Heat Stress: It's not just equine athletes at risk for heat-related injuries; educate clients

Heat Stress: It's not just equine athletes at risk for heat-related injuries; educate clients

Oct 01, 2001

This has been a difficult year for horses and weather.

First we had spring forest fires in the west that lead to emergency evacuations and disruption of pastureland for horses in that area.

Then the heat and cold snaps in central Kentucky lead to conditions ripe for cherry tree damage, caterpillar growth and the devastating problems associated with Mare Reproductive Loss Syndrome.

That gave way to nightly news reports about mosquito activity and the spread of West Nile Virus. And now we have the lasting remnants of a heat wave that affected most parts of the country.

Hot, sticky weather defines summer in much of the United States but this summer seemed to be a bit worse.

Though the temperatures do not seem to be that unusual (the hottest day so far this year in Atlanta occurred on the first of August and it still has not been over 95 degrees there yet), the combination of heat and humidity has made this a summer a killer.

Humans, too

Sports fans were saddened to learn of the death of Minnesota Viking standout tackle Korey Stringer during training camp. Stringer, a healthy professional athlete exercising under controlled conditions at the Viking's training camp, died on August 1 because of complications related to heat stress.

Not long after that college football players at Northwestern College and the University of Florida also died during training camp in the heat.

A south Georgia high school football player was the latest to die from heat-related stress and it becomes impossible to downplay the effect that heat and humidity can have on both horses and humans. Numerous cases of heat stress, heat stroke, cramps, exertional myopathy, exhaustion and even heat-related death occur in horses each year. Statistics kept for humans point out that such deaths are really more common than we think.

Statistics kept by the National Weather Service show that heat and solar exposure kill more people in the United States than all but one other natural force. Only winter cold, not lightning, tornadoes or hurricanes, floods or earthquakes, will kill more Americans-an estimated 175 per year. In a 40 year period from 1936 to 1975 more than 20,000 people were killed in the U.S. by the effects of heat or solar radiation. This figure only represents the direct deaths and does not take into account the heart conditions and allergic and respiratory problems that may have been so stressed from the heat as to lead to fatal complications as well.

In the devastating heat wave of 1980 it is estimated that more than 1,250 people died. No such statistics are kept for horses but, without the help of air-conditioning and with the additional burden of having to exercise and compete in the summer heat, it is likely just as devastating a factor for the equine population.

Tough call

Veterinarians are often called upon to determine if equine competitions can or should be held in hot conditions.

They must make recommendations to their clients as to measures to control heat stress and they must treat any cases that develop. A useful tool for evaluating the actual heat in the environment is the heat index (HI). This figure is measured in degrees Fahrenheit and is often referred to as the ambient temperature. It is calculated by adding the relative humidity to the actual air temperature. A temperature of 85 degrees with a humidity of 80 degrees produces an HI of 99. Ninety degrees with 80 percent humidity elevates the HI to 113. There are well-documented HI risk levels for humans. HI values between 80-90 will result in fatigue with prolonged exposure or physical activity. Values between 90 and 105 can cause sunstroke, heat cramps or exhaustion with prolonged exposure or exercise. At HI values between 105 and 130, people will likely experience sunstroke; cramps or exhaustion with prolonged exposure or activity and merely standing in an environment with a HI of more than 130 will cause heat stroke.

It is wise to remember that HI values are calculated for a shady location and full sun exposure can increase the value by 15 degrees.

Effect on horses

These precaution levels are calculated for humans and until the Atlanta Olympic Games in 1996 not much was definitely known about the responses of horses to conditions of heat and humidity. Veterinarians and researchers knew that the 1996 games were likely to encounter hot weather conditions so a number of studies were initiated to provide information on the physiology of horses and heat. Recommendations were sought for optimum conditions, for electrolyte treatment and for conditioning protocols.

It has been determined that during exercise the rate of heat generation in the horse increases by 50 percent. This tremendous amount of heat requires a huge increase in loss by the horse's body. The horse loses heat by convection (air moving past the body), radiation (heat given off to the environment around the horse), conduction (heat passing directly to objects in contact with the horse's body), and by evaporation (heat lost through sweat evaporation at the surface of the skin).

In hot environments there is little difference between the horse's temperature and that of the air or surroundings so conduction and radiation do not help much. Often in the summer there is little air movement so convection may not amount to much and evaporation may also be reduced. The volumes of sweat produced by exercising horses as an aid in evaporation can be enormous and fluid deficits can amount to 20 to 30 liters in these athletes. A joint study done by members of the University of Tennessee, Rutgers University and the University of Georgia found that horses competing in three day eventing lost so much water during the second day's endurance phase that they were unable to replace that deficit for the final day's stadium jumping phase.

They concluded, "this loss of water may contribute to fatigue during stadium jumping and may lead to injury". Horses maintaining a schedule of consistent exercise over a few days in a hot, humid environment will therefore deplete reserves without the ability to replenish and heat stress or related problems become a real possibility.

It is also well documented that, though the simple loss of water is important, the loss of sodium, potassium, chloride and other electrolytes in a horse's sweat is the main cause of cramping, exertional myopathy (tying-up), and exhaustion. Electrolytes should be a part of the daily routine for all horses in hot and humid environments, and not just for exercising horses. Horses in a hot pasture may lose so much sweat that they cannot replace their electrolytes through their normal diets alone. This is especially true for extended heat waves, which tend to reduce horses' appetites as well. Many commercial electrolyte preparations are available and one to two ounces are recommended daily.

Many similarities

The studies done in preparation for the Atlanta Olympic Games showed that horses, because of their weight to surface area ratio, are just as sensitive to heat and humidity as are humans and that the HI categories used for humans may be just as appropriate for horses.

Horses do acclimatize well and there are certainly those unique athletes, both human and equine, who can do well in the heat, but the majority of horses tested did not perform well at or tolerate conditions with a heat index much above 120.

At this level of HI horses had to be rested frequently, supplemented with fluids and continually monitored for stress. Veterinarians asked to make recommendations about holding competitions in the heat should consider the HI and then evaluate the type of stress that the horses will endure.

Jumping and dressage events do not require that a horse perform for long periods at a time. These horses can be cooled and rehydrated between rides so these events may occur at slightly higher HI levels. Endurance events or other horse-related activities where the horse must perform for longer periods of time should not be run if the HI level indicates that these athletes will be at risk.

Quick action needed

A heat stressed or exhausted horse must be treated quickly and aggressively.

Horses exhibiting such stress will show delayed heart rate recovery (pulse consistently above 64 and often irregular). These horses will have decreased gut sounds and mucous-coated feces indicating intestinal stagnation. There will be a decreased or absent appetite and little to no attention to surroundings. The horse will be depressed and will show a lack of thirst in the presence of clear signs of dehydration. The mucous membranes will show margination around the gum line, dryness and a muddy color. There will be poor jugular refill and a flaccid anal sphincter. Many horses will be ataxic, weak and uncoordinated in their movements.


Treatment of a horse in heat stress usually requires fluid replacement that can be done either orally or intravenously depending on the severity of the condition.

Cooling can be done with hosing and fans when needed and a cool water enema in severe cases. Correction and replacement of electrolyte losses should be done using blood values whenever possible. Many companies have begun producing hand-held blood gas and electrolyte analysis machines that use barely a drop of equine blood and yield a printout of values in minutes. These devices are invaluable to veterinary decision-making and treatment in field cases.

There are many ways to reduce heat stress in horses. Correct and consistent use of electrolytes prior to exercise allows the equine athlete to avoid fatigue and to maintain metabolic function for longer periods.

Misting fans, such as those used at the Olympics, can significantly cool the environment and reduce the HI. Regular fans will increase the horse's ability to lose heat through evaporation and convection. Screening materials can significantly reduce the sun's energy and make a stall or paddock safer for horses in the heat. A good physical examination may be warranted for horses experiencing heat stress because the heat can worsen underlying problems. Cardiac abnormalities, allergies and other respiratory conditions should be treated before complications can occur.


Heat related problems can be prevented.

Weather information is readily available and some simple thought and prior planning can keep an equine-related outing from turning tragic.

Client education must be done and an understanding of the HI and the levels at which horses are at risk should be communicated to all owners and riders. Electrolyte use should be stressed and quick recognition and proper treatment of cases of equine heat exhaustion can help tremendously.


Dr. Marcella, a 1983 graduate of Cornell University's veterinary college, was a professor of comparative medicine at the University of Virginia. His interests include muscle problems in sport horses, rehabilitation and other performance issues.