Unlike dogs and most other animals, horses have sweat glands throughout their skin. Sweating creates “evaporative cooling”---as water changes from a liquid to a gas, it absorbs energy from its surroundings. In this case, that energy is in the form of heat from the skin and the air just above, leaving it cooler. But sweating is actually a horse’s secondary cooling mechanism. What is the first?
a. dilation of the capillaries in the skin
b. heavy breathing
c. flattening of the haircoat
d. all of the above
Answer: a. dilation of the capillaries in the skin. As blood flows through the body of a horse at rest, heat is absorbed from the muscles and organs. When the blood reaches vessels that lie just under the surface of the skin, the excess warmth dissipates into the cooler outside air. When a horse exercises, the amount of internal heat generated by his muscles increases. To maintain a constant internal body temperature, the excess heat must be dissipated faster. To accomplish that goal, the capillaries become dilated so more blood will be sent to the skin. If a horse continues working to the point that the capillaries cannot keep up with the heat he is generating, only then will he begin to sweat. Panting, like a dog, and flattening the hair to allow air to reach the skin more readily also have some cooling effect, but these are not the primary mechanisms horses rely on.
When a horse begins to sweat, fluids from his bloodstream pass through the sweat glands to emerge onto the surface of the skin. But as he continues sweating, the blood left behind becomes more concentrated. Long before the blood becomes too dense for the heart to pump it, other fluid reserves within the body are drawn into the bloodstream to keep the critical red cells moving as the horse works. Which parts of the body are the primary sources for the backup fluid?
a. the interstitial spaces, the areas between the body cells
b. the gastrointestinal tract
c. the lymphatic system
d. the lungs
Answer: a. and b. The gastrointestinal tract, especially the cecum0 and large intestine, is an important reservoir of fluids that are rich in nutrients and electrolytes from the horse’s feed. The blood will also draw a lot of fluid from the spaces between his cells. If the horse continues sweating to the point where these reserves are running low, his body will start to draw fluid from inside of his cells. At this point, however, he is becoming seriously dehydrated. Horses who sweat too long without replenishing their fluids can experience a number of health issues, ranging from impaction colic to tying up. Fortunately, most horses need nothing more than rest and access to fresh water to make a complete recovery from an intense workout. A significantly dehydrated horse may require several days of rest and drinking, and a veterinarian may need to rehydrate the horse intravenously or through nasogastric intubation.
Test for dryness
If you’re worried that your horse may be getting dehydrated, there are two simple tests you can do. One is the skin-pinch test: Grasp a fold of skin on the point of his shoulder and pull it away from his body slightly. Then release it, noticing how long it takes for the “pinch” to flatten out. In a hydrated horse, the skin will snap back in less than a second. If the crease is still visible after two to three seconds, the horse is dehydrated; call your veterinarian if you can still see the pinched area after six seconds.
What is a second common test for dehydration?
a. press your fingertip against his gums
b. offer water to see if he drinks
c. turn him in a tight circle to test his coordination
d. take his pulse
Answer: a. press your fingertip against his gums. When you release the pressure, you’ll see a white spot; note how long it takes for the pink color to return. If the blood hydration is normal, the pink will return in less than two seconds. If the spot remains after three or four seconds, the horse may be dehydrated. (Note: this test is also used to help detect shock, blood loss and other conditions affecting circulation.) Other signs of dehydration include dark mucous membranes, dark urine and elevated body temperature. Keep in mind that a dehydrated horse may not be interested in drinking. He may have “gone off” his water or be refusing it for another reason. To get him to drink more, try offering a second bucket with dissolved electrolytes in addition to one with plain water. Flavoring his water with apple juice may also encourage him to drink. Call your veterinarian if your horse seems dehydrated but is also unwilling to drink.
Recharging the batteries
Electrolytes are minerals---calcium, potassium, sodium, magnesium, chloride, bicarbonate and phosphate---that play a role in most of the electrochemical processes that sustain life. And all are lost in prodigious quantities when a horse sweats. Fortunately, a horse can replenish his internal mineral supplies as he grazes or eats his normal feeds. Sometimes, however, administering an electrolyte supplement may be advisable to help a sweaty horse recover faster. Which of these horses is the best candidate for electrolyte supplementation?
a. the fit athlete completing a 50-mile endurance course
b. the fretful traveler coming off a trailer after a long ride to the show
c. the “weekend warrior” finishing a trail ride on a humid day
d. all of the above
Answer: d. all of the above. The common denominator among these horses is that all are likely to have been sweating for an extended period of time—at least an hour or two—without the opportunity to eat or drink. And all could deplete their internal electrolytes to critical levels, potentially causing fatigue, muscle tremors, heat stress and other problems. Equine athletes such as eventers or endurance horses are obvious candidates for electrolyte supplementation, but any show or pleasure horse who works and sweats extensively in hot weather can also benefit.
Around the block
Grass, hay, grains and commercial feeds are high in electrolytes, and most horses can readily replenish most of the minerals lost through sweat simply by eating their regular rations. There are, however, two essential electrolytes that are not abundant in natural feeds and must come from another source. What are they?
a. magnesium and potassium
b. phosphate and calcium
c. sodium and chloride
d. bicarbonate and magnesium
Answer: c. sodium and chloride. The two elements that make up common table salt are not abundant in grasses and other natural feeds. But horses have natural appetites for salt and will consume what they need from a salt block. Allowing your horse to have free-choice access to a salt block is essential, especially during hot weather.
It’s a hot, sticky morning in North Carolina, and you and three friends decide to ride in the mountains to cool off. You go early so you can be back at the trailers by the time the day really heats up. Still, in the parking lot after the ride, you notice something: Which of these horses is showing the red flags of a potentially serious problem?
a. the Quarter Horse with frothy sweat under his breastplate
b. the Appaloosa, who is drenched in clear sweat but is drinking from his bucket
c. the Thoroughbred, who is sweating only a little under his saddle and is “panting” like a dog
d. the Arabian, who isn’t sweating much but can’t wait to get at his hay and water
Answer: c. the Thoroughbred. A horse who isn’t sweating when others around him are may have anhidrosis, the inability to sweat. The causes of anhidrosis are not well understood, although the condition usually develops in horses who sweat for prolonged periods of time in very hot, humid conditions. In effect, the horse’s thermoregulatory system shuts down, and even slight exertion in hot weather can cause dangerous overheating.
An affected horse may sweat a little under his mane or under tack, but his coat remains mostly dry despite exertion in hot weather. He may breathe forcefully through his mouth in an effort to cool himself, and he may be lethargic and uninterested in food or water. He needs to be cooled off promptly: Move him to a shady area, and douse him with cold water.
Call your veterinarian if you suspect your horse may be developing anhidrosis. There is no proven treatment, but you’ll need to take long-term measures to keep him as cool as possible in hot weather. In some cases, relocating the horse to a cooler climate may be the best option. As for the other horses described above, the amount and quality of sweat they produce can vary according to their individual levels of fitness, physiques and other factors, but as long as they are alert, eat and drink readily, and recover fully from exertion within an hour or so, they are probably just fine.
Too hot to handle
Heat exhaustion, also called heat stress, is a life-threatening condition that develops when a horse is unable to cool himself by sweating. If his core temperature reaches 104 degrees Fahrenheit, his metabolic system will be affected, and if it goes to 105 degrees or higher, his organs and circulatory system may begin to shut down. Identifying the earliest warning signs of heat exhaustion is critical to avoiding devastating consequences. Which of these signs can be a signal of trouble?
a. the horse’s sweat becomes thick and sticky
b. his gums and mucous membranes become dark and/or discolored
c. his pulse is elevated but weak and irregular despite rest
d. he is depressed, with a low head posture
e. his breathing is either rapid and shallow or deep and gulping
f. all of the above
Answer: g. all of the above, especially when they appear in combination. A heat-stressed horse will sweat profusely. But once his internal fluid reserves start to run low, his perspiration may slow or stop, and its texture will become thick and sticky. Other early signs of stress include diminished gut sounds and an elevated pulse. Rapid breathing is a last-ditch attempt to expel heat. Any horse will be breathing hard after working in hot weather, but if the “panting” continues after several minutes of rest, he may be having trouble cooling off.
If your horse develops heat exhaustion, you’ll need to act quickly to save his life. Call a veterinarian right away, but even as you’re doing that, you can take a number of steps to start bringing your horse’s body temperature down. All but one of the actions listed here are good steps to take until the veterinarian arrives. Which one of the following actions should you avoid?
a. move the horse into the shade
b. douse him with the coldest water available
c. administer an electrolyte paste
d. press ice against his head and throat
e. encourage him to drink
Answer: c. administer an electrolyte paste. A horse who has already reached the point of heat exhaustion is metabolically stressed, and forcing him to swallow electrolytes may stress his system even more. Instead, focus on helping him cool down. Move the horse into any available shade, and douse him with the coldest water you have available: Drench the horse, scrape him dry and repeat—waiting for the water to evaporate won’t work fast enough. If you have ice or cold packs on hand, press them against the major blood vessels that run close to the surface under the horse’s throat—you’ll help to cool the blood flowing to his brain. You can offer him a bucket of water containing dissolved electrolytes, as long as you provide a bucket of plain water as well.
A stroke of misfortune
If a horse in heat stress isn’t cooled down quickly, his condition may progress rapidly to heat stroke. If his body temperature rises to 106 degrees Fahrenheit for a prolonged period, or if it tops 108 degrees for as little as 15 minutes, the damage to his body may be irreversible. Which organ is likely to show the first signs of the most serious effects of prolonged high temperatures?
a. the heart
b. the brain
c. the kidneys
d. the gut
Answer: b. the brain. Much of the metabolic work that goes on in the body is performed by proteins called enzymes, but these large molecules are heat-sensitive. If they get too hot, or stay hot for too long, their chemical structures can change, affecting their ability to function. The brain contains many proteins, and it is the site of many enzymatic reactions that influence the entire body. So the brain is one of the first organs to experience the effects of elevated heat, and the signs of damage to this organ can be widespread. A horse with heat stroke may be stumbling or have difficulty moving at all; his behavior may be anxious, irrational or erratic; he may be depressed, disoriented or oblivious to his surroundings; he may collapse or go into convulsions. If a horse survives the initial bout of heat stroke, he may still develop colic, laminitis, kidney failure, liver failure and other serious issues stemming from damage to his internal organs.
Keep it cool
Knowing how to help a dangerously heat-stressed horse is important---but it’s far better to keep him from overheating in the first place. Five of these actions are good ways to protect your horse from the heat---and one is a very bad idea. Which one of the following can put your horse in danger?
a. withholding cold drinking water
b. hosing him down before and during a workout
c. stopping to let him graze along the trail
d. keeping him fit
e. acclimating him gradually to work in hot weather
f. avoiding work during the hottest hours of the day
Answer: a. withholding cold water. The myth that hot horses who drink very cold water are at risk of colic is persistent. But research done for the 1996 Olympic Games in Atlanta definitively showed that offering a hot horse cold water posed no threat to his health. In fact, letting a thirsty horse drink is one of the best things you can do for him. When water isn’t available, allowing a horse to graze can also help—lush grass can contain up to 90 percent water as well as electrolytes. Hosing a horse down before and during rides is also a good idea; the more cooling he gets from the evaporative effects of the fresh water, the less sweat he needs to produce. A horse’s body requires time to adapt to changes in climatic conditions; if you travel to a hotter region, give him a week or so to adjust before resuming heavier workouts. Finally, when the forecasts call for high heat, especially when the humidity is also high, schedule your most rigorous workouts for the coolest hours of the day—or take the day off if you can.
This article first appeared in EQUUS issue #443.