats per minute), and an elevated respiratory rate (> 80 breaths/minute). Most horses with heat exhaustion, especially exercising horses, are dehydrated and fatigued. Mucous membranes are dry and tacky to the touch; capillary refill time is prolonged (> 3 seconds). The skin is normally dripping wet from excessive sweating, but may be dry and warm in horses with anhidrosis. Many heat exhausted horses "thump" or show a spasmodic jerking or thumping of the diaphragm and flanks. Fatigue often shows itself as a stiff, abnormal gait or muscle soreness similar to horses that have “tied up.”
Left untreated, heat exhaustion rapidly proceeds to heat stroke. Technically, horses with heat stroke exhibit both hyperthermia (temperature > 41oC/106oF) and evidence of central nervous system dysfunction. Additional clinical signs in these horses can include a weaving, staggering gait; rearing, falling, and scrambling to rise; seizures; coma; and death. Skin is no longer wet but rather dry and warm to the touch. Horses with heat stroke are often unaware of their surroundings and this abnormal mental status makes them a danger to themselves and those attempting to treat them. Complications such as laminitis, kidney or liver failure, colic, or respiratory issues including pulmonary edema and sudden respiratory distress are known sequella to heat stroke.
Horses suffering from heat-related problems need to be identified quickly and treated aggressively. All horses, even those showing only minor evidence of heat stress, should be moved to a shady area and put in front of a fan. Remove bits and other tack as soon as possible. Apply cool water, either from a sponge or hose, to the hose’s neck and body, paying particular attention to the large veins of the neck and thin-skinned areas in the groin. Water sprayed directly on the face and head usually does little good and further upsets a horse with neurologic impairment. Several pints of rubbing alcohol applied topically along the back and neck areas are effective in promoting heat loss. Cooling blankets, if available, should be used and changed frequently.Once the horse is stable, small amounts of cool—not cold—water should be offered. If the horse will not drink or is dehydrated, large volumes of IV fluids (> 60 liters) should be administered. Either a balanced electrolyte solution or 0.9% sodium chloride should be used to replace fluid losses and help balance electrolytes lost in sweat.
The use of antipyretic, anti-inflammatory drugs, such as flunixin and ketoprofen, is controversial as they may not be effective in lowering body temperature in horses with heat exhaustion or heat stroke. They will, however, protect the body against “heat shock” proteins and provide pain relief if needed. Typically at least one dose should be administered. Rapid-acting sedatives (detomidine, romifidine, xylazine) are recommended for agitated horses with neurological signs. The dose should be titrated to provide some relief from neurological signs and to protect handlers yet keep the horse standing and aware of surroundings. Glucocorticoids (methylprednisolone sodium succinate 2-4 mg/kg given inravenously) should be used in horses with severe heat stroke and rapidly progressing neurologic signs. Not only will they help prevent shock, but they stabilize cellular membranes and may be effective in preventing organ system failure or further problems such as disseminated intravascular coagulation (DIC).
Additional treatment depends on the severity of the clinical signs. Horses should not be moved until stabilized and then only to a cool, well-ventilated area. Perform blood work to identify kidney or liver problems, repeating as needed. All affected horses should be well rested and get no exercise for a minimum of 3 to 5 days. Horses that have recovered from heat exhaustion or heat stroke may be more susceptible to a second occurrence throughout their lifetime and should be monitored closely when exercising or when they’re confined to enclosed areas in hot, humid weather.
The overall prognosis varies depending on the severity of the process. Horses exhibiting minor heat stress or exhaustion that are treated appropriately have an excellent prognosis for a full recovery. Horses with more severe heat exhaustion or those that have full-blown heat stroke carry a poor prognosis. As a general rule, when central nervous system signs disappear during the cooling period, the prognosis is better. Continued lateral recumbency or the onset of seizures carries a very poor prognosis as does the onset of renal or liver failure, colic, laminitis, or DIC.
Dr. Hovda is director of veterinary services at SafetyCall International and Pet Poison Helpline in Bloomington, Minn. She's also Chief Commission Veterinarian for the Minnesota Racing Commission.
Pet Poison Helpline is a service available 24 hours, seven days a week for pet owners and veterinary team members who require assistance treating a potentially poisoned pet and can provide treatment advice for poisoning cases of all species, including dogs, cats, birds, small mammals, large animals, and exotic species. As the most cost-effective option for animal poison control care, Pet Poison Helpline's fee of $35 per incident includes follow-up consultation for the duration of the poisoning case. It is available in North America by calling 800-213-6680.