(1998) UNDERSTANDING WATER BALANCE AND STRATEGIES FOR HYDRATION IN SLED DOGS ...Arleigh J Reynolds, DVM, PhD, Cornell University, Ithaca, NY The late Dr Roland Lombard is often credited with being the first competitive musher to realise the importance of watering his dogs during the racing season. During his day, most mushers believed that their dogs got all the water they needed by just eating snow. The dependance of good performance upon good hydration is just one of the legacies left behind by the innovative and dominant force that was Doc Lombard. Over the past 25 years, racing and research have demonstrated the benefits of providing working dogs with sufficient amounts of protein, fat, vitamins and minerals. Still, a working dog may tolerate a dietary deficiency in one of these nutrients for several days or even weeks before any adverse effects on its performance or health are observed. In contrast, dehydration may lead to diminished performance and, in severe cases, even to death within hours of its onset. To understand why dehydration leads to such rapid and severe performance problems, one must first understand the many vital roles this important nutrient plays in the maintenance of nearly all body functions. A typical healthy dog has a total body water content of about 70% of its body weight. This water is divided into 4 compartments. Most (65%) of the total body water (TBW) resides within the individual cells of the dog's body. This water is the solvent in which all reactions of the cell take place. It thus facilitates the generation of energy, the synthesis of new cell materials, the storage of products and the detoxification of wastes. Intracellular water is also the medium of transport for all materials within the cells. The extracellular water is divided up between three spaces. The interstitial space (20% TBW) is the second largest compartment; it represents the water which lies immediately outside of the cells and bathes them. The main role of the interstitial water is as a transport medium for nutrients and other materials into, and wastes and other products out of, the cells. The plasma space (10% TBW) is the water found in the liquid or non-cellular part of blood. This water transports materials between all locations of the body. The transcellular compartment (5% TBW) is a conglomerate of all other extracellular spaces and is made up of the water found in the eyes, the joint fluid, the cerebral-spinal fluid (CSF), and the secretions of the gastrointestinal tract. This water acts as a medium for light passage in the case of the eyes, and as a source of lubrication and shock absorption in the joints and CSF. In the GI tract it is a solvent for digestion and a transport medium to facilitate absorption of digested nutrients. Under normal circumstances, water is free to shift between these compartments. The direction of that shift will depend on the conditions to which the body is exposed. During exercise for example, the metabolic changes occurring within the muscle cell increases the concentration of solutes or dissolvable particles within these cells. This increase in solute concentration causes water to move into the cell from the interstitial fluid. This loss of interstitial fluid volume is then replaced by the movement of water from the plasma compartment into the interstitial space. The result of exercise is thus an expansion of the intracellular compartment and a contraction of the plasma compartment. This fluid shift is in part responsible for the increased size of a weight lifter's muscles after a work out. They literally are pumped up. The small loss of plasma volume that normally occurs during short bouts of exercise does not usually adversely affect performance. If exercise is prolonged, or if a dog is losing significant amounts of water through other sources, the loss of water from the plasma may lead to a potentially dangerous contraction of plasma volume. As plasma volume diminishes, the heart has to work harder to circulate the blood because there is less fluid travelling through the vessels, and that fluid is more viscous or sticky. The result of these changes is a decreased delivery of oxygen and nutrients to and a slower rate of waste removal from muscle cells. In this situation working muscle cells have less fuel available and accumulate wastes more rapidly, a combination which restricts the sustainable intensity and duration of exercise. In severe cases plasma volume contraction can lead to major organ failure and even death. Dehydration is almost always easier to prevent than it is to treat. Still, early recognition of the problem gives the dog the greatest chance for a rapid and complete recovery. A dog's hydration status can be accurately assessed by a few quick observations. A well hydrated dog should have moist pink gums and it's capillary refill time (CRT) should be less than one second. CRT is measured by timing how long it takes the white spot created by pressing a finger on the gum to return it it's full pink colour once the finger is removed. Another simple test is to lift the skin over the withers up into a tent with your thumb and forefingers and see how long it takes the tented skin to flatten back into it's normal position. In a well hydrated dog the tent should disappear within 1-2 seconds. In mild to moderate dehydration where the dog has lost 3-7% of his TBW, gums will be slightly tacky, CRT may take 2-3 seconds, and tented skin can take 3 or more seconds to flatten out. After 10% of TBW is lost, dehydration becomes severe. In these cases the gums get very dry, CRT will take at least 3 seconds, it may take 5-10 seconds or longer for tented skin to flatten, and the eyes sink noticeably into their sockets. As dehydration progresses from the mild to the moderate and severe states, the animal's ability to correct the problem on it's own diminishes. Anyone unfortunate enough to have one or more dogs in their team suffer from advanced dehydration knows that these animals usually refuse to eat or drink. Such animals need veterinary attention immediately since loss of only 15% of TBW usually results in death. Correction of moderate and severe dehydration usually requires intravenous administration of fluids. Mildly dehydrated animals may be able to restore their water deficit by drinking but will often recover more rapidly if at least some of the fluid is replaced parenterally. To prevent dehydration, a musher must try to balance his or her dog's daily water loss with their animal's daily water intake and production. To achieve this goal one must first understand how water is added to and lost from a dog's body, and how each of these components of water balance change with changes in the dog's workload, environment, and health status. A dog may add to its total body water through its diet, by drinking water, and by burning fuels for energy in it's muscles. When you feed a dog a meat diet or a dry dog food soaked in water, 70-80% of what that animal eats is actually water. Water that is taken in as part of food is called "performance water." Many sled dogs get half or more of their daily water intake from preformed water. Most of the rest of their daily water intake comes from drinking water and eating snow. A small amount of water is also generated when fats, carbohydrates, and proteins are converted to energy in the muscle. For each 100 kcals of energy burned about 13ml of water is generated and for each gram of muscle glycogen used, 3-4 ml of water are produced. Water produced in the body by these processes is referred as "metabolic water." Metabolic water may contribute as much as 10% of the total water gained by a dog each day. Usually, the amount of water gained each day by a dog is exactly balanced by the amount lost. Each day a dog loses water. Through it's urine, faeces, saliva, breath, and sweat. Unlike humans and horses, dogs do not lose much water due to sweating. In fact, the only place a dog sweats is through it's foot pads. Evolutionary biologists believe that dogs may have retained the ability to sweat through their foot pads because it gives them a superior grip when running on smooth surfaces like ice and rock. The dog's inability to sweat from the rest of it's skin probably stems from it's large surface area to volume ratio. Water loss from such a large surface area would put the animal at constant risk of dehydration if it perspired from it's entire skin surface. In larger animals like humans and horses, a relatively small surface area and large volume inhibits heat dissipation. Dehydration due to sweating is less of a risk than heat accumulation in these larger animals. Most of the water a dog loses each day leaves it's body through urine, faeces, respiratory vapor, and saliva. The contributions of each of these factors depends greatly upon the dog's health, environment, workload, and diet. For example, consider the same 20 kg (44 lb) sled dog as a couch potato house dog, a sprint racing dog, and a distance racing dog. Each day the house dog, living in a climate controlled environment, loses about 1000 ml of water through urine, about 100 ml of water through its faeces, and about 300 ml of water through evaporation of respiratory water and saliva. If we move that dog outside and he becomes an open class sprint racing dog, he will lose about 1500 ml of water through his urine and 15 ml of water through his faeces. Assuming an ambient temperature of at least 0 degrees Fahrenheit, this dog will also lose about 300 ml of water from evaporation during a one hour run and about 800 ml of water from evaporation during the remaining 23 hours of the day. If this dog now becomes a distance racing dog his water loss to urine and faeces will increase to about 2250 ml/day and 250 ml/day respectively. Assuming he works 12 hours at about a 40% of VO2 maximum workload and rests 12 hours in an ambient temperature of 0 degrees Fahrenheit or below, he will lose between 2000 and 2500 ml of water during exercise and about 400 ml of water during rest to evaporation from his mouth and respiratory tract. As you can see the combination of exercise and living in a cold environment dramatically increases the dog's daily water requirement. The increase in this requirement is about two fold for the sprint dog and about four fold for the distance dog as compared to the house dog. The greater losses of urine and faecal water seen in working dogs are mostly due to their increased food intake. A greater food intake leads to an increased production of faeces which are usually 80-90% water. More food also means the generation of more iona and more metabolic wastes which must be filtered by and excreted from the kidneys. The excretion of these additional wastes results in an increased urine volume and consequently a greater urinary water loss. The most remarkable increase in water loss observed in working dogs is due to the increase in evaporation from the mouth and respiratory tract. Depending on the dog's exercise intensity, and the environmental temperature and humidity, evaporative water losses may increase 10-20 fold during exercise. At cold temperatures, the air a dog breathes in has very little moisture in it. When this cold air reaches the lungs it is saturated with water so that about 6% of every exhaled breath is water. In warm climates the inhaled air is more nearly saturated with water and so the dog loses less water from it's lungs with each breath. However, since dogs pant to cool themselves off, water loss through the evaporation of saliva often leads to evaporative losses in warm conditions equal to or in excess of those seen in cold environments. The numbers given above are estimates for specific cases but they give an idea of the influence that exercise and environment have on a dog's daily water requirement. Health problems may also greatly influence daily water loss. Urinary water losses increase dramatically in renal disease, systemic infections, diabetes and other hormonal abnormalities. Most of these dogs are sick enough that they would not be able to perform as sled dogs and would require veterinary attention. Increased water loss from the gastrointestinal tract is more common and often less serious. Nearly all kennels experience stress diarrhoea and the "flu" during the course of a season. Often, dogs will continue to perform well with these conditions as long as their hydration can be maintained. However, the rate of dehydration resulting from severe diarrhoea, as in the case of parvovirus infection, can be a life threatening situation. The severity of the situation can usually be assessed by the frequency and volume of fluid eliminated. In any case the fluid lost through the faeces must be replaced or the dog's health will deteriorate rapidly. The factors which contribute to water loss in the dog are complex and constantly changing. If we had to exactly calculate a dog's daily water requirement in order to hydrate it properly it would be nearly an impossible task. Fortunately nature takes care of that for us. As a dog's plasma begins to lose water, the increase in concentration of salts is sensed by a part of the brain which triggers the dog's thirst. Since it takes some time for water to be absorbed into the plasma, the quenching of a thirst does not immediately rely on the return of salt concentrations to normal. Instead, stretching in the stomach and a drop in throat temperature are the signals that lead to thirst satiation. This system is so well tuned that a healthy dog will adapt to changes in water loss just as quickly as these changes occur. Theoretically the dog will drink as much as it needs as long as water is available when he is thirsty. Therein lies the problem of keeping sled dogs well hydrated. They are not always thirsty when water is available, and water is not always available when they are thirsty. This is where the musher comes in. It is our job to anticipate the dogs needs and to make the water attractive enough so that he will drink it when it is available. As any marathon runner will tell you, if you wait to drink until you are thirsty, you cannot drink enough during a race to rehydrate yourself. Successful runners drink prior to and in the early stages of the race, before they become thirsty. For sprint teams this means watering well in the morning of a race and if the weather dictates warm temperatures watering a small amount (500 ml) 20-30 minutes prior to the event. For distance mushers this means more frequent water breaks when temperatures are extremely warm or cold. Baiting the water to make it more palatable has long been a successful way of getting dogs to drink when water is only available for short periods of time. Water is the most essential, yet often the most overlooked, of all nutrients. Each year most mushers spend a lot of time figuring out what and how much food they should feed their dogs. We must remember that training and racing change a dog's water requirements at least as dramatically as their requirements for energy, protein, vitamins and minerals. Dogs at highest risk are those living and exercising in relatively cold and dry, or relatively warm and humid conditions. Certainly prevention of dehydration is the preferred method of treatment. When prevention fails the severity of the dehydration and the rate and completeness of a dog's recovery depends upon the musher's ability to quickly recognise and treat the problem. Although the dependence of success on good nutrition is well established, it is generally even more important to the health and success of a team that it's dogs are well watered than that they are well fed.