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Dehydration and Electrolyte Losses in the Sport Horse

By: Dr. Melissa R. Mazan

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You can lead a horse to water but you can't make him drink. We're all familiar with this maxim that has long been used to underscore the apparent stubbornness of the horse. In the heavily exercising, or heat-exhausted horse, however, this refusal to drink has nothing to do with personality or temperament, and everything to do with physiology.

Dehydration occurs when your horse's body loses excessive amounts of water. Normally, both you and your horse lose body water on a continual basis, in the form of sweat, urine, and feces. However, on an ordinary basis, you replace these losses easily – your body tells you that you are thirsty, and you drink fluids. However, sometimes the losses are too great for the body to keep up with.

In the exercising horse, fluid loss occurs in the form of sweat. Early in dehydration, the horse can cope well with the fluid loss. Dehydration is estimated in terms of percentage of body weight that the horse has lost. It is very difficult to detect losses of 5 percent or less.

During severe dehydration, the skin becomes less elastic, which is seen with skin tenting: When you pull up a loose fold of your horse's skin, it takes a long time to return to its normal state; a horse with 10 to 12 percent dehydration, the skin fold takes 20 to 45 seconds to disappear. As dehydration progresses the heart rate rises, because there is less fluid in the blood vessels, so the heart has to pump the blood around faster to achieve the same effect. Your horse urinates less frequently, or not at all, and his performance deteriorates, as dehydration contributes to exhaustion. Eventually, with severe dehydration, your horse will no longer be able to perform, and may even collapse.

Electrolytes

At the most basic level, electrolytes are salts, such as table salt, that dissociate into separate ions when they are dissolved in water. Electrolytes are integral to nerve and muscle function, as well as to almost every other physiological function in the body. The most important electrolytes include sodium (Na+), chloride (Cl-), potassium (K+), calcium (Ca2+) and magnesium (Mg2+). These electrolytes are distributed throughout the body in a highly ordered way, and any disruption of this order can result in severe bodily dysfunction. For example, when the body's electrolyte levels are disturbed, your horse may have heart problems, his gastrointestinal system may not work properly, his muscles may develop cramps, and he may not even be able to think straight, because his brain may not function normally.

Your horse – or you or any mammal – is approximately two thirds water. So, for an average-sized 1000 pound horse, water accounts for 660 lbs of his body mass, which translates into 80 gallons of water. Of that 80 gallons, one third is extracellular, which means it is outside of the cells of the body in the form of blood and lymph fluid. Two thirds is intracellular, which means it stays inside the cells.

The electrolyte composition of the extracellular fluid (ECF) and intracellular fluid (ICF) is very different. The ICF is very high in potassium, whereas the ECF is very high in sodium. We use blood values to estimate whole body electrolyte composition, but they may be quite misleading, because they don't tell us about what is happening with the intracellular fluid.

All athletes lose water as sweat during exercise – it's part of an adaptation for getting rid of excess heat. As the sweat evaporates, your horse's body cools down. This is the primary reason why it is more difficult to exercise in high humidity – the sweat is still generated, but it doesn't evaporate, and thus doesn't cool effectively, so even more sweating is stimulated.

Equine sweat glands are different from human sweat glands, with the result that horses lose more electrolytes during sweating than humans do. Horses lose large amounts of both sodium and chloride in their sweat, with smaller losses of calcium and potassium. Water losses are high during exercise – up to 10 liters per hour. The rate of water loss will depend on the intensity of exercise as well as the ambient temperature.

Your horse's body gets the signal that he is thirsty when one of two things happens: either the blood volume drops or the sodium concentration in the extracellular fluid increases – that is, it gets concentrated, or salty. Horses get the thirst signal more slowly than humans, because in humans, sodium is less concentrated in sweat, so sodium becomes more concentrated in the blood, and the thirst signal goes out quickly.

In horses, the sweat glands are very poor at conserving sodium so even though the horse has lost a large amount of water and sodium, the signal does not go out for the horse to drink until a serious drop in blood volume occurs due to dehydration. Consequently, even though your horse is clearly dehydrated, when you lead him to water you can't make him drink. He's not stubborn – his body is just not giving him an early enough warning signal.

What to Watch For

After a challenging cross-country phase, a few chukkers of polo, or an endurance ride, your horse is likely to be quite dehydrated. Losses tend to occur early in the ride, even though your horse may look normal. Signs may include:

  • Dry mucous membranes
  • Sunken eyes
  • Fatigue
  • Colic
  • High heart rate
  • Respiratory rate that doesn't come down with proper cooling-out measures

    Your veterinarian will look for:

  • Weak pulses
  • Delayed filling of the jugular vein, which is the large vein that runs the length of your horse's neck
  • Skin tenting
  • Poor capillary refill time (CRT). In order to determine the CRT, your veterinarian will press the horse's gum lightly to cause blanching of the color, and determine how long it will take for the color to return. In the normal horse, this takes no more than 2 seconds.

    As dehydration proceeds, the horse may eventually collapse. In addition to the dehydration, your horse is also likely to have a whole body loss of electrolytes. Signs of low electrolyte levels may include

  • Nervousness
  • Fatigue
  • Muscle tremors
  • Stiffness

    Researchers at Washington State University have recently found out that the body's hormones still work to recover lost sodium the day after intense exercise – even though the horses shows no outward signs of electrolyte depletion.

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