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Overheating and Heat Stress
Overheating and Heat Stress in Horses
with Gary P. Carlson, D.V.M., PhD.

Heat Stroke
Heat stroke or heat exhaustion may occur in resting horses confined to hot, poorly ventilated quarters. Often this problem develops in horses that are poorly conditioned and overexerted in hot and humid climates.

The clinical signs of heat stroke are depression, weakness, lack of appetite and a refusal to continue exercising. Pulse and respiratory rates increase and rectal temperature may rise to 106-110 degrees Fahrenheit (normal temperature is 98 to 101 degrees Fahrenheit). The capillary refill time may also be prolonged which can be checked by pressing a finger against the horse's gums. When the pressure is released, the white space should quickly return to pink color.

Despite elevated body temperature, the sweating response is inadequate, therefore, hot, dry skin is indicative of impending heat stroke. Depression and weakness may progress to ataxia (inability to coordinate voluntary muscular movement), collapse, convulsions, coma or death.

In treating heat stroke, it is essential to lower the body temperature rapidly. First, the horse should be moved into the shade in a well ventilated area and preferably made to stand in a breeze or in front of a fan. Cool to cold water should be applied to the horse with either a hose, sprayer, sponge or towel. Ice packs and cold alcohol leg wraps may be of additional help. The most effective way to cool the horse's body is to apply water or ice packs over the large superficial vessels of the legs, head and neck instead of the large muscle masses. With severely affected horses, however, applying water of the entire body may be the quickest and most effective means of cooling the body. Ice water enemas can also be effective in some situations to reduce critically high temperatures.

The Exhausted Horse Syndrome
Fatigue results from relatively brief maximal exercise as in racing, and exhaustion results from protracted submaximal exercise as in endurance riding. During short-term maximal exertion, energy is primarily generated by the anaerobic (without oxygen) metabolic breakdown of carbohydrates. Muscle energy stores deplete rapidly and fatigue correlates closely with the excessive production of lactic acid.

Recognition of severely exhausted horses is relatively simple. These animals are usually severely depressed and show little interest in food or water despite their apparent dehydration. Most continue to sweat, although at apparently reduced rates. Pulse and respiratory rates generally remain elevated, despite a rest period. The pulse may be weak, heart rhythm may be irregular, intestinal sounds are usually absent or markedly diminished and anal tone is lacking. Some horses may also develop spasmodic colic.

Horses with signs of severe exhaustion need to be treated by a veterinarian. Treatment may require prompt and vigorous fluid therapy to restore adequate volume, correct electrolyte deficits and provide ready sources of glucose (blood sugar) for metabolism. The principal problems in these horses are dehydration and sodium depletion, so rapid restoration of the fluid volume with sodium-containing replacement fluids is essential

Muscle Problems
Exercising horses frequently develop muscle problems, most commonly seen as the tying-up syndrome (azoturia or "Monday Morning Disease") evidenced by sweating, stiffness in the hind end and reluctance to continue exercising. Recent studies have indicated that some forms of tying-up syndrome may be related to electrolyte imbalances. Screening tests using both blood and urine may be helpful for determining if electrolyte deficits exist and, if so, what type of electrolyte supplement would be beneficial.

Less severe muscle problems can also occur instead of tying-up. Among the muscle problems believed to be related to fluid and electrolyte imbalances are stiffness and pain associated with muscle cramping or spasms. These signs may represent one form of the exhaustive horse syndrome because muscle problems are more common in hot weather where dehydration and electrolyte imbalances play a role. These horses generally do not develop tying-up and the muscle cramping usually subsides with walking.

Those with tying-up syndrome in which damage to muscle fibers has occurred may become worse with walking. Muscle pain stiffness and sweating are generally more severe. Treatment consists of rest, analgesics, non-steroidal anti-inflammatory drugs such as bute, and fluid therapy to correct accumulated deficits and to assure adequate urine flow.

Summary
The medical problems associated with overheating and work stress in horses can be very serious. By recognizing the early signs of distress, one can initiate prompt treatment before it progresses too far. Rigorous training schedules several months before any competitive event are necessary to condition the entire horse's body. The extended training and conditioning period also provides a non-competitive period in which the horseman develops an understanding of what his horse can and cannot do.

Hot and humid weather create extra stress on a horse's system and horsemen should be particularly aware of their horses' physical responses during these conditions. If possible, avoid exercising during the heat of the day when temperatures and humidity are high. Horses should have access to shade, a well ventilated area, salt and clean feed and water. Clip long hair close if training continues in late fall or early spring. Experienced horsemen who know their mounts are often able to recognize impending problems and initiate proper care before the problem becomes serious.

Excerpts from paper prepared by Gary P. Carlson, D.V.M., Ph.D. through the Equine Research laboratory, School of Veterinary Medicine, U.C. Davis.