By Dr. Ken Brown and Dr. Seamans
It’s hot. Of course it’s hot, it’s summer. I love it when somebody says: “…but it’s a dry heat”. Yeah, so is a convection oven. I can suffer indoors by the air conditioner with a frosty beverage in my hand. My horses don’t have that luxury. However, we must remember that horses are big, strong animals with an astounding ability to adapt. The thermometer should not dictate our ability to enjoy our horses, but higher temperatures do demand more careful management to ensure their health and safety. Biologic systems are unbelievably complex, but we can acquire a basic understanding if we can grasp some simple concepts. Thermoregulation, or control of body temperature in exercising horses, involves water and salt and how they are lost, replaced, used and stored.
The physiology of both equine and human athletes have some similarities and some notable differences. We are all water-based, electric machines composed of 90% water. About half of the water is “free”, that which is easily lost or replaced. The rest of it is locked up in the various types of cells in the body. Virtually all of our physiology occurs due to the electric charge on specific minerals, (hence the term “electrolytes”) These are charged particles, or ions, of sodium (Na+), potassium (K+), magnesium (Mg2+), chloride (Cl–) and carbonate (HCO3–). They are essential for the function of muscle, nerve and blood tissue. The horse is basically a leather bag of salt water.
When it is hot, we sweat—although ladies from the South “glow”—it still looks like sweat on the rest of us. The reason for this is obvious: evaporation. This employs a basic physical property of water. As it changes from liquid to gas it takes energy in the form of heat with it, thus cooling the surface. There are some elegant mathematic equations that describe this, but you don’t have to be Albert Einstein to know a little breeze on a sweaty day feels pretty good. Electrolytes are also lost but, though this can become critical, the key is water. As the temperature goes up, the demand for water increases and the average horse can drink up to 50 gallons on a hot day.
Horse sweat may appear simple enough, but it represents some very complex physiology that is not well understood. When we think about life processes at a very basic level, the term “instinct” is a common part of the discussion. There are four of these that secure the survival of any species: eat, drink, protect, and reproduce. The motivating factors may seem obvious. You get hungry, you eat. You get scared, you run or fight. You want a baby…you get the picture. But why do we drink water? OK, thirst—but what drives this behavior? Dr. Harold Schott, professor of Equine Internal Medicine at Michigan State University has published some interesting research about how and why horses sweat and how they replenish water and salts lost in the process.
The old adage: “you can lead a horse to water…” is a mixture of myth and reality. Hot athletes sweat. This is a fairly efficient mechanism of removing body heat to the surrounding air. Logic would suggest that we would instinctively want to drink to replace the fluid lost in the process. While this does occur, it is not quite as simple as it appears.
Equine water metabolism is not just a sponge that can be squeezed dry or dipped in the creek to re-established hydration, so my “leather bag of salt water” analogy is a miserable failure here. It is a very complex system designed to conserve resources and to shift them to places where they are needed most. When we think of blood pressure, most of us imagine a measurement of the entire system. However, the body has the astounding capability of sustaining different pressure in several different systems at the same time. For example, fluids can be moved to supply enhanced requirements in selected tissue during certain activities. Your mom always told you not to go swimming right after you ate three chili dogs and half a gallon of ice cream. The reason for this is that, after a heavy meal, the body shifts blood to the GI tract to aid in digestion. Theoretically, this would shift blood away from skeletal muscle and impair your ability to swim, though there is little data to suggest drowning due to hot dog toxicity is a clinical entity.
This concept is also expressed in an exercising horse. The blood is shifted to skeletal muscle to supply energy and oxygen for the work of propelling the body forward. This happens due to neurologic pathways called the “autonomic nervous system”. This is an automatic system that controls much of our physiology—you don’t have to think about your heart and respiration, they function even when you’re sleeping. The tone, or pressure, exerted on blood vessels is part of this system and this is how the body shifts fluids from internal organs to areas with more pressing requirements, as during exercise, to skeletal muscle and skin.
When core temperature rises, more blood is shifted to the periphery (skin) for cooling. As the temperature continues to rise, the sweat glands are turned on so evaporation will improve the efficiency of the cooling mechanism. Although all athletes have a finite amount of water available for immediate use, and the ANS helps manage its movement for metabolism, the horse has an additional advantage.
The equine system has a fluid reserve around the gastrointestinal (GI) tract not found in other species. Fluid losses during endurance rides at distances of 25 or 100 miles are the same: ranging from 3% to 7%. Most of the body mass is lost during the first half of endurance competition, as replacement by eating and drinking appears to deter further losses. The fluid reserve in the GI tract of horses is about 10% to 12%, so the average, highly trained endurance horse will lose less fluid than his natural reserve. (Compare this with the 1% to 2% reserve in man.) This is part of why horses have such amazing stamina and can withstand a significant loss of water before true dehydration occurs.
Eventually, however, the horse will drink to replace water lost through sweat. This is often a challenge for horse owners—especially when they are away from home, as many horses are reluctant to drink in some situations. Why? In man, the level of electrolytes lost decreases with the duration of exercise. In the horse, a consistent amount of salt is lost regardless of the length of time they sweat. It appears that a drop in blood osmolality (pressure created by the presence of salt) is the primary stimulus of drinking behavior in horses. They won’t drink until their sodium (Na+) level gets low. In view of the GI reserve in horses, our stress about getting the horse to drink may be unwarranted in most cases. But this does beg the question about electrolyte supplementation.
Dr. Schott’s work indicates that horses drinking salt water prior to heavy exercise drank more plane water during recovery. However, this goes back to the initial concern about getting horses to drink at all. Interestingly, as the horse consumes plain water, this dilutes plasma Na+ levels which, in turn, diminishes the stimulus to drink. An oral slurry of concentrated salt solution (aka electrolyte paste) can stimulate horses to drink, but Holbrook et al. (2005) reported this could cause an increase in oral and gastric ulceration. Think about that. “Well, he’s drinking better, but now he has a gastric ulcer!” Decisions, decisions!
And, if that is not complicated enough, a study in South Africa demonstrated that human endurance runners recovered more quickly after a race by lying down and elevating their feet. This apparently aided the autonomic nervous system (ANS) in re-routing blood away from skeletal muscle and towards internal organs where the increase in blood pressure was needed. The improved recovery was not related to water or electrolyte intake, thus the ANS was actually more involved than was rehydration. It is interesting that a drop in systemic blood pressure has been recorded in some endurance horses after a race. (Some of the “natural horsemanship” systems include lying the horse down. Should this movement be incorporated in the training of endurance horses?) In another study, rehydration with intravenous fluids in volumes far less than the “calculated” requirement resulted in full recovery in horses. Thus it appears, as is seen in the human athletes, that the ANS is more a part of restoring physiology than was previously thought.
Although heat stress is most often seen in exercising horses, it can occur in horses housed in barns with poor ventilation or in paddocks without access to shade. Transporting horses in poorly ventilated trailers on very hot days is also risky. Symptoms can start with profuse sweating, elevations in respiration (greater than 40 breaths per minute), lethargy and loss of appetite. In severe cases, neurologic symptoms like stumbling, disorientation, and head pressing indicate heat stroke. Symptoms like these should be considered a life threatening emergency and veterinary help is imperative. Intravenous fluids and other treatments for shock should be started immediately.
There is a lot of mythology about horse care in general, and the management of overheated horses is no exception. In the recent thirty years or so, there have been some very good, carefully controlled studies about water and salt metabolism in horses that should have dispelled some of this lore, but some fables simply refuse to die. Here are just a few.
Never let a hot horse drink his fill, it may cause colic. False. A horse stomach holds up to two gallons, and the horse will stop drinking when he is full. The first sign of colic is when a horse stops eating and/or drinking, so he will stop when it becomes uncomfortable.
Never put cold water on a hot horse’s back. False. The best way to cool off a hot horse is with cold water and plenty of it. Keep him walking in the shade, this will increase the air flow over his body and improve evaporative efficiency.
Always use a blanket to prevent cooling from occurring too rapidly. False. After moving hay this summer, I have never once been tempted to put on a light jacket to cool off, I’m pretty sure my horse feels the same way.
Electrolyte paste will prevent dehydration. False. In fact, adding electrolytes (salt) above the daily requirement to the diet of the horse probably has no benefit. Dehydration, by definition, is a deficiency of water in the blood stream. Putting high concentrations of salt (electrolyte paste) in the stomach of a stressed horse may draw more water out of the blood stream and actually increase dehydration. This is why soaking a leg in water and Epsom’s salts will reduce swelling—the water in the inflamed tissues is attracted by osmosis (remember high school biology?) to the salt in the soaking solution.
Electrolyte supplementations for horses remains somewhat controversial. To date, there are no controlled, randomized studies clearly indicating salt-containing supplements, exceeding the recommended daily requirements, are beneficial to horses. The regular diet fed to horses will usually contain plenty of electrolytes. If a horse is seriously dehydrated, he will need intravenous fluids to replenish both water and electrolytes rapidly. This requires a large volume and a few hours to deliver it. For example, a 1000 pound horse (450Kg) dehydrated 10% will need 45 liters of fluids—that’s about ten gallons. Since the stomach of the horse only holds about two gallons…you can see the problem.
Heat stress can usually be prevented by using a little care and common sense when riding. Avoid riding during the hottest part of the day, usually from noon to just before sundown. Allow easy access to water, and take breaks from strenuous exercise. Stay in the shade. Add fans and misters to barn isles and stalls. When possible, haul after dark or in the early mornings.
Summer is a great time to enjoy our horses. With a little common sense, we can keep it safe in all kinds of weather.
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