Preventing Heat Stress In Horses

On summer days, it can be difficult to distinguish normal fatigue and sweatiness from dangerous heat stress in horses. Here’s what to look for and how to safeguard your horse’s well-being.

By Heather Smith Thomas

Each summer we are reminded of an unfortunate fact: The very same warm, sunny weather we so much enjoy can be difficult for our horses to cope with. A horse must expend considerable energy just to keep cool as temperatures climb.

That doesn’t mean, of course, that you can’t enjoy most of your favorite equestrian activities even on relatively hot days. But you’ll want to take precautions and be vigilant in watching for the earliest signs of heat stress in horses. Extra care also needs to be taken to protect horses being transported or kept in dry lots or paddocks with little shade. Heat stress in horses can range from poor performance to potentially fatal heat stroke. And the progression from the first stage of trouble to the last can be swift. That makes it critical to recognize the signs of heat stress in horses and to intervene immediately when you spot them.

Stage 1: Dehydration

Anytime a horse sweats, he loses fluids first from his bloodstream, then from his gut and the spaces between cells. If he continues to perspire, fluid is eventually drawn from within the cells. If he is mildly to moderately dehydrated, a horse with access to water will usually drink enough on his own to recover. In some cases, however, overexertion, illness or severe dehydration prevents a horse from taking in enough water to compensate for his fluid losses. Two simple tests can be used to identify dehydration: To do the skin-pinch test, grasp a fold of skin at the point of the shoulder and release it. If the horse is well hydrated, the skin should snap back into place in less than a second. The longer it takes for the skin to flatten, the greater the level of dehydration. However, skin elasticity varies among horses, so you want to learn what is normal for your horse when he is rested and fully hydrated. For the capillaryrefill test, press gently on the horse’s gum just above an upper incisor and note how long it takes for the pink color to return to the blanched spot. Two seconds or less is normal; longer times indicate dehydration. 

 Other signs of dehydration can include

  • dark urine
  • a body temperature that is elevated by a degree or two
  • a dull expression or behavior.

When a horse works too long without replenishing fluids and critical electrolytes, he may develop synchronous diaphragmatic flutter (“thumps”) or exertional rhabdomyolysis (“tying up”).

Stage 2: Heat Exhaustion

Once a horse has started to dehydrate, his capacity to control his body temperature diminishes. If his main cooling system–sweating–begins to fail, and his body temperature reaches 104 to 108 degrees Fahrenheit, he enters the danger zone, where physiological changes start to occur. This is known as heat exhaustion.

“Enzymes that control the chemical reactions within the body are sensitive to heat,” says David Marlin, PhD, of Hartpury College in Gloucester, England, an equine exercise physiologist who helped prepare standards of care for horses competing in the 1996 Atlanta Olympic Games and the 2008 Olympic equestrian games in Hong Kong. “Enzymes are proteins, and if you start to heat proteins, they change their structure, which usually changes their function.” He adds that enzymes can return to their original structure and function if the heat was not too extreme or prolonged. Enzymes function best within a temperature range of about 98 to 104 degrees Fahrenheit.

Warning signs that a horse is approaching heat exhaustion include a reduction or cessation of perspiration or a thickening of sweat while the skin remains hot to the touch–all indications that his fluid reserves are running low. “This doubles or triples the threat and risk for heat stroke,” says Barney Fleming, DVM, who practices in Custer, South Dakota, and monitors endurance rides all over the country. The horse has run out of ways to cool himself, so his temperature will shoot even higher.

Other indications of heat exhaustion include:

  • a skin pinch that takes longer than six seconds to flatten or does not flatten at all
  • dark and/or discolored gums and mucous membranes
  • a capillary refill time of several seconds or longer
  • an elevated but weak and irregular pulse that remains high despite rest
  • diminished or absent intestinal sounds
  • depressed posture, with lower head carriage
  • rapid shallow breathing, called primary panting, or deep gulping air intake, called secondary panting, immediately after exercise. “The main reason a horse is blowing is to try to get rid of heat,” says Marlin. “Second-phase panting is slower and deeper, and there is clearly an effort. The horse is drawing in as much air as possible, trying to maximize respiratory heat loss. This is an easy way to recognize a hot horse.” A horse who is blowing at a rate of around 60 to 80 breaths per minute is struggling to cool himself. Primary panting is more likely to be seen in horses in dry lots or at endurance events. Racehorses and eventers are more likely to be seen using secondary panting, especially just after pulling up.

Sometimes, however, the signs of impending heat exhaustion are difficult to recognize: “The scary part is that a lot of horses have a tremendous amount of heart and will continue to work even though they’re distressed,” says Middleburg, Virginia, sports medicine expert Kent Allen, DVM, who served as a veterinary coordinator for the Atlanta Olympics in 1996. “Those horses will keep going but lose some impulsion. Where normally the horse’s ears would be pricked forward and he shows enthusiasm for the job, he may now be lop-eared and droopy, without a focus anymore. Don’t keep pressing the horse or he may keep going until he collapses.”

Stage 3: Heat Stroke

Heat exhaustion can advance to heat stroke quickly. If a horse’s body temperature rises to 106 degrees Fahrenheit for prolonged periods or if it increases to about 108 degrees for more than 15 minutes, the consequences can be serious. “If you heat enzymes too much, or to a slightly higher temperature for a long time, they may change irreversibly,” says Marlin. “Then a lot of the reactions that should be taking place in the body stop happening. This is heat stroke.”

The brain is extremely vulnerable to elevated heat and will be among the first organs to show effects. It contains a lot of protein and is the site of many crucial enzyme reactions that influence the entire body. “One reason we start to see neurological signs in people with heat stroke is due to these changes,” says Marlin. “They get headaches and balance issues, and we see these problems in horses as well.” Signs of heat stroke vary, depending on the nature and extent of any brain damage, but include

  • stumbling or difficulty moving at all
  • anxious, irrational or erratic behavior
  • depressed, disoriented or oblivious mannerism
  • collapse. “He may then go into convulsions, due to swelling and damage in the brain,” says Marlin. “These horses become comatose and may die. Other things that can happen if the horse doesn’t die from the heat stroke include development of associated problems like colic, laminitis, liver failure, kidney failure or tying up.”

If you suspect your horse is developing heat stroke, call a veterinarian immediately and take extreme measures to cool your horse quickly; his life is in serious danger.

Cooling Off

Halting the downward spiral of an overheated horse requires active intervention the moment you recognize the problem. The extent of the measures you need to take depends on how hot the horse has gotten. If he’s still alert, still sweating normally, and his rectal temperature reads 104 degrees Fahrenheit or lower, he is overheated but not in danger. “He just needs walking around, letting him drink, and some washing down with cool water,” says Allen. “It’s normal for a horse to heat this much while working.”

But, when a horse starts edging toward severe overheating, more extreme cooling measures are necessary:

• First, stop riding, remove the saddle and move the horse into the shade.Keep him walking, to encourage circulation that will bring more heated blood to the surface of the skin for cooling; if there’s a breeze, walk him in circles to expose him to the cooling air on all sides.

• Let the horse drink his fill as you walk him out.A hot horse needs to take in as much water as he wants to replace what he lost through sweating. And don’t worry about the temperature of the water. One myth that still crops up is the notion that letting a hot horse drink cold water will cause colic and muscle cramps. But there’s no scientific basis for that fear.

• Splash or spray cold water onto the horse to aid evaporative cooling.Another false notion is that putting cold water on hot muscles will constrict the blood vessels and lead to cramping; however, studies done in preparation for the 1996 Olympic Games in Atlanta failed to identify any ill effects from the practice. “We disproved the myth that if you put cold water over the big muscles the horse would tie up,” Allen says. 

“Cooling the horse with [room temperature] water all over the body is fine in a hot, dry climate,” says Marlin. The water will evaporate quickly into the dry air. “If it’s hot and humid, you need water that’s lower in temperature.” Add ice to buckets of water to cool it to as low as 40 degrees Fahrenheit before applying it to the horse. If the horse’s body temperature is edging upward into the danger zone—106 degrees or higher—douse as much of his body as possible with the coldest water available.

Continue to monitor your horse’s temperature as you walk and cool him. Within 10 minutes, you ought to see around a 2 degree Fahrenheit drop. Stop using the cold water once his temperature drops to 101 degrees Fahrenheit or lower, his respiration approaches normal and the skin on his hindquarters feels cool to the touch after a walking period. If the horse is not back to normal and drinking readily within an hour, then summon immediate veterinary assistance. He may need intravenous hydration and other measures.

Summer is prime time for riding, and the chance to spend a sunny day in the saddle is hard to pass up. But if you’re going to ride when it’s hot, plan ahead for cooling off–keep buckets of cold water beside the arena for your horse to drink or choose a trail that goes through shaded areas and crosses water–and be alert for signs of heat injury. With a few basic precautions, and a bit of attentiveness to your horse’s needs, there’s no reason why the hot days can’t be safe and comfortable for both of you.





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