Sweating the small stuff

As we move into the heart of the show season, temperatures and humidity in many parts of the country are on the rise. Replacement of fluid and electrolyte losses in the exercising horse becomes critical ...

OK, so you have heard the message countless times before.

You already know about the difficulty horses have in dissipating temperature and their poor ability to evaporate in humid conditions. You have read about the massive electrolyte losses in equine sweat and the need to replace sodium, potassium, chloride and magnesium. You know all about the correct ways to both prevent electrolyte imbalance and heat stress and to re-hydrate the equine athlete following exercise in hot, humid conditions. Or do you?

Weighing horses during competitions has allowed researchers to record more than 50 pounds of fluid lost by some athletes in hot, humid conditions. Some trained horses being supplemented with electrolyte solutions during exercise, however, have shown an ability to drink enough fluids to maintain body weight and reap the benefit of improved performance.

There may just be a few things about equine sweat and electrolyte balance in the performance horse that might surprise you. More and more research is being done in this area and the increasing popularity of endurance riding and the ever-growing number of other equine athletes working and competing in the summer is forcing us to do a better job dealing with heat and humidity. And that means learning more about equine sweat.

Losing heat

Exercising horses produce a large amount of both heat and sweat. The heat is the byproduct of muscular work, and the sweat is a physiologic adaptation that allows the body to dissipate or lose that generated heat. In ideal conditions, sweat is evaporated off the body surface and cools the horse.

Evaporation accounts for 65 percent of heat dissipation, with the heat transfer in exhaled air from the lungs accounting for another 25 percent and the last 10 percent taken care of by a host of less effective methods. Horses are not the most efficient animals at producing energy and getting rid of heat. It is estimated that only about 25 percent of the fuel (food, water and oxygen taken in) in the horse is transferred to work. A large amount of the remainder (about 80 percent) winds up as heat that must be eliminated.

Dr. Kerry Ridgway, a noted endurance veterinarian, addressed this tremendous heat production in a presentation he made as part of "On To Atlanta '96". This publication was the result of the FEI Samsung International Equine Sports Medicine Conference in Atlanta in 1994 as part of an effort to learn as much as possible about holding equine events in conditions of high heat and humidity. The Atlanta Olympic Games of 1996 marked the first time in the modern era that a significant competition was hosted in a location during a season that almost guaranteed weather extremes. The desire to be able to hold an event worthy of the Olympics while ensuring the safety and well-being of the equine athletes prompted this conference and the large amount of research that preceded the successful Atlanta games. In his presentation in that publication, Ridgway wrote, "To give you some idea of the incredible heat production occurring in the exercising horse, a horse racing at 13 meters per second (about 28 miles per hour) produces enough heat to bring 8 liters of water from room temperature to a boil. That is enough heat to bring more than 2 gallons of water to a boil after just a two-minute race.

Even more astounding, an endurance horse traveling at about 17 km/hr sweats approximately 12.5 liters every hour and produces enough heat in that hour to bring about 25 gallons of water from room temperature to a boil."

Sweating like a racehorse

Sweat and evaporation are the horse's main means of losing heat. Because of the horse's low body surface area to weight ratio, however, this task is a definite challenge. Humans have a three times greater advantage in this ratio compared to horses, and it turns out that when we say, "I'm sweating like a racehorse," we are probably not being very accurate. Humans sweat at a rate of 2 to 3 liters per hour, which cannot compare with the equine's rate of 10 to 15 liters per hour. This elevated sweating rate allows the horse to attempt to compensate for its poorer body weight to surface ratio and to try to dissipate more heat. Additionally, human sweat is hypotonic — it contains a lesser concentration of electrolytes than in plasma.

Equine sweat is hypertonic causing them to lose extraordinary amounts of electrolytes—principally sodium (Na), magnesium (Mg), Chloride (Cl) and Potassium (K). Another difference in species is seen in the response of horses and humans to exercise. Human athletes are much more efficient than horses in conditioning the sweat glands to respond to exercise. Conditioned humans adapt to conserve sodium (Na) and can drastically reduce the amount of that electrolyte lost during exercise. Horses can improve a bit through training and a reduction of Na from 140 mmol/lt. to 120mmol/lt. has been shown following 21 days of adaptive exercise in hot, humid conditions. Human training, however, can lower the Na level to 50 to 70 mmol/lt. Overall, fit horses and fat horses tend to lose just as much sodium in sweat and have similar requirements for replenishment. Many owners assume that their fit horses are better able to handle exercise in hot weather and may not pay as much attention as needed to the correct replacement of electrolytes that even these elite animals will require.

The volume of water and electrolytes lost by a horse during exercise is often surprising. An endurance rider, writing in Endurance News, weighed his horse during a mountainous 50-mile event held in North Carolina in March of this year. "Magic's starting weight was 912 pounds," writes Jim Holland. He continues to observe, "at the first vet check (16.5 miles) he had lost 74 pounds." This was a well-conditioned horse and Magic began drinking on trail so that by the end of the ride he had regained 14 pounds for a net loss of 60 pounds in 50 miles. Not all of that loss was water but estimates of 25 to 50 pounds of water loss are considered "normal" for horses exercising in the heat.

You can lead a horse to water ...

Most riders and horse owners assume that horses will simply drink what they need and replace these tremendous volumes naturally. This does not seem to be the case. Dr. Harold Schott and a group of researchers at the Veterinary Medical Center at Michigan State University have been looking into methods of replacing lost fluids and electrolytes in horses.

"Both human and equine athletes," Schott says, "fail to completely replace body fluid losses by voluntary drinking during the first few hours of a recovery period following exercise, despite free access to various solutions." This condition is called "voluntary dehydration" and has been attributed to blunted thirst. Management factors also enter into this rehydration problem as many owners and trainers often prevent horses from drinking after exercise or offer only a small initial drink immediately following work.

"This traditional practice has no basis in science," Schott writes, "but has been propagated over the centuries." Blunted thirst, however, has more to do with the actual science of drinking. Schott explains that "the primary stimulus for thirst is an increase in plasma tonicity." Because horses lose so many electrolytes in the vast amounts of sweat they produce, and because that sweat is much higher in electrolytes than plasma, horses often do not receive adequate signals to drink to rehydrate even after they have experienced significant fluid loss. This finding prompted Schott's group to investigate methods of making horses drink. Some of their findings, presented at the 2003 American Association of Equine Practitioners meeting in New Orleans showed that following high-intensity exercise (racing) most horses showed a dramatically increased respiratory rate that interfered with water intake. Also noted was the fact that most horses drank the same volume when initially offered fluid of whatever composition and that offering more fluid did not result in more intake during that time period.

The researchers concluded that this initial intake is related not to dehydration but to the size of the equine stomach. Once filled, the horse will not drink until the stomach was emptied. This may be the reason that the time-honored practice has been to offer a small amount of water to a recently exercised horse. The researchers agreed that this practice was not detrimental to the horse provided that additional fluids were made available to the horse in the 15-20 minutes following exercise. Schott's group also found that offering a solution of salty water (0.9% NaCl) initially following exercise resulted in a greater amount of water being consumed during the hour following exercise than if normal water were given immediately.

They encouraged this practice at rest breaks during exercise as well. Horses in these studies also preferred water near ambient temperature (20 degrees C) rather than cooler of hotter.

Unexpected findings

Research in horse sweat has produced some unexpected findings. Professor Malcolm Kennedy of the University of Glasgow has been looking at the protein content of equine sweat. He believes that equine sweat proteins may have applications in other fields. Horses are known to produce a large amount of protein in their sweat and unfit horses have higher levels of this protein than do conditioned horses. Professor Kennedy explains that this unique protein has detergent-like properties. It is this protein and its properties that makes horses lather-up during exercise.

"Horse sweat is unique," says Kennedy, "in that it contains a detergent-like protein that helps the perspiration to interact with the normally water repellent hairs of the coat." This detergent allows the sweat to spread over the body and enhances the surface area available for cooling.

Kennedy's interest in horse sweat extends to what it may be able to offer as a natural non-destructive detergent for environmental use. "A lot of trouble with oil spills is not the oil itself, which is bad enough, but the detergents that are put onto the slicks which can be very destructive," Kennedy writes. Researchers at the University of Glasgow are investigating the DNA code for equine sweat protein with the hopes of growing that protein in bacteria and easily producing an environmentally friendly detergent.

Researchers such as Schott, Ridgway, Kennedy and many others have provided us with increasing amounts of information concerning equine sweat, electrolyte loss and methods of replacing lost fluids in performance horses. As we continue to use horses in hot, humid conditions, we will need new and better strategies to compensate for their inefficiency in this type of weather. Most veterinarians recognize the need for electrolyte replacement during and after exercise. There are many brands of replacement products that can be given in the feed, the water or orally as a paste or slurry to help replenish the horse.

One must become familiar with these products in order to provide advice to clients. Simple electrolyte preparations can be made from items found at the grocery store (1 pound table salt, two10 oz. containers of lite salt, 2 tablespoons of Epsom salts mixed together and given at the rate of 1 to 2 oz. daily in feed) that provide a general balance supplement. Schott's recommendations of offering a small amount of salt solution immediately following exercise, followed by large volumes of room temperature water may be helpful to clients as well.