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

This has been a difficult year for horses and weather.

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

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

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

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

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

Humans, too

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

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

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

Statistics kept by the National Weather Service show that heat and solarexposure kill more people in the United States than all but one other naturalforce. Only winter cold, not lightning, tornadoes or hurricanes, floodsor earthquakes, will kill more Americans-an estimated 175 per year. In a40 year period from 1936 to 1975 more than 20,000 people were killed inthe U.S. by the effects of heat or solar radiation. This figure only representsthe direct deaths and does not take into account the heart conditions andallergic and respiratory problems that may have been so stressed from theheat as to lead to fatal complications as well.

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

Tough call

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

They must make recommendations to their clients as to measures to controlheat stress and they must treat any cases that develop. A useful tool forevaluating the actual heat in the environment is the heat index (HI). Thisfigure is measured in degrees Fahrenheit and is often referred to as theambient temperature. It is calculated by adding the relative humidity tothe actual air temperature. A temperature of 85 degrees with a humidityof 80 degrees produces an HI of 99. Ninety degrees with 80 percent humidityelevates 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 orphysical activity. Values between 90 and 105 can cause sunstroke, heat crampsor exhaustion with prolonged exposure or exercise. At HI values between105 and 130, people will likely experience sunstroke; cramps or exhaustionwith prolonged exposure or activity and merely standing in an environmentwith a HI of more than 130 will cause heat stroke.

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

Effect on horses

These precaution levels are calculated for humans and until the AtlantaOlympic Games in 1996 not much was definitely known about the responsesof horses to conditions of heat and humidity. Veterinarians and researchersknew that the 1996 games were likely to encounter hot weather conditionsso a number of studies were initiated to provide information on the physiologyof 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 generationin the horse increases by 50 percent. This tremendous amount of heat requiresa 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 environmentaround the horse), conduction (heat passing directly to objects in contactwith the horse's body), and by evaporation (heat lost through sweat evaporationat the surface of the skin).

In hot environments there is little difference between the horse's temperatureand that of the air or surroundings so conduction and radiation do not helpmuch. Often in the summer there is little air movement so convection maynot amount to much and evaporation may also be reduced. The volumes of sweatproduced by exercising horses as an aid in evaporation can be enormous andfluid deficits can amount to 20 to 30 liters in these athletes. A jointstudy done by members of the University of Tennessee, Rutgers Universityand the University of Georgia found that horses competing in three day eventinglost so much water during the second day's endurance phase that they wereunable to replace that deficit for the final day's stadium jumping phase.

They concluded, "this loss of water may contribute to fatigue duringstadium jumping and may lead to injury". Horses maintaining a scheduleof consistent exercise over a few days in a hot, humid environment willtherefore deplete reserves without the ability to replenish and heat stressor 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'ssweat is the main cause of cramping, exertional myopathy (tying-up), andexhaustion. Electrolytes should be a part of the daily routine for all horsesin hot and humid environments, and not just for exercising horses. Horsesin a hot pasture may lose so much sweat that they cannot replace their electrolytesthrough their normal diets alone. This is especially true for extended heatwaves, which tend to reduce horses' appetites as well. Many commercial electrolytepreparations are available and one to two ounces are recommended daily.

Many similarities

The studies done in preparation for the Atlanta Olympic Games showedthat horses, because of their weight to surface area ratio, are just assensitive to heat and humidity as are humans and that the HI categoriesused 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 ofhorses tested did not perform well at or tolerate conditions with a heatindex much above 120.

At this level of HI horses had to be rested frequently, supplementedwith fluids and continually monitored for stress. Veterinarians asked tomake recommendations about holding competitions in the heat should considerthe 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 longperiods at a time. These horses can be cooled and rehydrated between ridesso these events may occur at slightly higher HI levels. Endurance eventsor other horse-related activities where the horse must perform for longerperiods of time should not be run if the HI level indicates that these athleteswill 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 (pulseconsistently above 64 and often irregular). These horses will have decreasedgut sounds and mucous-coated feces indicating intestinal stagnation. Therewill 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 presenceof clear signs of dehydration. The mucous membranes will show marginationaround the gum line, dryness and a muddy color. There will be poor jugularrefill and a flaccid anal sphincter. Many horses will be ataxic, weak anduncoordinated in their movements.

Treatment

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

Cooling can be done with hosing and fans when needed and a cool waterenema in severe cases. Correction and replacement of electrolyte lossesshould be done using blood values whenever possible. Many companies havebegun producing hand-held blood gas and electrolyte analysis machines thatuse barely a drop of equine blood and yield a printout of values in minutes.These devices are invaluable to veterinary decision-making and treatmentin field cases.

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

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

Prevention

Heat related problems can be prevented.

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

Client education must be done and an understanding of the HI and thelevels at which horses are at risk should be communicated to all ownersand riders. Electrolyte use should be stressed and quick recognition andproper 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. Hisinterests include muscle problems in sport horses, rehabilitation and otherperformance issues.