Heat Stress Prevention

Editor's Note: Dr. Godman, DVM, MS, graduated from Auburn's vet school 1993 and received his Masters at University of Kentucky's Gluck Equine Research Center. He currently works for the Kentucky Horse Racing Authority.

Nice weather has finally arrived. After months of cold weather, it's a nice sunny weekend and you're ready to saddle up and ride! But, is your horse ready? We can simply take off a coat when it warms up, but what does your horse do?

It is important for horse owners to understand some basic principles about equine thermoregulation before working their horses. Mammals must maintain their body temperature within certain limits in order for their organ systems (cellular machinery, enzymes, etc.) to work properly.

During cold weather, the horse relies upon its thermoregulatory system to conserve heat. It does so by decreasing blood flow to the skin, increasing insulating fat layers, and thickening the hair coat--ll of which makes heat loss to the environment more difficult.

During hot weather, the same system must transfer heat to the environment to prevent excess accumulation. This is accomplished by acclimation to the warmer temperatures. Acclimation is basically reversing the mechanisms of conserving heat-eveloping increased vascularization to the skin, decreasing fat layers, and reducing the hair coat. These contribute to the evaporative cooling of sweat, which is the primary means of cooling in the horse. Additionally, horses experiencing larger heat loads (be it from excessively hot weather or strenuous exercise) must also expand their blood volume to accommodate sweating. This is accomplished by hormone release in response to exercise.

All of these physiologic changes take time--days to weeks depending upon the degree of workload and fitness level. During this "acclimation" period, the horse's body shifts from conserving heat to releasing heat and any introduction of excess heat predisposes the horse to "heat stress."

As horses exercise or work, energy is produced by muscle metabolism, and approximately 80% is released as heat. Under normal circumstances, this heat is released to the environment without consequence. However, when the temperature is high, the workload is excessive, or the horse is not acclimated, heat load might create a problem. Under those circumstances, the horse might not have adequate blood volume to supply working muscles and blood vessels to the skin while maintaining perfusion to the vital organs. When vital organs don't receive enough oxygen, their function becomes compromised.

The clinical manifestations can include poor performance or inability to follow commands. In serious cases, it might lead to erratic behavior or even collapse. This condition is known as "heat stroke" and results from lack of oxygen to the brain. Resulting behavior will depend upon which area of the brain is affected and what degree of hypoxia exists. Other organ systems are also likely deficient in blood flow, but they are not as susceptible and seldom show obvious clinical signs.

Significantly decreased blood flow to tissues can result in lowered oxygen levels. Cellular functions such as energy production can be seriously compromised. Under these hypoxic conditions, waste products might accumulate that damage cells and cause their desired function to be temporarily altered.

Counteracting Heat Stress
Treatment is aimed at restoring blood flow to the brain. Obstacles to heat loss such as saddles, pads, blinkers, etc., should be removed immediately and the horse should be liberally doused with cold water. This causes vasoconstriction of the blood vessels in the skin and allows blood volume to be returned to the core circulation. External cooling should be continued until the horse has fully recovered. If the horse is down, ice water is more effective at peripheral vasoconstriction, but that might not be necessary for less severe cases. In severe cases, consult your veterinarian to see if your horse has fully recovered or if any other organ systems (such as the kidneys) have been affected.

Prevention of heat stress involves properly acclimating your horse to exercise in warm weather. Moderate exercise causes heat from muscle metabolism to be released into the system and induces the thermoregulatory system to set into motion the physiologic changes mentioned earlier. Fit horses are able to adapt more easily to these changes.

In Summary
Allow the horse to be properly acclimated. Heat stress can occur even in very mild temperatures if the horse's body is still set to conserve heat instead of releasing heat.

Have the horse athletically fit. Fit horses are able to adapt to temperature changes much easier.
Use proper husbandry and nutrition leading up to an athletic event. Animals should be on a proper plane of nutrition and health care. The hair coat might need to be clipped if there is a sudden increase in temperature just prior to your event. Vaccinations should be addressed as infectious disease can reduce the horse's ability to thermoregulate. Consult your veterinarian on which specific vaccinations best suit your situation.

Use proper husbandry after an athletic event. Give the animal the proper opportunity to cool down following the event. This might require external cooling such as hosing and scraping to go along with hand walking and watering.


About the Author

Robert Lee Godman

Robert Lee Godman, DVM, MS, graduated from Auburn's vet school 1993 and received his Masters at University of Kentucky's Gluck Equine Research Center. He currently works for the Kentucky Horse Racing Authority.

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