Introduction/Causative Agents: Heat stroke is a problem that can occur with almost any horse on almost any day of the year. The problem is most often associated with hot days; however, a horse that is overworked on a cool day can also suffer from the effects of heat stroke. A horse develops heat stroke when its body temperature rises to 104 degrees Fahrenheit and remains there.
Heat and humidity, if too high, will break down a horse's ability to cool itself. Sometimes, if the animal is not in condition for what it is doing, it will overwork and subsequently, overheat. At other times, the environmental conditions are too much for a horse even in adequate condition for what it is doing. A horse can produce as much as 50% more heat than average when it is under intense exercise. Transporting horses in enclosed vans or trailers on hot days or stabling them in poorly ventilated stalls also invites the onset of heat stroke.
Clinical Signs: The first signs of potential heat stroke are related to "heat stress" that is not corrected. These animals will sweat profusely, breath rapidly, and have a high heart rate. If the "heat stress" is not corrected, then signs of true heat stroke will appear. These signs include hot dry skin, a heart rate that is very high, and accelerated respiratory rates. In working animals, one will note obvious attempts to stop working, loss of strength, trembling, stumbling, inattention and decreased urination. If the horse is resting, symptoms are different. The horse may refuse to eat, is restless or skittish and, again, has decreased urination. Take care to monitor a horse even after stabling it after a work out. Symptoms of heat stroke may not be visible until after the horse is stabled.
Because heat stroke causes such a change in a horse’s normal temperature, heart and respiratory rates, the following information is given to help determine when a problem exists:
Diagnosis: Observe the above mentioned symptoms. If any of the above items (temperature, respiratory or heart rates) remain elevated after the horse is rested, then the horse is suffering from heat stroke. A horse owner can also perform a pinch test to see if the horse is dehydrated. The skin of a dehydrated horse will not recoil immediately when pinched. See page B115 for details.
Treatment: Immediately attempt to cool the horse by providing fresh water to drink, a bath with cool (not cold) water, and place them in a cool, well-ventilated environment. Then call a veterinarian. If the horse's own ability to cool has completely shut down, it is important to try and cool the horse down in a fairly quick, but controlled fashion. For example, do not use icy cold water. Cool water is sufficient and must be used to help prevent the horse from going into shock. Allow the horse small drinks of water or any potable liquid that is available. A hot horse can suffer colic if it drinks too much water at once.
Place fans on the horse or some sort of ventilation to aid the horse’s own ability to expel heat. Air flow is vital for a horse even under normal conditions to cool off. Hose the horse down and apply cool packs to its head and neck. There are large veins in the legs, head, and neck that are near the surface of the skin. When these large vessels contact the cool water, heat will naturally be released from the horse’s body.
A sweating horse, just like humans, will lose electrolytes. This means that the electrolytes will need to be restored with either commercial or homemade solutions. Purchased electrolyte replacers may come in a tube or in a powder form. When mixing and administering these products, be sure to follow the label directions exactly. If access to a commercial electrolyte product is not available, some table salt plus "lite salt" can be used. Be sure to not mix the salt too concentrated or the horse may reject the water or feed. It is helpful to use molasses when trying to get the electrolyte mixture to adhere to concentrates. This is not the best option, but can encourage the horse to drink and replace some of the lost electrolytes. In severe cases of dehydration, a veterinarian may need to place an intravenous (IV) catheter and administer IV fluids. Details about fluid administration and electrolytes can be found on page E148.
Prevention: Make sure all horses have plenty of fresh water, a mineralized salt block and are in appropriate condition for their expected level of activity. Pay attention to the horse in hot weather, especially if it is muggy, and note if its behavior changes. Another method to prevent heat stroke is to use an effective temperature test. When the ambient temperature in degrees Fahrenheit and the relative humidity add up to about 150, use caution in exercising horses. Most long or intense riding activities should be postponed if that figure is around 180. For horses that are used in long, drawn-out rides or activities, electrolyte supplementation is very helpful. Furthermore, owners should never neglect a "cool-down" period, even if the effective temperature is below 150. The time it takes a horse to expel extra heat depends on individual situations.
Note: Some horses have little or no ability to sweat, a condition called "anhydrotic." Since the combination of moving air and evaporation (sweating) is the horse's main method to cool itself, anhydrotic horses must be especially watched for heat stress and stroke.