Endurance riding is increasing in popularity every year. The sport offers opportunities for just about every level of competitor, from the rider who likes to enjoy the trail, those seeking to find their personal best, and those who aim for first place.

What is an Endurance Ride?

Endurance rides typically range from 25-100 miles. Rides less than 50 miles long are called limited distance rides and are a fast growing segment of endurance riding. Rides are divided into divisions based on the weight of the rider for adults and an open division for junior riders 16 years old and under. Competitions are sanctioned by the American Endurance Ride Conference (AERC). Horse welfare is paramount to the AERC. They have established strict veterinary checks at the start of and throughout the ride in order to ensure that all horses are fit and able to continue. Many studies have been conducted on endurance horses in order to determine what happens to them physiologically during extreme distances and what we can do in order to keep them healthy and safe.

What happens to the horse during an Endurance Ride?

One of the most noticeable things that occurs when horses exercise and especially when they exercise for a long period, is that they sweat. Sweating is designed to help cool the body from the tremendous amount of heat that is produced during work. As the sweat evaporates from the skin, it causes a decrease in temperature. Equine sweat is made from not only water, but also the electrolytes sodium, chloride, calcium, potassium, and magnesium, and proteins. The proteins are what make horses “lather.” As a horse becomes more fit, the amount of protein loss decreases. Unfortunately, this does not happen with the electrolytes. There is a negligible decrease in the amount of sodium lost as a horse gets fitter, but a fit horse loses about the same amount of sodium as a fat horse. And horses lose a lot of electrolytes in their sweat, much more than humans do. In fact, their sweat is hypertonic, that is, it has more electrolytes in it than does their blood.

We often say “Boy, I am sweating like a pig!” but we really should be “I am sweating like a horse.” Horses lose up to 5-7 liters of sweat per hour in cool temperatures and can lose up to 10-12 liters of sweat/hour in hot, humid conditions. An average 1000 lb horse normally consumes between 40-50 liters per day. In hot weather, he can lose all of that water intake in just a few hours of exercise.

So why is the loss of water and electrolytes in sweat so important?

Water loss will decrease the plasma (water) component of the blood, which in turn causes a decrease in blood volume and blood pressure. This can lead to decreased perfusion of the tissues and organs of the body. As a result of the decrease in blood volume, the heart will have to work harder and faster to pump the smaller volume of blood throughout the body in order to maintain proper oxygen levels. Dehydration then starts a competition for fluid between the cardiovascular system and the thermoregulatory system. When fluids are trying to be conserved for blood volume there is a decrease in blood flow to the skin and a decrease in sweat and evaporative cooling. Hyperthermia and hypovolemia can become severe and occasionally fatal.

The loss of electrolytes can cause a whole host of disturbances in the body. Electrolytes help in many of the homeostatic processes within the body, including maintaining proper nerve conduction, proper muscle contraction and relaxation, maintaining normal body pH, and regulating body temperature. Loss of electrolytes can affect muscle cells and cause cramps and tying-up. Cardiac muscle can also be affected and result in cardiac dysrhythmias (irregular heart rhythms). The smooth muscle of the gastrointestinal tract can be affected and result in colic. One unusual condition associated with electrolyte loss is synchronous diaphragmatic flutter, commonly called “thumps.” Chloride loss in sweat can cause metabolic alkalosis, an increase in the blood pH. This in turn causes an increase in the protein binding of ionized calcium in the blood. Ionized calcium is needed for nerve function. When it is decreased due to increased calcium binding nerve irritability increases. The phrenic nerve runs from its origin from the spinal cord at C3-5 and passes through the chest (near the heart) and then innervates the diaphragm. When the phrenic nerve is irritated due to low ionized calcium and then gets stimulated by the beating of the heart muscle, it will fire with each heartbeat, causing the muscle of the diaphragm to contract. We see this as the “thump” of the diaphragm.

Exhausted horse syndrome is one of the most dangerous consequences of electrolyte and water losses. This occurs in horses that are exercised beyond their fitness level or in environmental conditions (hot, humid) that prohibit proper cooling and discourage water intake. Clinical signs include elevated temperature, pulse, and respiratory rate; depression; anorexia; unwillingness to continue to exercise; dehydration; weakness; stiffness; hypovolemic shock; exertional myopathy; synchronous diaphragmatic flutter; atrial fibrillation; diarrhea; colic; and laminitis.

Treatment includes stopping exercise; rapid cooling; rapid large volume intravenous or oral fluid administration; and nonsteroidal anti-inflammatory drug administration. Occasionally, death can ensue. Some horses show obvious signs of problems, such as collapse, unwillingness or inability to move, or outright colic. Others may be on the cusp of a metabolic problem but are not showing signs yet.

How is a horse with a potential problem identified?

Veterinarians have developed the Cardiac Recovery Index to help identity these horses. This is an evaluation of the horse’s metabolic status. The horse’s resting heart rate is taken. He is then trotted 125 feet out and back. The pulse is taken again exactly one minute from the beginning of the trot. Failure of the heart rate to recover to or below the original value indicates potential problems. This horse should be carefully monitored so that treatment, if necessary, can be instituted as quickly as needed. As you can see, there is a complicated and delicate of balance between electrolyte levels and normal bodily functions.

How can we avoid these issues?

Studies have been conducted to understand the effects of exercise and environmental conditions on electrolyte levels and hydration in horses. The 1996 Olympic Games in Atlanta were the impetus for many of these studies. Equine veterinarians were concerned about what toll the hot, humid temperatures of summer in the Southeastern U.S. would have on the equine athletes. These studies sparked many others that have helped all horses involved in endurance sports. Drs McCutcheon, Geor, et.al conducted a study that showed that training or competing in hot, humid conditions caused losses in sodium, chloride, and potassium in sweat that exceeded the daily dietary intake of these electrolytes. This suggests the need for appropriate supplementation.

So it would seem that the answer during an endurance ride would be to make your horse drink more water and give him electrolytes. But it is not that simple. It has been found that horses don’t always drink well after exercise. This is due to heavy breathing, their relatively small stomach size, and a blunted thirst response. Once a horse takes a drink and fills his stomach, he will not drink anymore until his stomach empties. Thirst is driven by a decrease in blood water volume and an increase in blood electrolyte concentration. Horses lose their thirst drive when they sweat excessively because they lose so much electrolytes in the sweat, thus there is no increase in blood levels to stimulate drinking. Dr. Harold Schott or Michigan State University found that giving a horse electrolyte water (0.9% sodium chloride) immediately after exercise will stimulate later thirst in the next hour post exercise. This will improve electrolyte levels and help to hydrate a horse better than giving plain water. The water should also be at ambient temperature. Horses preferred this to either warm or cold water. This brings up the old wives tale about not letting a hot horse drink. There is no science to support that allowing a horse to drink after exercise causes colic, laminitis, or any other problems. More likely, withholding water will result in these problems.

One caveat that was found during these studies was that it is not a good idea to give a horse electrolyte pastes prior to or during exercise if you know that he will not drink. If your horse ingests large amounts of electrolytes without proper water intake, he can develop a whole host of metabolic disturbances due to too high levels in the blood. Dehydration will also be exacerbated.

Electrolytes are essential for life. It is important to know how they affect the systems of the body and what can happen if there are disturbances in their levels. Knowing when and how to supplement electrolytes for the endurance horse can help to make the finish line an easier place to reach.

Registered 2011 by Equestrian Collections
Author:  Sallie S. Hyman, VMD, DACVIM, CVA

Information in this article is for educational purposes only and is not a substitute for evaluation by an equine professional. In particular, all horse owners should seek advice and treatment from a licensed veterinarian, such as TEVA, for their horses' medical care.

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