Heat Stress in Horses: Symptoms and Prevention

Summer is picture-perfect weather, putting outdoor riding at the top of American Quarter Horse owners' lists of favorites. But the effects of hot weather can be brutal to our equine friends. When temperatures soar into the 90s and 100s, your horses' health can quickly cross into the danger zone, especially when exercising.

Heat builds up during exercise and must be released. In hot conditions, avoid heat stress, which can progress to heat stroke, by observing your horse's condition and minimizing excessive exercise.

Hot and humid weather can cause horses to become overheated. Heat stress and hot-weather dangers are nothing to take lightly. When a horse overheats, along comes a whole host of horrors, including heat exhaustion and heat stroke.

Jump ahead to frequently asked questions about heat stress in horses.

Horses produce large amounts of heat, mainly through digestion of feed and muscular activity during exercise. If the air is cooler than the horse’s body temperature, blood is shunted to the skin where the horse easily rids himself of the excessive heat. However, if the air temperature is warmer than the horse’s body temperature, blood shunting is not enough, and sweating becomes the primary means by which the horse cools himself. The horse is the only mammal, other than man, that cools itself primarily by sweating. This wets the body so cooling, due to evaporation, can occur.

Problems develop and the cooling system breaks down during hot, humid weather when sweat doesn’t evaporate and adequate cooling cannot take place. The result is heat stress, which often leads to heat exhaustion or life-threatening heat stroke.

What Heat Stress in Horses Looks Like

Symptoms of heat exhaustion include:

• Profuse sweating or less sweat than expected.
• Hot skin (might progress to cold if skin circulation shuts down).
• Muscle weakness.
• Stumbling.
• Rapid breathing. (Normal breathing rate for an adult horse is 8-18 breaths per minute.)
• Rapid heart and pulse rates that don't recover after exercise.
• Increased body temperature of 102 degrees to 106 degrees F. (Normal temperature is 98-101 degrees F.)
• Signs of dehydration, including loss of skin elasticity, sunken eyes, tacky membranes and cessation of urination.

The horse continues to sweat profusely, and despite the fact that his skin often feels cool, his body temperature is elevated and may remain elevated even after he has rested.

A much more serious condition is heat stroke, where the affected horse’s skin is hot and dry, breathing is extremely rapid, and the horse’s body temperature may reach 106-110 degrees F. If untreated, horses experiencing heat stroke may collapse, go into convulsions and die.

What to do if a Horse Overheats

If a horse becomes overheated, the horse owner should not attempt to differentiate between the two syndromes but should immediately:

1. Stop riding the horse, place him in the shade and call a veterinarian, as the situation may quickly become an emergency.
2. If there is no breeze, provide air movement with a fan, if possible.
3. Starting with the feet and legs, gradually wet the horse’s entire body, including the head, with cool water.
4. Avoid using cold water and never attempt a cold-water enema.
5. Small amounts of drinking water can be given at 15-minute intervals until the veterinarian arrives.

Preventing Heat Stress in Horses

Heat stress can be prevented with proper management and conditioning. Calculating the heat index may alert the horse owner to unfavorable weather conditions where the horse should not be worked.

To calculate the heat index:

• Add the temperature in Fahrenheit and the percentage of relative humidity.
• If the sum is below 120, there should be no problem exercising a horse.
• If the sum is between 130 and 150, the horse will probably sweat but should not experience any problems if he gets plenty of water to replenish fluid lost during sweating.
• When the heat index exceeds 180, do not exercise a horse because the horse's heat dissipation systems will not be adequate to prevent heat stress.

Horses at Risk for Heat Stress

Certain horses are more susceptible to heat overload:

• Poorly conditioned horses.
• Overweight horses.
• Geriatric horses.
• Horses in direct sunlight when the temperature is more than 100 degrees F.
• Animals in hot, poorly ventilated stalls or trailers are always at risk.
• Horses not consuming enough water and not getting sufficient salt or electrolyte supplements.
• Horses transported from cooler climates that have not had time to adjust to hot weather. (These horses should be provided adequate time to adjust to the warmer weather conditions.)

How Feed Affects Heat Stress

Rations high in protein generate extra body heat during the digestion process and make horses more susceptible to overheating.

Horses produce large amounts of body heat during chewing, digesting and metabolizing feed, so avoid riding them too close to feeding time during hot weather. Heat generated during food digestion added to the heat generated by exercise could push them into heat stress. As a rule of thumb, feed horses three to four hours before exercising and wait at least two hours after they have been ridden before feeding.

Salt and Water: Crucial to Preventing Heat Stress

The most important single factor in preventing heat stress is providing plenty of clean, fresh water and trace-mineralized salt to all horses. The average American Quarter Horse will normally drink 8-10 gallons of water a day. High temperatures, even without exercise, will double this amount. Therefore, if water is being supplied in buckets, add an additional bucket per stall during the summer. Check the buckets at least three times per day to ensure the horse has plenty of water. Research has shown that horses working hard, such as endurance horses, can sweat nearly 4 gallons per hour under conditions of high heat and humidity.

Under normal conditions, a balanced ration and free access to mineralized salt are sufficient to replace electrolytes lost during sweating. However, equine sweat contains a large amount of sodium and potassium, and horses sweating excessively might not be able to replenish these electrolytes by merely consuming a salt supplement. Therefore, provide a good electrolyte supplement to the feed of any horse working hard in hot weather.

Dehydration

Dehydration is an unmistakable sign of heat exhaustion. When a horse loses water through perspiration faster than it can be replaced, he becomes dehydrated. A simple "pinch" test can help you determine if your horse is dehydrated. Pinch a small section of skin on your horse's neck or shoulder and release. If the skin snaps back into place, your horse is in the clear. If there is a delay, he could be dehydrated.

Dehydration has a profound and immediate impact on the well-being of a horse, more so than the lack of any other nutrient. To put it in context, the body can lose nearly all of its fat and more than half of its protein content and survive, but a loss of just 1/10th of the body’s water can result in serious consequences.

Feeding large amounts of hay or grain usually increases water needs. Conversely, horses grazing on lush, green pastures may meet most of their water requirements from the grass, as it may contain 60 to 80 percent water.

Exercise increases the amount of heat the horse must dissipate in order to function. Horses dissipate heat load primarily through evaporative cooling. Air movement also helps with evaporative cooling. While the horse will still lose heat via evaporative cooling at ambient temperatures greater than body temperature, high humidity severely depresses the evaporative efficiency.

To find out if your horse is at risk, add the outdoor temperature to the relative humidity. If the total is more than 150, there is a potential for overheating, and extra caution should be observed.

Signs of dehydration include a slow capillary refill time and a decrease in skin elasticity. To measure the capillary refill time, press your finger on the horse’s gums, then release your finger and determine how long it takes for color to return. Normal capillary refill time is 1.5 to 2 seconds.

Monitor the hydration status of your horse. Know his normal temperature, pulse and respiration. Remember that each horse is an individual.

Guidelines

• Provide free access to palatable water. Cool water is preferred because the stomach empties faster.
• During exercise, allow the horse to drink as often as possible.
• After exercise, a hot horse should be cooled before being allowed free access to water.
• Provide salt to encourage adequate water consumption and help maintain electrolyte balance. If pastured, one to two ounces per day is good.
• A powdered electrolyte is a good choice to use when adding electrolytes to the feed. If a quality electrolyte is not available, a mixture of table salt and lite salt in equal amounts may be used.
• Give your horse free access to salt even when using electrolytes.
• It cannot be over-emphasized to supply sufficient water any time you give electrolytes.

Supplying sufficient water, judiciously using electrolytes and using common sense during hot weather will maximize the performance of your horse while ensuring his safety.

Smooth Sailing: Have a Plan For Emergency Situations

Going to competitions during the hot months, can increase your horses chances of heat stress. Along with the stress of travel. the heat could be the factor that makes a fun trip to the horse show a fatal endeavor. Making sure you are prepared for your show is key for a successful and enjoyable riding and your horses health. 

All trips can benefit from preparation and planning, helping to ensure you're ready when the truck and trailer reach the destination. Here are some ways to help make your trip a success:

• Equipment: One or two days before leaving, make sure all tack, clothing and grooming supplies are packed and on your trailer. This includes water buckets, supplements like electrolytes etc. 
• Paperwork: Be sure to bring current health certificates, proof of a negative Coggins test, and copies of association membership cards and registration papers. 
• Travel safety: Plan your travel route and timing early, allowing for traffic and any inclement weather. If it is a long trip, have your stops planned accordingly to make sure you are offering your horse water, especially if the temperature is hot. 
• Backup plan: Even if you are only traveling a short distance, check your emergency kit to be sure it includes necessary tools and medical supplies.

Stresses, such as travel, can lead to equine stomach ulcers. When a horse has ulcers, some horses will consume more water, while others will consume less. If your horse is consuming less water, your horse is at higher risk for heat stress. Learn more about how to manage horses with gastric ulcers.

FAQs About Equine Heat Stress

Extreme heat can lead to dehydration, tying up, heat exhaustion, colic and even death. Dr. Justin Voge of Hartman Equine in Whitesboro, Texas, and Dr. Elaine Carpenter, formerly of Cave Creek Equine in Phoenix, Arizona, strongly recommend using common sense when determining whether your horse is at risk for these conditions.

What temperature is too hot for a horse?

“We advise to not exercise your horse in the heat of the day,” Dr. Carpenter says. “Pick early morning or late evening when it’s cooler to ride or work your horse.”

That said, even in cooler parts of the day during the summer, take extra care to properly warm up and cool down your horse.

“Just be sensible. If it’s hot out and you’re hot, think about how hard you work your horse. A fit horse can handle it better than a horse that’s not fit,” Dr. Carpenter says.

How do you cool down a horse after exercise?

At the end of your ride, you’ll want to make sure that you are walking for the last 10-15 minutes of your ride. Then, after your ride and you’ve untacked, you should offer your horse water, as much as your horse wants. Then, the policy at Voge Quarter Horses is to always rinse the horse after exercise in the summer. Scrape all the excess water off of your horse when you are done rinsing. It cools them down faster, gets the dirt and sweat off and, then when placed in a well-ventilated stall, keeps them cool longer.

How do you avoid dehydration in horses?

It’s critical to always provide clean drinking water for horses, but in the summer, it can be a matter of life or death.

“In the summertime, we feed a little bit of Morton lite salt every day,” Dr. Voge says.

It’s a combination of salt and potassium chloride designed for human consumption. It can be found at most grocers and helps horses want to drink more.

Electrolytes can also help your horse stay healthy in the heat. Electrolytes can be administered in an oral paste, or mixed in feed or water. 

"Be careful when you put electrolytes in the water, though,” Dr. Carpenter warns. “Always offer water without electrolytes, as well. Some horses will stop drinking because they don’t like the taste. And you definitely don’t want them to stop drinking.”

What are warning signs that a horse has overheated?

Your horse might need serious medical attention because of the heat. Call the vet right away if your horse has any of these symptoms:

1. The horse stops sweating.

   Some horses suffer from anhidrosis, which is the inability to sweat, and a condition that needs to be diagnosed by the veterinarian. In a normal horse, it is a sign of extreme heat stress. If it’s hot out and your horse stops sweating, call the vet right away.

2. Increased respiratory rate.

   Anything above 50 breaths per minute warrants a vet call.

3. Increased rectal temperature.

   If the horse’s temperature rises above 102 degrees Fahrenheit, call the vet. If the horse’s temperature shoots up right away after a few minutes of exercise, that is a sign that the horse is having trouble cooling itself.

4. Muscle stiffness.

   A horse that seems stiff and/or is reluctant to move around may be starting to tie up. Call the vet.

5. Lethargy.

   If the horse acts depressed, is disinterested in food or lacks coordination, call the vet.

While waiting for the vet to arrive, stop activity, offer the horse water, rinse the horse with cold water and keep him in the shade and in front of a fan.

Sponsored by Boehringer Ingelheim

Horses are living, breathing works of art and caring for them requires an entire team – veterinarians, farriers, trainers, riders and grooms. Boehringer Ingelheim is proud to help provide tools to help the team keep them performing at their peak. Learn more at www.boehringer-ingelheim.com/animal-health.