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Heat Stress: Too Hot To Trot?
By Dr. Jenifer Nadeau, Equine Extension Specialist, University of Connecticut on Coalition to Ban Horse Drawn Carriages
During hot summer weather, heat can be a concern for horse owners. Horse owners need to provide extra care during hot weather to decrease stress and maintain the health and well being of the horse. Heat and humidity affects the horse, and in severe cases, can result in death. Heat production estimates can increase as much as 50% during periods of intense exercise as compared with heat production when the horse is at rest. Only about 25% of the energy used in the performance horse's working muscles is converted to actual movement; the remaining 75% loss of efficiency is represented by waste heat that is difficult for the horse to dissipate in hot and humid weather.
Normally, the horse cools itself by sweating. Heat is lost and the body cools as sweat evaporates from the skin's surface. Less moisture evaporates in times of high humidity, however, and this causes the cooling mechanism to become less efficient. Prolonged exposure to high temperatures results in dilation of surface blood vessels. When dilation occurs without an increase in blood volume, circulatory collapse may also occur. The cooling mechanism of the horse is most effective when the sum of the ambient (outside) temperature and relative humidity is less than 130. Efficiency of cooling decreases between 130 and 150. When the sum of the ambient temperature and relative humidity is greater than 150, the horse's ability to cool itself is greatly reduced. When the sum is greater than 180, horse owners need to be cautious, since these conditions could be fatal if the horse is stressed. Some horses are anhydrotic, meaning they have little or no ability to produce sweat. These horses are prime candidates for heat stress.
Some common terms for overheating in the horse are hyperthermia, heat exhaustion, heat cramps, heatstroke or sunstroke. Hyperthermia or overheating in the horse is due to a disturbance in the heat regulating mechanism of the horse's body that can result from hot weather, high humidity, poor stable ventilation, prolonged exposure to direct sunlight, overwork, transport and obesity. Some signs of overheating include muscular tremors, profuse sweating and collapse. Dark urine, dull expression and behavior, dark mucous membranes, and slow capillary refill are also signs of overheating or dehydration. Normal vital signs in the horse are a temperature of 99.0 - 100.8° F, pulse rate of 32-44 beats per minute, and a respiration rate of 8-16 breaths per minute. Heat exhaustion will cause the temperature to rise to 105-109° F, the pulse rate will rise to 50 to 100 beats per minute, and the respiration rate will rise to more than 30 breaths per minute. Heat cramps are most commonly found in horses doing hard work in intense heat that are sweating profusely. These occur due to a loss of electrolytes (severe salt loss). Heatstroke or sunstroke is more critical. Horses undergoing prolonged hard or fast work during hot weather, horses exposed to direct sunlight without shade, young, poorly conditioned horses and horses with long hair coats are susceptible to heatstroke or sunstroke. Signs of heatstroke or sunstroke are rapid breathing, weakness, incoordination and refusal to work. Temperature is increased to 106-110° F, sweating stops and the skin dries. Delirium and convulsions may also result. Death can occur within a few hours if the horse is not cooled and does not receive emergency veterinary care. Treatment for overheating depends on the condition that occurs. In cases of heat exhaustion, the horse should be sprayed with cool water and moved to a shady area or cool, well-ventilated barn. The same treatment would be used for heatstroke as well as placing ice packs on the horse's head and large blood vessels on the inside of its legs. Horses with heat cramps should be cooled, rubbed down, and given electrolytes. Other management suggestions for overheated horses include using fans, allowing the horse to have a few swallows of cool, clean, fresh water every few minutes, and calling the veterinarian, because the horse may need to receive an intravenous injection of fluids. Electrolytes should be replaced after physical exertion, since sodium, potassium, calcium and chlorine are lost in the urine and sweat. Loss of electrolytes can lead to metabolic problems, a decrease in the thirst response and loss of interest in eating and drinking which leads to more problems. A simple source of these electrolytes is feeding a 2-ounce mixture of 3 parts lite salt (potassium chloride), and 1 part limestone on a daily basis. This is also a good source of sodium.
Prevention of overheating includes limiting strenuous riding to late evenings or early mornings when the temperature is lower, providing adequate ventilation in stables, properly conditioning horses, removing blankets or sheets from stabled horses during extreme heat, using fans in the barn or stall, providing shade for all outside horses, clipping horses with long hair, sprinkling the aisle of the barn with water to aid in cooling in areas of low humidity, transporting horses at cool times of the day and providing them with adequate ventilation, and providing plenty of clean, fresh water. At 0° F, the horse will drink about 1 pint of water per pound of dry feed consumed and this increases to 1 gallon of water per pound of dry feed consumed at 100° F. The skin pinch test can be used to determine if a horse is properly hydrated. When a section of skin on the neck or shoulder is pinched, the skin should recoil in 1-2 seconds in a normally hydrated horse. A delay indicates some dehydration. Walking a hot horse is also important, because it protects against placing a horse in an area void of airflow where sweat will not undergo convection and evaporation. One study examined the effect of feeding 10% corn oil (a fat source) and 3% soybean meal instead of 13% of the cracked corn ration in the control diet. Using fat as an energy source produces less metabolic heat for the energy produced when compared to carbohydrate as the energy source - this results in the horse having less heat to dissipate.
Four common myths have also been found in equine publications of the years past. They are 1) "never let a horse drink more than one or two swallows of water at a time" 2) "never give ice-cold water to a hot horse either inside or out," 3) "never let a hot horse cool out without a blanket or sheet" and 4) "never let a hot horse cool out in a drafty area." Each of these misconceptions prevents the overheated horse from cooling out the way nature intended. Although allowing a hot horse to consume unrestricted amounts of water may lead to problems such as colic due to hyperdistension of the stomach, a horse's stomach can hold between 2 and 4 gallons of fluid without being distended, so one or two sips of water at a time is overly restrictive when the hot horse is rapidly losing water to try to keep itself cool. The horse should be allowed to have a few swallows of cool, clean, fresh water every few minutes. The second myth has been the source of controversy over the years because certain horse trainers believed that ice cold water placed on a hot horse's body will "shock" the horse's thermoregulatory system into shutting down blood flow to the skin. This belief has been found to be wrong based on extensive research conducted during 1995 at the University of Illinois and University of Guelph and at the 1996 Olympic Summer Games in Atlanta. Researchers proved conclusively that horses working under hot and humid conditions were better able to maintain core body temperature within an acceptable range or even reduce it and also reduce heart rate during rest periods after intense phases when ice water baths were used. Horses also trotted more freely after the baths. Liberal application of icy cold water to the overheated horses helped to dissipate heat not only by providing more water to evaporate from the skin, but also by direct conduction of the horse's body heat into the water that runs off the horse, carrying excess heat with it. Blocking the evaporation of water from the skin by using a blanket or sheet in hot weather is also a very bad idea in hot and humid conditions. Finally, restricting access to moving areas during hot and humid conditions also makes little sense. Regular dry fans work to increase evaporation and dissipate heat by the cooling process known as convection. Misting fans take advantage of the additional cooling property of blowing water onto the horse when it is changing phase from liquid to gas.
As a knowledgeable horse owner, you are capable of preventing overheating from occurring. Prevent this condition by following the tips mentioned in this article. Know how to recognize the signs of overheating and what to do when the signs occur. Be aware of the myths regarding overheating treatments and share correct information with others. Please call (860) 486-4471.
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