Heat Stress in Horses
Beneath your helmet, your head feels hot and sweat drips off your face as you ride. Your horse's neck is soaked, and your reins are slippery and lathered. The more you call for an effort from your horse, the more sluggish he seems. Despite moving across firm ground, it's as if his legs suddenly are mired in deep footing, with the ground holding him down. Your horse has run out of steam--or more correctly speaking, his body is boiling over with too much heat. He has reached a dangerous state of exhaustion. You pull him up, yet his muscles remain quivering, he's panting, and his nostrils are flared. Could you have foreseen this development? Could you have prevented your horse from pushing the red line into the danger zone? Let's examine how heat stress develops, what signs you can monitor, and how you can prevent it.
The Buildup of Heat
A distinction can be made between heat stroke and heat stress. Heat stroke can occur over a relatively short period of time, as in the case of an unfit athlete worked strenuously in high ambient temperatures or horses confined in poorly ventilated, hot trailers. Heat stress, also known as heat exhaustion, usually results from protracted fluid and electrolyte loss during exhaustive exercise. The exercise might become exhaustive as a result of high ambient temperature, poor conditioning, lack of normal sweating, etc. A sunny day contributes to high ambient temperatures. Warm air temperature and high humidity prevent a horse from adequately dissipating internal heat from his body (i.e., the mechanisms of heat dissipation are overwhelmed and/or inadequate).
With each stride, the muscles of an exercising horse flex and strain from the effort--vast amounts of heat accumulate from the metabolism of working muscles. Over half of the energy used for muscular activity and locomotion (movement) in a horse is converted to heat.
Left unchecked, continued heat buildup stimulates a decline towards exhaustion. If increased metabolic and oxygen demands cannot be met, muscles fatigue. At higher temperatures, muscles (and all body tissue) demand more oxygen for energy utilization. If the horse gets too hot, the body's demand for oxygen can exceed the amount available via respiration (more on this later).
A tired horse presents graver risk than simply sore muscles the next day; loss of muscular control and strength from fatigue can lead to serious accidents. An exhausted horse might stumble and fall, placing both horse and rider in jeopardy. Left untreated, a horse with heat stress might exhibit neurologic signs including seizures and suffer serious damage to his heart, muscles, and kidneys.
Hours of protracted exercise or high-intensity exercise for short periods are conditions that particularly tax the ability of a horse to move heat out of his body quickly. To remove muscular heat, your horse sweats, pulling heat from the interior of his body to his skin in a process known as evaporative cooling. Around 70% of the heat of locomotion is normally dissipated from the body using this process.
Hot weather, particularly if it's humid, compromises a horse's ability to shed heat from his body. He sweats, but it's not always enough to stay ahead of the heat buildup.
The inherent problem in prolonged exertions is the persistence and duration of your horse's sweating process. A horse suffers body fluid losses and electrolyte imbalances with sweat. A horse which sweats during a mile-long track race loses lots of body water and some small degree of electrolytes, but the exertion is quickly over. In a short time, he easily replenishes what was lost. But a horse which exercises for protracted periods continues to dehydrate as heat from the continually working muscles is eliminated through sweat and evaporative cooling.
Heart and Respiratory Rates
As internal body temperature rises, sweat is not the only means to dissipate heat. Rapid breathing is a far less effective mechanism, but it can eliminate up to 15% of the heat load in your horse. Your panting dog uses this route almost exclusively to rid his body of heat. As your horse breathes rapidly, warmed blood flowing from heated skeletal muscles circulates to the heart and through the lungs. With each incoming breath, cool air (and oxygen) is exchanged for warm, exhaled air.
Heart and respiratory rates might remain elevated for a short period once exercise stops; blood flow coursing through his body flushes the bulk of heat to the skin, while respiration serves a minor role in cooling.
Fitness and good circulation are essential for efficient heat dissipation. If the horse isn't fit, his heart rate and respiratory rates are high following an intense effort. His nostrils continue to flare and his flanks rapidly move in and out with each breath, giving the impression that he is "panting." When the number of your horse's respirations is faster than his heart rate per minute, this is called an inversion. This is a sign of high internal body temperature and the respiratory tract is attempting to dump some of the heat load. (For more on normal heart rates after exercise, see "Is Your Horse Fit for the Task?" in the April 2002 issue, article #3406 at www.TheHorse.com. For more on normal respiratory rates after exercise, see "Recharge Your Horse's Batteries" in the June 2002 issue, article #3618.)
A fit horse that is performing aerobic exercise returns to a heart rate below 60-64 beats per minute (bpm) within 10-15 minutes. No matter the intensity of an exercise effort, both heart and respiratory rates should recover to below 64 beats/breaths per minute within 30 minutes following cessation of exercise. Dehydration notably slows heart rate recovery.
Persistent elevation of heart rate indicates that the horse is not coping well with the demands of the effort and could be in metabolic distress. Poor recovery often signals an impending metabolic collapse due to the combined effects of dehydration, energy depletion, electrolyte losses, and heat buildup in the muscles.
Horses with heat exhaustion might exhibit irregular heart rhythms as well as high heart rates. Affected horses might develop synchronous diaphragmatic flutter (SDF, also known as thumps). During episodes of SDF, the horse's flank twitches with every heart beat. Thumps occurs during heat stress, and is associated with acid-base and electrolyte abnormalities, namely alkalosis and low blood calcium concentrations.
Many of us spend time strengthening our own bodies for the athletic demands of riding. Whether we accomplish this foundation through aerobic sports, jogging, biking, or riding, we take pride in our stamina and fitness. Our horses deserve no less of a conditioning program, and might even require a more tailored approach.
Training often concentrates on skills essential to the intended discipline. Yet, another training ingredient is essential to the success and well-being of any athletic horse: Fitness of the cardiovascular system. As the muscles train to better efficiency, less work is needed to achieve a certain level of athletics, with less heat generated by the body.
Hot weather is not the only factor contributing to developing inversion (higher respiratory rate than heart rate), dehydration, or other signs of heat stress. A horse ridden at too fast a speed for his level of condition generates excess body heat. A horse being asked to climb a particularly intense hill or mountain, or to put forth an extraordinary work effort in jumping or galloping, will tend to overheat. A horse which is ridden too long without a rest also can build up an excess heat load in his muscles.
A horse sporting a full winter coat is at risk of overheating since the hair that keeps body heat in during cold weather also keeps in excess body heat from exercise. Hairy horses should be clipped to accommodate taxing weather conditions. Heavily muscled horses, such as Warmblood breeds and Quarter Horses, are at greater risk of retaining heat in the working muscles than leaner-breed horses such as Arabians or Thoroughbreds (thus the preference for these breeds in endurance racing). This is because they have a lower ratio of body surface area for cooling relative to their body mass that's generating the heat.
An overweight horse with abundant fat layers beneath his skin cannot dissipate heat effectively. Not only does excess body weight interfere with normal cooling processes, but it reflects a lack of fitness. Adequate preparation and training develop a horse into a sleek physique, building muscle where once there was fat. Conditioning also expands capillary beds (the networks of tiny blood vessels linking arteries and veins) and blood flow within skin and muscles to improve circulation of oxygen in the tissues and flushing of heat to the skin surface.
Transporting a horse in an enclosed van in hot weather can contribute to dehydration and heat stress. Additionally, a horse which was shipped to a warmer climate and has not been acclimated to exercise in hot and humid conditions is ill-prepared to deal with the added stress of the new environment no matter how fit he is. Most horses need at least three weeks in a warmer climate to allow their bodies to adapt and dissipate heat more efficiently. Some horses shipped to hotter climates experience anhidrosis, an inability to sweat normally.
Monitoring the Cardiovascular System
Some basic physical parameters can be examined to monitor how well your horse is coping with the stress of exercise. Heart rate recovery is an important one. In addition, a horse's hydration status is a good indication of how he's handling exercise.
A racehorse running one mile in two minutes can lose as much as 2 1/2 gallons of sweat as he "cools" his body. Consider, then, the dramatic fluid loss in an unfit horse which is in sustained work under adverse climatic conditions. Even a well-conditioned horse loses as much as two to three gallons of fluid per hour with exertional demands in the face of high heat and humidity.
A rough estimate of dehydration can be made by gently pinching up a fold of skin on the point of the shoulder or an eyelid, then noting how quickly it snaps back into position. It is considered normal for the skin to snap back immediately. Skin that remains "tented" and refuses to return to its normal position represents serious dehydration of 7-10% of body fluids. There are many levels in between.
Mild dehydration of 2-3% might be accompanied by a dry mouth and dry mucous membranes. At about 5% dehydration, eye sockets appear sunken, skin elasticity is markedly reduced, and the horse is weak with a dull, listless attitude and posture.
A peek at mucous membrane color and capillary refill time of the gums provides a good impression of blood perfusion throughout the body. The gums should be a healthy pink color. After pushing on the gums with a fingertip, this color should return within two seconds. A normal pink color with a normal capillary refill time indicates an adequate cardiovascular state, confirming pumping of blood throughout the body tissues. With poor circulatory perfusion, gums appear darker pink, implying stagnant blood, and capillary refill time slows.
The moistness of the gums and a skin pinch test are but crude assessments of hydration status. It is easy to be fooled that all is well simply by measuring capillary perfusion time, gum color, or skin tenting. You might not be able to obtain a clear picture of mild clinical dehydration using only these physical inspections. Mild dehydration of as little as 2-3% is associated with a decrease in performance. Your horse might not be in direct danger of metabolic collapse, but his ability to compensate for further dehydration, electrolyte losses, or heat buildup becomes taxed as exercise continues.
Rectal temperature is another valuable parameter for monitoring a horse's well-being. As a mammal, an internal set point is regulated in a horse's midbrain to maintain body temperature within a very narrow range. Part of the body's temperature control process relies on losing heat generated by working muscles and normal digestive metabolism.
An exercising horse typically works within a rectal temperature range of 101-103°F. Should rectal temperature surpass 103.5°F, the horse is overheating.
Once a horse has been pulled up to rest, rectal temperature should decline steadily over 20 minutes back to the normal resting value of 99-100°F. Initially in the workout, as internal temperatures rise, the bulk of blood from cardiac output is diverted to the skin away from the working muscles to facilitate heat dissipation. Internal heat continues to rise if surface evaporation (sweating) is no longer able to keep pace with the heat buildup. As muscle temperature elevates, contractile function of the muscle fibers is impaired, further contributing to fatigue and exhaustion.
Loss of vital fluids through the skin causes a steadily progressive state of dehydration unless this "water" is replenished. Blood flow diminishes to the subcutaneous layers of the skin to reduce sweat fluid losses, further limiting sweating action in an effort to conserve body water. If exercise continues and water isn't offered, heat continues to build in the horse with no outlet.
A rectal temperature above 105°F is abnormal in any horse and poses a dangerous situation; rapid cooling measures should be initiated at once. The higher the internal temperature, the more metabolic demands are placed on the system; this metabolism needs to be fueled by oxygen. If body
temperature exceeds 106°F, the body's demand for oxygen might surpass the amount that can be supplied by the respiratory system. An oxygen deficit then occurs in the tissues (hypoxia), potentially leading to kidney, heart, liver, and brain damage.
At temperatures greater than 107°F, a horse in severe heat stress can go into convulsions or a coma, then die. The objective is to avoid these scenarios.
Under any exercise conditions, following some simple strategies will assist your horse in cooling out. As you finish a workout, bring your horse to a walk. Hop off and spend a minute or two walking him so blood flow continues to flush metabolic waste products and heat from his muscles. If an overheated horse abruptly ceases working, blood pools in the muscles (decreasing that in the circulation), contributing to relative dehydration. A fatigued horse might refuse to move. Provided he is not tied-up with muscle spasms, you can assist circulation in the muscle by massaging major muscle groups in rhythm with the heartbeat.
If the heart rate returns to 64 bpm or below within 30 minutes, but the respiratory rate remains elevated, such an inversion does not necessarily imply your horse is in danger. It means he needs help ridding his body of the extra heat. Both the respiratory rate and heart rate should return toward resting rates within 10 minutes of stopping exercise. You can assist your horse in cooling in a couple of ways.
In warm weather, copiously bathe his head, neck, and legs with cool water. Large blood vessels in these locations flush heat to the skin surface, and rapid evaporative cooling is achieved by continual sponging of these areas. Apply cool water and as it heats up, scrape it off of major muscle groups, such as over the loin and hindquarters. Draping wet towels over the head and neck might be counter-productive to cooling as the towels serve to insulate the horse rather than allowing heat to escape, particularly if the water on them remains warm.
Continuously apply and scrape water away until the horse's skin feels cool to the touch. His respiratory rate should decrease as his internal body temperature descends into the normal range. All horses will need some assistance with cooling in the summer months even if the respiratory rate is not inverted or elevated.
Ideally, the body temperature of an overheated horse should be decreased by 1°F every 30-40 minutes by bathing his head, neck, and back with water. Cooling him down too rapidly can cause him to chill.
Keep monitoring rectal temperature with a thermometer, and muscle tone, as you cool out your horse. Once the rectal temperature reaches 101°F, you can stop and see if he stabilizes without further assistance.
In hot and humid climates, cold or ice water can be applied to the entire body with less risk of muscle cramping. The danger in cooling these large muscle groups too rapidly lies in the tendency of blood vessels to constrict away from the surface while retaining metabolic by-products that need to be carried out from deep muscle tissues. Diminished blood flow to the skin surface further allows heat to persist within deep muscles, causing heart and respiratory rates to remain elevated.
Besides exhibiting poor metabolic recoveries, the horse might develop "tying-up" syndrome, with sudden cramping and muscle spasms (more on this shortly). An affected horse might refuse to move, or exhibit colic-like signs due to painful cramps. Heart and respiratory rates further climb in response to pain, and as muscles spasm and contract, more heat is generated in a horse's already overheated muscles.
Offer a bucket of water to your horse immediately following exercise. Traditionally, horse owners have avoided letting "hot" horses drink because of a perceived risk of colic and cold-water founder (laminitis). However, with the possible exception of very hard galloping exercise (e.g., racing), horses can safely drink right after exercise. He'll want more water at this time than he will later, so give it to him to best rehydrate him. Offer water as soon as is practical and let him drink up to two or three gallons during the initial 15 minutes of recovery.
Find a shady spot for an overheated horse, preferably with decent air circulation from a light breeze or fan. An enclosed space with stagnant air adds to heat retention. Fans are helpful for convective cooling--as the air flows across the horse's body, it pulls heat off the skin. Periodic, short walks also help the muscles pump heat out of deeper tissues.
A dangerously overheated horse might need to be dunked into a nearby pond, or soaked entirely with a hose or buckets of water. Intravenous fluids might be necessary to treat severe dehydration and shock, and also to cool the internal organs and muscles. Severely affected horses need to have their acid-base balance and electrolyte status evaluated and their deficits corrected using oral and intravenous fluid therapy. Administration of the non-steroidal drug Banamine might be helpful to decrease inflammation and to prevent endotoxemia in very debilitated patients.
Some horses in hot, humid climates lose the ability to sweat--a syndrome known as anhidrosis. It is thought that overworked sweat glands exhaust their ability to sweat. During exercise, such a horse loses the ability to cool himself. In addition to a reduced tolerance to exercise, you might notice that your horse's skin is dry and hot to the touch. There might be a damp area of sweat beneath the mane and saddle or in the groin region, but no moisture is felt elsewhere. He pants with the slightest effort, and seems fatigued. Rectal temperature will rise.
Exercise must be stopped immediately, the horse moved to a cool location, and aggressive cooling techniques implemented immediately. Such horses are in great danger of heat stroke. Early recognition is important to restrain the horse from further physical exertion and so appropriate medical attention can be initiated.
Horses need to be acclimatized over a period of several weeks to high heat and humidity. Some simply stop sweating for no specific reason. There is no way to avoid this if the horse's body fails to turn on his sweat glands. Affected horses need to be moved to a different environment either to an air-conditioned barn, or out of the geographic area. Some horses benefit from electrolyte administration. One electrolyte/ nutrition supplement containing the amino acid L-tyrosine was developed specifically for horses with anhidrosis.
Seek More Knowledge
Using these guidelines, you can help keep your horse out of trouble. Pay close attention to your conditioning program, and use cardiac and respiratory recoveries to guide advances in speed and intensity. If your horse is laboring under the effort you ask, rethink your training strategy, learn more about equine cardiovascular conditioning, and use appropriate cool-down techniques during training and competition.
About the Author
Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.