Horse Heatstroke and Exhaustion
A horse asked to perform strenuous exercise often is pushed to the limits of his body's mechanisms to recover. In most cases, these mechanisms allow the horse to finish the exercise with no problem. Sometimes, however, the horse's ability to recover may be inadequate and will result in possible heatstroke or exhaustion. This means that more than one organ system might stop functioning properly, such as the muscles, kidneys, central nervous system, or clotting system. Without prompt intervention the horse might suffer irreversible damage.
When the sum of outside temperature plus the relative humidity is below 130 (e.g., 70 F with 50% humidity), most horses can keep their body cool. The exception will be very muscular or fat horses. When the sum temperature and humidity exceeds 150 (e.g., 85 F and 90% humidity), it is hard for any horse to keep cool. If the humidity contributes over half of the 150, it compromises the horse’s ability to sweat. When the combination of temperature and humidity exceeds 180 (e.g., 95 F and 90% humidity), the horse’s cooling system is ineffectual and very little cooling takes place even if the horse is sweating profusely. At this stage, exercise can only be maintained for a short time without the animal’s body temperature— especially in the muscles— rising to dangerous levels. When the horse’s body temperature has reached 105 F, the blood supply to the muscles will begin to shut down. After this occurs, the blood supply to the intestines and kidneys will also shut down. The blood supply to the brain and heart are spared until last, but severe and permanent damage may have already taken place.
The body maintains its normal temperature in hot weather by moving heat through the muscles and out through the skin. Blood also removes heat as it circulates through the body and releases it through lung tissue, skin and expanding blood vessels. This is why blood vessels may appear larger and more distended during hot weather. This dilatation and resulting perspiration serves to cool the skin and body as the sweat evaporates. Horses that cannot sweat will usually overheat and heatstroke very rapidly, even in cooler weather with a small amount of exercise.
The chance of overworking a horse (exhaustion) rapidly increases when any of the following conditions exist: heat and humidity, poor fitness, obesity, the presence of another disease or lameness, high altitude, rough or steep terrain, rider inexperience, or if the horse has the inability to sweat. Exhaustion is noted by fatigue and inability to cool himself alone. In contrast, heatstroke resembles a horse tying up and or in shock, but can resemble a horse with exhaustion when they collapse. An exhausted horse might be distressed and anxious. He might have a high heart and respiratory rate that does not decrease with rest and his skin might feel hot and dry. Signs of shock with heatstroke, however, include pale, dry mucous membranes, increased capillary refill time, increased jugular vein fill time, a weak, irregular pulse, and no gut sounds. Some horses become stiff and experience pain due to muscle cell damage, which can be detected by observing red or brown urine (hemoglobin and/or myoglobin uria). Horses affected this badly might go down or develop other, often life-threatening conditions such as laminitis, kidney failure, or diarrhea. A badly effected horse also might appear wobbly or demented. Signs of heatstroke may include any of the above including the following:
- Temperature as high as 105 to 107 F
- Rapid breathing, rapid pulse
- Stumbling, weakness, depression
- Refusal to eat or work
- Dry skin and dehydration
- In severe cases, a horse may collapse or go into convulsions or a coma
It should be noted this list of clinical signs could also be present in other disease conditions such as; “tying up” which might be any one of the following: “exertional -rhabdomyolysis” (ER) chronic or sporadic, “polysaccharide storage myopathy” (PSSM) a congenital condition, or “recurrent exertional rhabdomyolysis” (RER).
Treatment is aimed at lowering body temperature and restoring electrolyte imbalances as well as controlling and monitoring organ damage that might have occurred. When possible, place the animal in the shade. A breeze can be added with a fan, in order to cool the body. Ice packs, alcohol or cold water from a hose will cool down the blood as it circulates through the body. It acts as the “antifreeze” and cooling system as it circulates. Avoid icing the large major muscles of the loin and hind legs. These muscles are already lacking blood circulation and it may make the condition worse. You may ice the forehead since the brain contains the temperature control center for the body, and this will help to cool the horse. Strong efforts should be made to lower the rectal temperature below 100.5 degrees Fahrenheit. Small amounts of water should be provided to re-hydrate the horse. Electrolytes may also be given orally. In severe cases intravenous fluid therapy is necessary to treat dehydration, electrolyte loss and shock.
Your veterinarian may also need to administer other medications directed at other organ systems that might be involved. Another source of heat is feed. All feed releases heat as it is digested. For example approximately 50% of the energy in oats is released as heat. This is why small frequent meals are preferred to large meals prior to strenuous exercise. Therefore, it is probably wise to avoid feeding a large concentrate meal before a competition. Instead, small amounts (one to two pounds) of concentrate can be fed one to two hours before the event and at each rest stop (if this applies to your sport). In events that span more than one day, these small amounts of concentrate should be fed every few hours until competition begins again.
Keep in mind that strenuous exercise on a hot, humid day can lead to problems in a short period of time for even the best-conditioned horse. Horses with heavy muscling or excess fat or in poor condition will have more problems. Based on the conditions listed above, there are a number of ways to help prevent exhaustion and/or Heat Stroke in the horse. For example, horses should be thoroughly prepared over the same type of terrain over which the event will be held. If the event involves transporting them to a hot, humid climate (Florida or south Texas in August for example), they should be given at least three to seven days to get used to the conditions in the new location. Being able to identify and assist an exhausted or Heat Stroked horse--as well as knowing how to prevent the condition--might save your horse or the horse of a friend or competitor. Consult your veterinarian for other ways to bring your horse home from your particular competition in the best physical condition.
Your veterinarian might recommend a specific electrolyte paste, top-dress, or mixture be added to a second bucket of water just before and during exercise to protect against electrolyte losses. Horses should have free-choice access to water (and roughage) during training, before the event, and at each period of rest. It generally is accepted that horses (and humans) do not voluntarily drink enough water or take in enough electrolytes to completely prevent dehydration. Therefore, every effort should be made to replace fluid and electrolyte losses during the overnight portion of events.
Dr. Preston Hickman