Symptoms Give Warning of Heat Stress in Horses

By Donald Stotts

STILLWATER - Daytime temperatures for parts of Oklahoma already have hit 100 degrees Fahrenheit or greater, and that means horse owners will need to monitor their animals closely. Heat is produced as a normal by-product in the daily metabolic processes of horses, said Dave Freeman, Oklahoma Cooperative Extension Service equine specialist.

During exercise, there is a significant increase in the amount of heat produced by working muscles. Heat production estimates can increase as much as 50 percent during periods of intense exercise as compared with heat production when the horse is at rest. In response, a horse increases its sweating rate, moves more blood to the capillaries under the skin and increases its rate of breathing in an effort to release this build-up of heat.

"Heat stress from exercise can result when the environmental temperature is high," Freeman said. Commonly observed signs of heat stress are profuse sweating, rapid breathing rate and rapid heart rate. Furthermore, some horses are anhydrotic, meaning they have little or no ability to produce sweat. Since heat loss is mainly dependent on convection (wind) and evaporation (sweating), anhydrotic horses are prime candidates for heat stress.

Freeman said heat stroke can progress rapidly from heat stress if work intensity, environmental temperature, humidity or anhydrosis overloads the horse's ability to cool itself.

Symptoms include skin that is dry and hot, pulse and respiratory rates much higher than normal and unusually high rectal temperatures. "Heat stroke is life threatening," said Freeman. "The owner should call an equine veterinarian immediately." Freeman said the horse should be moved to a shady area with fans or wind to provide ventilation. Cool water should be sprayed on the legs of the animal's body to help the evaporation process.

"In critical situations, ice packs should be placed on legs and other areas that exhibit large veins surfaced on the horse," Freeman said. Veterinarians normally will give large amounts of fluid to the animal, and possibly give cold water enemas or drenches if the core temperature is extremely high.

"Normally, a horse's rectal temperature is around 101 degrees Fahrenheit," Freeman said. "The critical temperature, one that is characteristic of a life-threatening situation if maintained for any length of time, is around 104 degrees Fahrenheit."

The best recommendation is for equine owners to know how to identify heat stress in a horse before it progresses to heat stroke. Relieving the horse from exercise and cooling the animal's body by fans and shade will help stop the onset of heat stroke. "Also, care must be taken that the horse doesn't become dehydrated during long bouts of exercise," Freeman said. "Large amounts of fluid can be lost through sweat."

Freeman said the long-accepted practice of limiting drinking water to exercising horses has little scientific backing. "Generally, horses should be allowed to drink as frequently as they desire, even during periods of exercise, unless they are showing definite signs of heat stress," he said.

A hot horse may colic if given large amounts of water; since horses should not drink large amounts when they are hot, riders should offer small amounts of water to the horse in frequent intervals before, during and after exercise.

A simple test that can be used to determine marginal water loss in a horse is the pinch test. When a section of skin on the neck or shoulder is pinched, the skin recoil will be immediate in normally hydrated horses. Dehydration will delay skin recoil.

Another practical test is the "effective temperature" test, used to help determine the environmental conditions most likely to result in heat related illness in an exercising horse. This test combines ambient temperature with relative humidity.

"When the sum of the ambient temperature in degrees Fahrenheit and the relative humidity is around 150, the rider should use caution in exercising the horse so heat build-up doesn't become critical," Freeman said. Most riding activities involving long or intense exercise should be postponed when figures approach 180.

Finally, it is important not to overlook cool-down periods following exercise bouts, even when environmental temperatures are well within normal parameters, Freeman said. "Large amounts of heat build up in a horse during work," he said. "This heat must be released from the horse's body through respiration and sweat."

Heat loss through sweat requires convection and evaporation. Freeman said the commonly used practice of walking a hot horse guards against placing it in an area void of air flow.

"Air flow is vitally important for convection of heat off the horse's body," Freeman said. The length of cool-down procedures will depend on the amount of work, the environmental conditions and the individual horse. Freeman said horse owners who use these simple procedures and who know the signs of heat stress in horses can help prevent heat stroke in their animals.