August 6, 2020 
Summer has arrived, and with it comes hot, humid temperatures.
Toasty summer days aren’t merely a discomfort for horses, they can be downright dangerous. Heat stress, or the more serious heat stroke, can lead to hazardous, (and potentially fatal) physiological conditions. In fact, the FEI has published a 26-page document on the history and dangers of heat-related illnesses, and how riders can better acclimatize their performance mounts in the competition environment.
Sport horses aren’t the only ones at risk, however. Even pleasure horses and “pasture ornaments” are susceptible to climbing temperatures.
What is heat stress?
If you’ve ever worked outside in the summer and pushed yourself too hard while taking in little fluid and sweating a lot, you’ve likely felt the effects of heat stress. Nausea, flushed skin, elevated heart rate, and increased respiration are all signs of an increase in core temperature. It’s very similar to what occurs in our equine partners.
Heat stress (also referred to as heat exhaustion) is a condition when the core temperature of a horse rises due to exercise, climate, or a combination of other factors, and negatively affects functions within the body.
During normal exercise, a horse’s heart rate will rise, increasing blood flow to muscles and skin. Sweat will form and, as it evaporates, help cool the horse (known as evaporative cooling). This effect is aided by increased respiration and the flow of air through the horse’s nasal passages.
Heat stress occurs when the horse is not able to cool (for example, through sustained environmental conditions or continuous exercise).
What are the signs?
A horse in heat distress may exhibit quivering muscles, dripping sweat, flaring of nostrils, or visibly blowing (breathing hard and deeply). Heat stroke may also have additional signs such as extreme tiredness, dehydration (as evidenced by a skin tent test that lasts 4–10 seconds), or rectal temperatures above 103 degrees Fahrenheit. The horse may even seem confused, or there may be a lack of reaction to anything happening around the horse.
It’s worth noting that dripping sweat is not necessarily a danger sign by itself. Horses can lose two to four gallons of water through their sweat while working. And they don’t just lose sweat, they’re losing electrolytes as well, which might need replacing through the addition of salt to their diet. If a horse is sweating less than a reasonable amount (especially on the hindquarters), this can also be a cause for concern.
Nasal flaring or hard breathing alone is also not a reliable indicator of the presence of heat stress when it exists as the only sign. In addition to sweating, horses have been found to cool their bodies through increased blood flow and oxygen to their nasal sinuses during exercise. This process helps them use a technique referred to as “Selective Brain Cooling,” to reduce the temperature of their central nervous system during work.
While any one of these signs alone may indicate another medical condition, the presence of several of them together (along with consideration of environmental conditions), may indicate that your horse is suffering from heat stress. This is especially true if the signs remain and don’t disappear shortly after work ceases. Rectal temperature should decline steadily over 20 minutes post-exercise; the normal resting value is 99–100°F.
How do I know if my horse is at risk?
Given the right (or wrong) environmental conditions, any horse may suffer from heat stress. Common risk factors include:
Size: Horses that are overweight or out of shape, large or well-muscled, or suffer from anhidrosis (a condition where the horse is unable to produce sweat) are higher risk. These horses may heat up quickly, and be unable to cool themselves down through the normal sweating mechanisms.
Work load: For the athletic horse, it may be easy to assume that the horse is fit enough to handle a day of work in the sun. But that’s not necessarily the case. If work is suddenly increased beyond the horse’s capability, if they’re returning to work after a lay off period, or if the temperature and humidity for that day are excessive, fit horses are just as likely to suffer from heat stress as non-fit horses. In fact, they may actually be at increased risk due to the amount of heat produced by muscles.
Performance horses in particular are at risk for additional complications, as well. Not only do “thermally stressful conditions reduce performance,” but horses that are overly tired from competing or exercising are more likely to injure themselves or fall seriously ill, such as with colic or laminitis.
Breed: A study that attempted to decipher the best horse for use by the Brazilian army found a significant difference between breeds with respect to how the horses handled thermal increases in ambient temperature or thermal regulation during exercise. Brazilian Thoroughbreds were generally less adaptable to the climate and exercise than the Breton.
Changes in location: If the horse has recently changed climates and hasn’t had the chance to acclimate (a process which can take 15–21 days), even a simple workout can lead to trouble. Horses that have acclimated to humid heat show increased sweating efficiency, and better physiological responses to heat than even aerobically fit horses.
Discipline: According the the FEI’s Optimising Performance report, jumping and dressage horses are often larger and heavier, and the riders in those disciplines often tend towards longer warm-ups. Eventing and driving, disciplines that traditionally involve periods of intense exercise, can quickly lead to the production of high heat, especially if the horse involved is larger.
Length of warm up: One study found that when exercise was escalated quickly without a proper warm-up period, the rise in body temperature of the horse was too fast for the sweating response to begin and compensate. On the flipside, too long of a warm-up can cause increased core temperatures without benefit.
Hot and humid temperatures: Even in the absence of exercise, the onset of heat stress may arise when the horse is exposed to a hot and humid environment for extended amounts of time. For example, when a horse spends the day in a sunny pasture with an empty trough and no shelter, or when a horse is in a hot, enclosed space for a period of time, like poorly ventilated trailer or stall.
What can I do to prevent heat stress?
Prevention is the best medicine for heat stress. Prior to beginning work, look at the current temperature, as well as how hot it will eventually get while you’re riding, and consider whether you’ll be in the sun or shade.
Keep an eye on days that have both a heat and humidity component—especially stagnant days where there is no wind to assist with evaporation. The sweat on your horse’s coat won’t freely evaporate in these conditions, preventing them from using their normal cooling processes. So even if they are sweating buckets, it’s not helping them to stave off potential heat stress.
The most accurate method to determine the effect of the heat is through the WBGT, or Wet Bulb Globe Temperature (which is what the FEI uses when determining when to run events in hot climates). The WBGT takes all factors into consideration, like temperature, humidity, wind speed, sun angle, and cloud cover, and gives a fairly complete picture of the weather state for the day.
Note that WBGT is not the same as the heat index or “feels like” temperature, which should not be used for determining the impact the heat and humidity will have on your horse.
If you don’t have the WBGT available, it’s recommended that you add the humidity and temperature (F) together to determine whether work would be dangerous. If the sum of the two is 130 or more, the horse has some decreased ability to cool itself. At 150, you should consider avoiding hard work, as the horse as limited ability to cool itself (especially if the humidity represents more than 50% of the number). At 180, almost none of the horse’s intrinsic cooling system still works (“Heat Stroke”). This may sound like a high number, but in 80 percent humidity, it only needs to be 70 degrees to quickly reach unfavorable conditions.
Of course, the best protection for you can offer is pay attention to your horse. Know what their normal vital signs are, and what a normal level of exertion looks like during exercise. Evaluate whether your horse has any additional risk factors for heat stress, as well. Dehydration can quickly lead to heat stress, since the horse needs to be well-hydrated in order to be the most efficient at producing sweat.
If you do have to ride in less-than-ideal conditions (a reality for many of us in the summer months), try to ride in the cooler portions of the day, or break the work up. Take frequent walk breaks, keep the canter and trot sets to a minimum, and promptly remove tack when you’re done. If a long warm-up is preferred, consider dividing the warm-up into 20-minute sessions with a brief cooling and rest period between.
Once exercise is finished (or if your horse is heated from their pasture, trailer or stall), give your horse a cool bath until his temperature has returned to normal and breathing is no longer labored. The FEI recommends continuous hosing or application of cold water, rather than repeated sweat scraping, as research indicates scraping while hosing does not demonstrate any added benefit.
If you have access to ice, an ice bath is an effective (and safe) method of cooling horses. Focus on the ribs, head and neck, where there is a high concentration of blood vessels, for the most efficiency.
In the pasture, ensure your horse has access to shade and is able to get out of the sun, and that they have plenty of clean, cool water. A salt lick or electrolyte supplement may also be a wise choice, if they are sweating frequently. If stalled, a properly installed fan can help horses stay cool in stagnant air.
And that old wives’ tale about not giving your horse water during or after work? It’s just that. Cool water can help replace fluid lost through sweat, and will help reduce the temperature of your horse during times of hard work.
Take home message
Heat stress can be a dangerous condition for even the fittest of performance horses, and requires education and preparation on the part of the rider to prevent. By setting your horse up for success and taking risk factors and environmental conditions into consideration, your athletic horse can perform at his peak during all seasons.
And remember, when in doubt, if you’re overheated, they’re overheated.
