Water, the most important nutrient, is a part of all biochemical processes within a horse’s body. Normally, water intake and loss (breathing, sweating, urine, and manure) are in balance (IN=OUT). When deprived of water, the vital work of cooling, elimination of waste products via the kidneys, transport of nutrients to and waste products from the muscle, and function of the intestine are adversely affected. When water loss continues without replacement, the blood volume is diminished. When this happens ample oxygen and nutrients cannot be furnished to body cells, and carbon dioxide and waste products cannot be efficiently removed. Every organ of the body is affected, including the brain and nervous system.

Dehydration refers to the signs and symptoms shown by a horse that has not received sufficient water to meet its metabolic needs. Most of the characteristics of dehydration are not difficult to detect and do not require special diagnostic equipment and tests.

Dehydration Symptoms
Following are some indications of dehydration:

• Failure of the skin to return to a flat plane when pinched and held briefly at the point of the shoulder, then released. The skin normally flattens out in one second or less. Dehydration is indicated when it takes the skin two to three seconds to flatten; over four seconds indicates the danger level of fluid loss is being approached.
   

• Pressing the gum above the upper corner incisor tooth is another good test for dehydration. Press the gum using a finger and hold for a few seconds, then release and time how long it takes the color to return to the blanched area. Normal color refill time is one second or less, longer time indicates problems may be starting.
   

• Jugular vein refill demonstrates the effect of dehydration upon blood volume. A veterinarian can demonstrate how to obstruct return flow of the blood from the head to the heart by pressing on the vein in the jugular furrow. It should immediately fill with blood and “bounce” when it is stroked while being held off.
   

• Urine volume and color are indications of available water in the body for waste removal. If a horse is dehydrated, urine output decreases and the color becomes darker because it is more concentrated. Very dark, coffee-colored urine indicates the potential for a life threatening condition involving muscle degeneration and mineral imbalance accompanying dehydration.
   

• The color of the gums and the inside of the eyelid are helpful aids in evaluating the hydration status of a horse. The normal color of gums and inside of the eyelid is pink or pink with a very slight yellow tint. Under increasing stress and dehydration the gum and eyelid lining become a bright brick red. The next step is progression to a bluish color which indicates oxygen is unable to be distributed to the vital structures of the body and carbon dioxide levels are too high.
   

• The mouth becomes dryer as dehydration progresses and, in severe cases, sweating ceases.

Critical Dehydration
Sweating stops when dehydration reaches the critical level when the body attempts to preserve enough fluid to maintain life essential kidney function and enough blood volume for the heart to continue functioning. If dehydration is the result of muscle work and no sweat is produced, a very important cooling mechanism is lost and associated complications of rising body temperature can occur. The eyes become dry, dull, and sunken as tear production slackens. The respiratory rate increases, even to the rapid shallow characteristics of panting, which allows the body to cool itself by loosing heat through the lungs and by air passing over the many large blood vessels that line the nasal cavity. These physiological processes occur when the horse is too dehydrated to sweat efficiently, but must rapidly loose heat generated from exercise.

In progressive dehydration, the horse that is normally an eager, aggressive eater will refuse grain, hay, and even lush, tender grass. The animal’s body recycles water from the kidneys and the intestine by an intricate and complicated mechanism that is not able to function when severe dehydration occurs. As dehydration progresses, very dry fecal balls may be passed, sometimes covered with a thick elastic mucous that indicates inadequate water is present in the intestine for digestion to progress normally. This can lead to a condition called impaction where the manure in the large intestine becomes so dry and hard that it cannot be pushed through a section of intestine that is already weak from inadequate blood flow.

With dehydration, the blood literally becomes thicker as blood cells and proteins become concentrated due to loss of the liquid fraction. When this occurs, it is very difficult for blood to penetrate the very tiny blood vessels called capillaries, which are present in every body structure.

Horse Reaction
Another sign of severe dehydration is mental unpredictability or unresponsiveness—a result of inadequate circulation of blood through the brain. This can cause a normally gentle, quiet horse to be unpredictable and even dangerous to handle. Some horses become very dull, disinterested in their surroundings and other horses.

Causes of Dehydration
A number of situations can cause dehydration. The following situations comprise experiences over many years of veterinary practice. Cold weather may initiate dehydration when a horse consumes coarse, overly mature hay. The result may be development of impactions due to slow, weak intestinal activity combined with inadequate water intake or lack of access to water. When these factors are present, sufficient water is not available in the intestinal tract to keep the mass of hay soft and moving through the tract. This can be partially prevented by keeping free-choice salt and mineral available.

I also believe wet molasses mixed into a grain ration stimulates water consumption. Problem horses with a history of minimal water intake during cold weather may benefit from the addition of one level teaspoon of salt daily to the grain ration. Addition of more than this quantity of salt can be counter productive and cause more fluid loss than is intended. A few horses benefit from being allowed tepid water at least once daily when the weather is very cold.

One common cause of dehydration is simply water is unavailable to the horse. A spring or creek can run dry during a period of drought or freeze in unseasonably cold weather and the caretaker fails to check the water supply. Failure of automatic waterers is not uncommon and can go unobserved if installed in a position where they cannot be easily seen twice daily.

Another cause of dehydration is associated with electrical heating devices which malfunction due to stray voltage, short circuits, inadequate grounding, and rodent chewed wire, causing the horse an electric shock when attempting to drink. These dangers are easily overlooked because the horse will not repeatedly subject itself to the pain of electric shock. This situation can be fatal if a horse receives the full 110 volts while standing on damp soil, which is a good electrical ground.

Misjudgment is an occasional cause of dehydration. The mistaken belief that horses can eat enough snow to survive, and trusting people, especially children who do not have the responsibility and commitment needed to care for such a totally dependent animal, can put a horse at risk of dehydration. Some people fail to provide adequate horse care when the weather is unpleasant or when it is simply not convenient for them.

Another contributing cause of dehydration is failure to allow the horse to drink from creeks, ponds, ditches— all natural sources of water which many horses never encounter at home. Some horses have never drunk from anything other than a bucket or waterer, which has conditioned them to drink only from those water sources. It takes patience, but stop the horse after it has worked a bit and give it the opportunity to drink or just to play in the water. Horses learn by watching and being with other horses that are experienced drinkers from natural sources.

One courtesy that is so very important, but often neglected to the detriment of the horse, is not waiting until each horse has had ample opportunity to drink its fill when riding with a group. The herd instinct is so strong in horses that they will follow the group even though their need for water is critical. Consideration and good judgment are essential when riding with other people, you must not allow someone else’s ignorance or inexperience to put your horse at risk.

One sadly common cause of dehydration is the misinformed idea that a horse should not drink water when it is hot. It seems this myth has been around forever. Water is precisely what a hot horse requires to ensure the vital body processes will continue to function when it is hot due to exercise, high environmental temperature, high humidity, or a combination of these factors. The only horse discomfort I have observed is when the horse drinks a large amount of water from an ice cold source, like a spring. This requires the body to shunt blood from the skin and muscle to the stomach and small intestine to warm the near freezing water. This can cause momentary cramping while the water is warmed. It is extremely important to allow a horse to have all of the water it will drink at every opportunity, never let its tank “run dry.”

Water Founder Myth
Based upon many years of experience with distance rides and observation and participation in trail and field events of every variety, I feel that water founder is a myth. I believe founder/laminitis occurs not from a horse drinking water when it is hot or stressed, but from circumstances that occur during exercise. One circumstance is excessive trauma and concussion to the hoof, often on unfit or overweight horses. I believe the main cause of founder/laminitis is inadequate flow of blood through the hoof. This happens when the blood has been shunted to the skin and muscle for heat loss and waste removal, or the liquid content of the blood has been so depleted by sweating and rapid breathing that there is simply insufficient blood volume to supply all body structures.

Unfortunately, the lamina which attach the hoof to the coffin bone are very vulnerable to damage from diminished blood flow, even for an hour or less. Without blood and the oxygen it carries, part of the lamina starts to die, initiating a chain of events which may lead to separation of the hoof from the coffin bone. The resulting damage can cause severe pain and lameness. Recovery may take many months. The laminar damage starts during the exercise period, the horse drinks, and water is blamed.

I do feel water is part of the equation of exercise related founder/laminitis, but only because it was not supplied as needed and the resulting dehydration contributed to the damage. My experience has proven to me that no harm is done by allowing a horse to drink its fill. The risk is in limiting or withholding water from a dehydrated horse which is in dire need of it. My feeling is that one should never deprive a horse of water if it is willing to drink.

Exercise Associated Dehydration
Dehydration associated with exercise can be more devastating because there is an increased demand to nourish body cells and carry waste away from various organs, especially the muscle, and to help dissipate heat generated from muscle contraction. The muscle reacts to fluid loss and the electrolyte imbalance that accompanies it by becoming less responsive and less forceful during contraction. This is demonstrated by the horse trembling, by loss of animation and impulsion, by reluctance to move when given the command, and by stumbling and appearing incoordinated. Uncorrected, dehydration can put the horse at risk with muscle degeneration leading to azoturia (tying-up). The heart muscle can be adversely affected, resulting in abnormal rhythms with diminished cardiac output and, in some cases, circulatory failure.

Another dramatic example of dehydration’s affect on muscle is “thumps,” which occurs when the diaphragm starts to contract with each heart beat. Another potentially devastating result of reduced blood circulation to muscle is the build-up of heat within the body. Insufficient blood volume cannot sufficiently remove body heat by carrying it to the skin surface and the lungs where it can be eliminated. The build-up of body heat, which can cause the body temperature to rise to 106°F or 107°F, can cause damage to delicate tissues and to enzyme systems within the body that are vital to life.

Lack of Thirst
A perplexing situation occurs when an obviously dehydrated horse in distress and in immediate need of water will not drink when it is offered. The reason for this is the animal has lost minerals called electrolytes from the blood along with water via sweating. Thirst is stimulated as water is lost from the blood and the remaining minerals, especially sodium, become more concentrated in the blood. The blood becomes “saltier.” This signals the thirst center in the brain to cause the horse to drink. Consumption of water will dilute the salty blood back to normal.

If a horse will not drink, much sodium (which makes the blood salty) has been removed from the body via sweat along with water. Without sufficient minerals in the blood, the thirst center cannot be stimulated. Without this stimulation, dehydration cannot be corrected immediately by the horse drinking enough water to replace the deficit. If this situation occurs, immediate veterinary care is required.

Prevention
Conservation of water and prevention of dehydration can be facilitated by preventing excessive fluid loss using good horse management. Just tying a horse in the shade where there is a breeze can keep the horse cool. Feeding hay, especially moistened hay, with free access to water before trail and field events is helpful. The hay in the large intestine acts like a sponge and serves as a reservoir of water the horse can draw upon. Sponging water onto the skin of a hot horse lessens the fluid and electrolyte lost via sweating. Sponging is most effective where large dilated vessels are evident on the skin surface. The purpose of these large blood vessels is to carry heat to the body surface. Always ensure the horse has unlimited access to good-quality water.

A horse that is fit will sweat quicker when heat is produced by exercise because the mechanism that controls heat loss is more responsive. Generally, fit horses have learned to drink frequently to keep their fluid level constant. The thickness of sweat is more a function of duration of exercise than fitness level. The sticky mucous-like substance that makes sweat linger on the skin rather than quickly dripping off allows the body to cool by evaporation which is an effective mechanism of heat removal. When sweat becomes very watery, it only means the glands that make the mucous substance are exhausted at that time.

Conclusion
The rule should be to allow the horse to drink whenever water is available and schedule the performance or work period so that water is accessible. A horse that isn’t drinking sufficient amounts of water may eat hay soaked with water or lush, tender grass. Some horses will accept off-flavored water due to sulfur, iron, or chlorine content by adding a little gelatin to disguise the unfamiliar taste and smell. Determine which gelatin flavor is most acceptable to a horse before trailering the horse to events.

Water is truly the essential nutrient. If it is not available when and in the quantity needed, performance will be diminished and, if not supplied, serious life threatening consequences may result.

John H. Thomas, D.V.M., has an equine practice in Martinsville, Indiana, where he and his wife, Chris, also operate Springcliff Farm and Boarding Stables. His personal interest is in distance riding. Dr. Thomas manages and judges competitive rides throughout the midwest.

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