There are a number of physical responses that occur in the a mammal’s body when it is exposed to heat. It is important to not only understand what thermoregulation is, but the physiological and/or anatomical thermoregulatory responses that allow sustained exercise in horses.
Thermoregulation is the control of body temperature within certain limits even when the surrounding temperature is very different. This enables the body to function effectively and is known as maintaining homeostasis, which is a dynamic state of stability between an animal’s internal environment and its external environment.
A relatively constant body temperature is necessary for the efficient functioning of the complicated brain of higher animals. Extreme temperatures alter biological molecules and disrupt body functions resulting in illness such as hyperthermia or hypothermia, which if not treated can lead to death. Mechanisms have subsequently evolved in mammals to enable body temperatures to stay within certain limits.
All mammals are endothermic meaning they maintain and regulate their own body temperature. Mammals and birds maintain a constant body temperature which is usually above the environmental temperature, known as homeothermic.
Adapting to the Environment
Mammals live in a number of widespread environments around the world, forcing them to face daily and seasonal fluctuations in temperatures. Some mammals live in harsh environments, such as arctic or tropical regions, and must withstand extreme cold and heat. In order to maintain its correct body temperature, a mammal must be able to produce and conserve body heat in colder temperatures, as well as dissipate excess body heat in warmer temperatures. Some mammals have adapted to their environment by increasing their surface area in the extremities, such as large ears on the Zebou cattle.
Animals that are exposed to the cold have heavier organs, and their skin color is dependent upon the amount of radiation they are exposed to. In colder climates, fat under the skin provides mammals with necessary insulation. Due the to surface to volume ratio, a large animal has the advantage over smaller animals since less skin is exposed to the elements.
Surviving the Heat
While fat is necessary in colder temperatures, it is also crucial for mammals living in warmer climates. The Zebra cattle deposit fat deep in the body to aid greater heat tolerance, while camels deposit fat in their humps.
In warmer climates, excess body heat can accumulate and cause life-threatening problems for a mammal, making it important for the body to dissipate heat. This is accomplished when circulation near the skin’s surface releases heat into the environment, and when moisture from sweat glands or respiratory surfaces evaporates and cools the mammal. In dry regions where water loss is dangerous for mammals, evaporative cooling is less effective, forcing the mammals to seek cover during the hotter daylight hours, and resume activities at night. Smaller animals have an advantage over larger animals because the larger surface to volume ratio allows for greater heat loss.
A Delicate Balance
A mammal’s body temperature results from a balance between production and loss of heat. In this balance, heat is constantly produced and lost, and production and loss of heat will be equal, resulting in the temperature remaining constant.
The hypothalamus is responsible for the regulation of a mammal’s body temperature. This temperature control requires sensors, a control center and effectors. The two types of sensors which respond to hot and cold are found throughout the body in the skin, body core, and brain. The control center is in the hypothalamus of the brain, and acts as a thermostat which has a temperature set point. The effectors produce more heat (increased metabolic rate, shivering, brown fat metabolism), and change heat loss (blood vessel dilation or constrictions, erection of hair, curling up, sweating).
The skin is primary organ for removal of metabolic heat by cooling the body through the sudoriferous (sweat) glands. There are two types of sweat glands- the apocrine, which secrete pheromones, and the eccrine, which are tubular, secrete sweat, and are found over the entire body of most farm animals. Approximately 90% of body heat is lost through the skin, and if the body temperature is too high, the skin can dilate blood vessels, increasing blood flow by 150 times. In cold temperatures, the skin constricts blood vessels in order to reduce heat loss.
Heat loss is by radiation, conduction, convection and sweating. Sweating can be used to lose enormous amounts of heat, as the sweat glands which are activated by the sympathetic nervous system release secretions on the skin surface. If the ambient temperature is higher than the body temperature, sweating is the only way heat can be lost.
Physiological and Behavioral Mechanisms
There are a number of physiological mechanisms involved in temperature regulation. These mechanisms include dry heat loss, radiation, and convection, which are dependent on environmental factors, evaporation, excretion and panting.
Behavioral mechanisms are concerned with the heat gains and losses in animals and include convection, evaporation, and radiation.
Types of Thermoregulation
There are two types of thermoregulation that are used by animals- physiological regulation and behavioral regulation.
In physiological regulation, an organism changes its physiology to regulate body temperature, such as sweating to cool the body and shaking to create heat and warm the body.
In behavioral regulation, an organism changes its behavior to changes in body temperature, such as finding shade when the body gets too hot in order to cool down.
Dangerous of Heat
If there is a failure of temperature regulation in hot regions, an animal can suffer from heat exhaustion and heat stroke. Heat regulation failure can be triggered by loss of fluids, since animals can lose up to 1.5 liters of water an hour as sweat. If the fluid is not replaced, the blood pressure will fall, heat regulation failure will occur and there will be a rise in body temperature.
Heat exhaustion is characterized by wet, cool skin from sweating and is treated by replacing lost water and salt. Heat stroke is characterized by dry, hot skin and is treated by rapid cooling of the body. Heat stroke is a failure of the sweating mechanism, is very dangerous and can be life-threatening. The body temperature rises so fast during a heat stroke that brain damage and death may result if the body is not cooled quickly.
Heat Stress on Horses
Heat stress occurs when heat production exceeds evaporative capacity of the environment or evaporative mechanisms become impaired due to great loss of body fluid and reduced blood volume, making it necessary to cool the entire body.
A horse that is suffering from heat stress may be distressed, tired and unwilling to continue exercising, lethargic, blow hard continuously, or stand without showing interest in surroundings or grazing.
Since heat stress can lead to heat stroke, anyone exercising a horse should know the warning signs of heat stress as a means of preventing the life-threatening condition. It is crucial to provide extra care and attention to a horse being exercised on hot days, since there is a significant increase in the amount of heat produced by working muscles. Heat production can increase as much as 50% during intense exercise as compare with heat production when the horse is at rest. A horse increases its sweating rate to move more blood to the capillaries under the skin and breaths much harder in an effort to release this build-up of heat. The most commonly observed signs of ‘heat stress’ are profuse sweating, rapid breathing, and a rapid heart rate. Some horses have a condition leaving them little or no ability to produce sweat, and are referred to as ‘Anhydrotic’. Since heat loss is mainly dependent on sweating and its evaporation, anhydrotic horses are prime candidates for heat stress.
If a horse begins to exhibit the signs and symptoms of heat stress, a vet should be called, the horse should be moved to a shady area with ventilation, the legs and lower body should be sprayed with cool water to lower the internal temperature, and in critical conditions, ice packs or cold water soaked towels should be placed on legs and other areas that exhibit large veins on the horse. If a horse is showing signs of heat stress, it should not be given large quantities of water, since a ‘hot’ horse has the chance to colic.
To determine marginal water loss in a horse, a pinch test is performed. When a section of skin on the neck or shoulder is pinched, the skin will recoil immediately in a normally hydrated horse, however skin recoil will be delayed in a dehydrated horse.
Fluid Loss and Electrolytes
Sweat not only contains water, but salts which when broken down into their chemical components are known as electrolytes, such as sodium, potassium, chloride, magnesium and calcium. These components each carry an electrical charge which allows electrolytes to govern the transfer of water through cell membranes into or out of the cells. If a horse sweated out these electrolytes in the same percentage in its sweat as in its circulating fluids, the body’s concentrations of these electrolytes would remain in balance, even though there would exist a depletion of both total fluids and electrolytes. However, this balance is not maintained, and a horse’s sweat is more concentrated in electrolytes. If there is not a minimum electrolyte replacement, a horse can suffer from ailments such as kidney impairment, cardiac arrhythmias, and poor tissue perfusion.
Heat Exhaustion and Olympic Horses
In 1992, at the Barcelona Olympic games, several horses suffered from heat exhaustion, which lead to concern about the welfare of the horses at the 1996 Atlanta games, due to the hot and humid climate. While there were talks about modifying or canceling the equestrian events, the Animal Health Trust in Newmarket, England set up a project in order to make recommendations on transport, acclimatization periods and modifications to the competition to ensure that horses were not placed under unnecessary stress. Prior to departure, all countries competing were circulated with detailed recommendations relating to preparation (including diet, training, cooling and pre-acclimatization), transport and subsequent acclimatization in the U.S.A. There was also additional advice on management of horses and riders before, during and after competition (e.g. rehydration, cooling, etc.).
The effectiveness of these measure was to a large extent dependent on acceptance, interpretation and implementation by individual teams and was therefore essentially optional. There were other measures implemented to ensure the welfare of the animals including air-conditioning of the Atlanta quarantine station, well ventilated stables, portable shade from mesh screens, fans and a large covered warm-up area. The Three-Day Event had changes made to its format including an earlier starting time, a reduced distance, cooling stops on the second roads and tracks and an extended ’10 minute box’ to allow more time for cooling and monitoring the horses.
Decreased Exercise Capacity
A research team for the Atlanta Olympics found horses have a marked reduction in exercise capacity in extreme heat and the optimum acclimatization period is 10 to 14 days. The team made this determination after exposing thoroughbred horses to high heat and humidity and then measuring various physiological responses. The main criterion was how fast the body temperature increased, and if it was slower at the end of three weeks, it was indicative of better heat dissipation.
The researchers also compared acclimatization responses in horses and humans to determine if they were similar. They found that after 20 days of acclimatization, humans have an increased exercise capability through an integrated series of physiological responses. Horses and humans are the only mammals that primarily use sweating as a cooling mechanism, and while horses sweat at two to three times the human rate, they have less surface area over which to lose it.
Three Weeks of Acclimatization
It was recommended that horses participating in the 1996 Olympic games spend three weeks in Georgia prior to competition in order to acclimate to the hot temperatures and high humidity of the region. The horses were fed grain and fat, which have a lower heat increment than fibrous feeds such as hays, to assist them in the hot climate.
The efforts were rewarded by only minor, non-heat related problems with horses competing in the Dressage and Show Jumping competitions, with no cases of anhydrosis (loss of sweating). Two-thirds of the horses which started the Three-Day Event Speed and Endurance test finished in both team and individual competitions, which is considered normal. Examination of the body temperatures of horses throughout the day showed that horses that started early, finished with very similar temperatures to those that went late, providing good evidence that the effects of the climate on the horses had been greatly reduced.
Cold Water Cooling
Cold water cooling is a method that, if it is applied correctly, can cool down hot horses rapidly after competing, thus speeding recovery times and reducing the chances of heat stress. Horses that are hot (above 40 degrees C) and competing in hot environments (above 26 degree C) and are cooled quickly during or after competition, are less likely to suffer heat stress, will recover more quickly, will not become as dehydrated and are almost certain to perform better.
The cold water cooling technique cools horses using two of the three ways they normally lose heat- convection and evaporation. The technique is started as soon the horse finishes exercising, while taking the animal’s rectal temperature. Cold water is applied to the horse, with special attention to the large muscles in the quarters, which get extremely hot during movement. It is crucial to continuously apply water to the animal for 20-30 second periods, and then walk the horse for 20-30 seconds in order to promote blood flow to the skin and cooling by convection, as the air movement aids cooling by evaporation. The rectal temperature should be periodically taken, and should fall about 1 degree C. In 10 minutes. It is important to provide the horse with water during exercise in order to help with the cool down process and reduce the effects of dehydration. The process should be stopped if the rectal temperature is less than 38-39 degrees C, the skin over the quarters is cool to touch after a walking period, the respiratory rate is less than 30 breaths per minute and if the horse shivers continuously. It is important to concentrate on cooling as much of the body surface as possible with cold.
There are other things which should be considered when exercising a horse. A horse should not have an excessive amount of grease applied to its body prior to a cross-country event, since grease acts as insulation, limits or prevents sweating, and limits sweat evaporation. A horse should not be allowed to stand still for prolonged periods. Since water is rapidly expelled from the stomach, the horse should be allowed to drink small amounts of water during competition, and water should be left in the stable until 15-30 minutes before exercise. Muscles work more effectively when they are warm, so horses need to be warmed up prior to exercise. While a moderate increase in body temperature is not harmful, a horse will warm up faster in hot weather.
A horse must be properly cooled-down following exercise work-outs, and because the built up heat must be removed through respiration and sweat, air flow is vital. Walking a hot horse allows the air movement to continue to help evaporate heat, but if the horse is left standing still, the lack of air movement could force the internal temperature to rise higher.
When a horse is engaged in exercise, only about 25% of the energy used in the horse’s working muscles is converted to actual muscle movement, while the remaining 75% is represented by waste heat that becomes very difficult for the horse to dissipate in hot and humid weather.
While radiation of heat from a horse’s body into the atmosphere is a potential mode of heat dissipation, it most often works in the opposite direction during sunny days, with horses
(especially dark ones) gaining radiant heat from the environment.
The single most effective means a horse has for getting rid of the enormous heat load generated during exercise is evaporation, accounting for 65% of the heat dissipation, with the lungs accounting for about another 25%.
“While horses have a sweating capacity that is almost double that of humans, the low surface area-to-mass ratio of the species imposes great demands on the thermoregulatory system during moderately intense exercise. Selective brain cooling occurs in the nasal cavity of the horse and serves to cool venous blood. When a horse sweats, the temperature declines by 2.5 C,
and dissipates 60% of total heat produced (McConaghy 1994).”
Thermoregulation is essential for a mammal to survive in extreme hot and cold conditions. Horses and humans are the only mammals which sweat to control body temperature, and understanding thermoregulation is extremely important when it comes to exercising and cooling down horses in various climates.
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