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Heat stress occurs when the horse’s internal cooling mechanisms stop working efficiently and the ability to maintain a normal body temperature is affected. This is common and in hot and humid conditions, or following vigorous exercise. [1]
Without intervention, heat stress can progress into a life-threatening condition called heat stroke, or hyperthermia. If the horse’s body temperature exceeds 40.5°C (105°F), blood supply to the muscles and organs can be affected. [2]
Many factors can determine a horse’s ability to tolerate heat, including acclimatization, breed, age, feeding plan, fitness, energy expenditure, hydration and humidity.
If your horse is experiencing heat stress, there are several ways to reduce the risk of heat stroke including moving them to a cool area and providing adequate hydration.
If symptoms worsen or do not improve within 20 minutes of cooling, contact your veterinarian immediately. [3]
Signs of Heat Stress in Horses
Horse owners need to know how to identify heat stress and heat stroke in order to quickly help their horse. Horses in hot weather or undergoing heavy work should be frequently monitored for signs of overheating.
If you notice any of the following clinical signs of heat stress, cease all physical activity and bring your horse somewhere cool: [2]
- High rectal temperature (103 – 107°F or 39.5 – 41.5°C)
- Increased heart rate at rest
- Rapid breathing and flared nostrils at rest
- Dehydration: loss of skin elasticity, tacky gums, sunken eyes, and reduced urine output
- Exhaustion or lethargy
- Excessive sweating and hot skin
- Reduced feed intake
Signs of Heat Stroke
Heat stroke occurs when the horse becomes severely overheated. Call your veterinarian and cool your horse down immediately if they demonstrate any of the following signs of heat stroke: [4]
- Very high rectal temperature (105 – 107°F or 40.5 – 41.5°C)
- Very rapid heart rate at rest (more than 60 beats/min)
- Very rapid breathing at rest (more than 40 breaths/min)
- Stumbling, reluctance to move
- Incoordination
- Dehydration with prolonged skin tent
- Agitation and distress
- Shock
- Collapse
If the horse’s core body temperature exceeds 41°C, many proteins, including those in muscles can denature or break apart. Other severe consequences can occur, including colic, renal failure or hypotension. [3]
Heat regulation in the Horse
The normal body temperature for a horse is between 37.5 and 38.5°C (99 – 101.5°F). [1] Whether your horse is in a hot or cold environment, they keep their body temperature within this narrow range by controlling thermoregulation processes that produce and dissipate heat.
Heat is produced by many processes including digestion, metabolism, and exercise. In cold weather, horses can increase their metabolic rate to generate heat.
Conversely, in hot weather and with heavy exercise, the horse relies mostly on sweating to cool down.
Horses can lose anywhere between 2 and 4 gallons of sweat (7.5 – 15 liters) an hour when working or training in a hot environment. [5]
Sweating and evaporation of sweat from the skin allows the body to dissipate heat off its surface. Horses lose between 65 – 70% of their body heat through sweat and 29% through evaporative respiration. [6][7]
Ability to regulate heat
Several environmental and animal factors affect a horse’s ability to regulate their body temperature.
Ambient temperature (air temperature) and humidity are the main environmental variables that influence heat regulation. Other environmental factors are: [8]
- Precipitation
- Wind velocity
- Solar radiation
Horse characteristics and management factors also impact the ability to thermoregulate, including:
- Age: Young foals and senior horses are more susceptible to cold and heat stress
- Body condition: Fat insulates the body making it harder for overweight horses to cool down and for underweight horses to stay warm
- Coat condition: A thick coat and blanketing make it harder to dissipate heat
- Health status: Some illnesses affect thermoregulation, particularly if the horse develops a fever
It is important to consider these factors when exposing your horse to hot climates or exercise programs that could put them at risk of heat stress.
Is my Horse Hot if I’m Hot?
If you’re feeling hot while riding, your horse is likely feeling the heat even more and earlier. [3]
Although horses are incredible athletes, they are less heat tolerant than humans due to their size and muscle mass.
Compared to humans, horses have more muscle mass and generate more heat during exercise. They also have proportionally less skin surface area to dissipate the heat. [5]
It only takes 17 minutes of moderate exercise in hot, humid conditions for the horse to exceed its normal core body temperature range. Humans can handle up to 10 times longer duration exercise in the heat. [3]
Risk Factors for Heat Stress
Any horse can develop heat stress. Senior, obese and unfit horses are less likely to tolerate heat and have a greater risk of heat stroke.
In addition, horses trailered over long distances may be at increased risk due to poor access to drinking water and absence of airflow in a trailer. [4]
Foals cannot tolerate heat as well as adult horses, which makes them susceptible to heat stress. Monitor foals closely in hot and humid weather, even if their mare appears fine. [2]
Exercise
During an exercise bout, the horse’s muscles contract and produce a large amount of heat. Heat generation increases with exercise or training intensity.
Strenuous exercise in hot conditions can cause body temperature to spike and exceed normal levels, promoting substantial sweat and electrolyte loss. [5][9]
The onset of fatigue naturally protects the body from heat stroke, as exercise slows or stops when the body gets too hot. This limits the duration of high-intensity exercise.
Although horses in low-intensity exercise take longer to fatigue, they may still experience harmful dehydration. They can experience persistent sweat loss over long periods of time, which may be less apparent to the owner and lead to insufficient rehydration.
Temperature & Humidity
Horses lose the ability to cool themselves efficiently when air temperature and humidity are high.
Too much humidity in the air prevents heat dissipation through evaporation, keeping the sweat stuck to the body. [8]
Feed
The processes of digestion and metabolism involves chemical responses that produce heat in the horse’s body.
Fiber digestion and fermentation by microbes in the hindgut also generates heat, which can be detrimental to the horse in hot climates.
Excess protein in the diet needs to be metabolized and excreted, which generates heat. A diet that meets without significantly exceeding protein requirements can help reduce the risk of heat stress in exercising horses.
Exercising horses with high calorie needs benefit from added fat in the diet to replace some calories provided by fibre. Since fat is digested in the foregut, it does not rely on microbial fermentation and generates less heat through digestion. [10]
Anhidrosis
Anhidrosis, or dry coat syndrome, is a condition that limits the horse’s ability to sweat. This can drastically affect thermoregulation and performance.
Sweat glands fail to function and the horse is unable to cool themselves effectively through sweating. The horse may begin to breathe rapidly and with difficulty. [6]
Any horse can develop this condition, regardless of breed, age, genetics or physical fitness. [11] The onset of anhidrosis can occur rapidly or gradually.
Horses with anhidrosis should be monitored closely in warm conditions and be given unlimited access to cool drinking water. Severely affected horses may need to stay indoors with a cooling system or in a shaded paddock on hot days. [12]
Diagnosis
If your horse is showing signs of heat stress, contact your veterinarian immediately for examination. It is important to recognize heat stress or heat stroke early in order to help your horse quickly.
Diagnosis is usually made based on clinical signs and rectal temperature. A rectal temperature above 103°F / 39.4°C indicates heat stress, and a temperature above 105 or 106°F (40.5 or 41°C) indicates heat stroke.
Skin Pinch Test
A skin pinch test (skin tent, turgor) can help identify dehydration. Gently pinch the skin in front of the horse’s shoulder using the thumb and forefinger.
If the skin snaps back after a second or two, the horse is likely hydrated. If the skin remains elevated, the horse may be dehydrated. [4]
Horses with heat stress may have a skin tent that lasts for several seconds, whereas equids with heat stroke can have skin tents that last up to 10 seconds. [13]
Treatment
If your horse is displaying signs of heat stress, cool them down immediately and monitor them closely for improving or worsening symptoms.
If your horse progresses to heat stroke, contact your veterinarian immediately. Prompt medical intervention may be necessary.
Hosing & Scraping
Overheated horses with a rectal temperature above 103°F can usually be cooled by hosing or sponging them down with cold water. Spray the horse with a continuous stream of cold water because the water will heat when it touches the skin. [4]
Using cold water and ice on a hot horse to cool them down was once thought to induce tying-up. However, this myth has been disproven. Rapidly applying cold water and ice to the body may cause mild cramping but it is safe and can quickly relieve mild or moderate heat stress. [14]
After the horse is sprayed and starts cooling down, excess water can be scraped off of the body using a sweat scraper. This is purported to remove water trapped in the hair and further helps cool the horse. However, research studies have found no difference in cooling between continuous hosing and hosing with scraping. [15][16]
After hosing, the horse should be walked in a cool, shady area. If there is nowhere to walk the horse out of heat or direct sunlight, continue to cool them with water or ice.
Applying Ice
Horses with rectal temperatures exceeding 105°F may require further measures to manage symptoms of heat stroke.
Apply ice to the horse’s forehead, head, neck and back to cool the major blood vessels and the blood traveling through the body. As blood circulates, this will help bring the horse’s core temperature down. [2][4]
Avoid cooling the muscles in the hind-end and around the rump. These muscles could already be experiencing low blood flow and cooling them can worsen the condition.
If the horse collapses from severe heat stroke, continue cooling them down from a safe distance until veterinary help arrives. Getting too close to a collapsed horse can be dangerous for both the handler and horse if the horse tries to get up.
Electrolytes
Because the horse’s sweat is hypertonic (more concentrated than their blood) they lose a significant amount of electrolytes through sweat.
These electrolytes and the water lost through sweat need to be replaced. Providing plain drinking water can make the problem worse by further diluting electrolytes in blood.
It is critical to provide electrolytes in the drinking water to support rehydration and electrolyte balance. [3]
Intravenous Fluids
Some horses with heat stress may require intravenous (IV) fluids to replace lost water and electrolytes. IV fluids are usually administered by a veterinarian via a catheter in the horse’s jugular vein. [17]
Prevention
The best way to prevent heat stress in your horse is to limit heavy exercise in hot, humid conditions. However, for many performance horses, it is impossible to completely avoid training in hot weather.
Fortunately, there are several management practices that can be implemented to help horses regulate their body temperature as well as avoid dehydration and heat stroke.
Acclimatization
Major equine sporting events that require horses to be in peak physical and mental condition are often held in hot climates, which can lead to significant physical and mental strain. Horses that are not accustomed to humidity and heat are highly susceptible to heat stress.
Give horses traveling to areas with different climates time to acclimatize in their new environment. Horses have several physiological adaptations that help them cope with exercising in heat, including increased plasma volume, and reduced sweat fluid and electrolyte loss. [20]
It is generally recommended to give horses traveling from cool, dry climates to reside or compete in hot climates 15 to 21 days to acclimate and increase their heat tolerance.
Horses should be monitored closely for signs of heat stress during this time. [4]
Exercise
Strenuous exercise and training should be avoided if possible when the combined heat index is over 150. The heat index measures air temperature and relative humidity, indicating perceived temperature. [4]
A heat index below 130 is safe for exercising most horses because they are able to dissipate body heat effectively and prevent heat stress. However, unfit or overweight horses can struggle in this heat.
If the heat index is above 170 – 180 or humidity is above 75%, the horse’s ability to lose heat via evaporation is significantly affected. Owners should take serious precautions and all exercise should be avoided. Exercise horses in the morning or evening when it is cooler outside. [18]
Cooling Equipment
To reduce the risk of horses overheating, cooling equipment can be installed in stalls, paddocks and pastures. Fans and air conditioners are commonplace in barns and have the added benefit of improving ventilation and airflow through the facility.
Other cooling equipment that can be installed in barns and equine facilities including mist curtains and mist fans. [7]
You can also use ice cooling vests and cooling sheets to dry the horse after a sweaty exercise bout. Cooling sheets should never be placed on a horse showing signs of heat stress as they can affect the horse’s ability to sweat. [4]
Water & Electrolytes
Horses should be given access to clean, cool water at all times.
The average 500 kg (1100 lb) horse will consume anywhere between 21 – 29 L (5.5 – 7.5 US gallons) of water daily. In a hot climate with moderate to heavy exercise, a horse may need to drink more than 72 L (19 US gallons) of water per day. [12]
Electrolyte Loss
Rehydration after exercise is essential to prevent dehydration. But if a horse does not have adequate electrolyte levels following exercise, they will limit water intake to maintain ion balance. This can worsen dehydration.
Electrolytes are essential minerals that regulate muscle contractions and maintain fluid balance. The major electrolytes found in sweat are sodium, chloride and potassium. [9]
Horse sweat is highly concentrated in electrolytes, means they lose very large amounts of these minerals when sweating. This is why sweat can appear white on the horse’s coat. [3]
It is critical that these lost electrolytes are replaced, both for exercise recovery and overall health. [3]
Electrolyte Supplementation
Supplementing the horse’s diet with a balanced electrolyte solution helps replenish body fluids, sustain sweating and promote cardiovascular function in hot, humid conditions or following a bout of exercise. [5]
Some horses need to be taught to accept the taste of electrolytes in water. Introduce electrolyte supplements slowly by adding a small amount to water or feed over a few days or weeks.
Clipping
Horses can overheat if they are subjected to warm or hot weather with long coats. Clipping the coat removes the insulating hair layer from the body, which may improve heat dissipation and evaporation. [19]
Some owners opt to clip their horse’s coat in the summer, especially if they tend to grow thick hair year round. Clipped horses are better able to cool down following strenuous exercise and may have a lower risk of heat stress. [19]
Summary
Horses in hot, humid climates are at risk of heat stress, especially when exercising. You can support your horse’s horse’s heat tolerance and reduce the risk of serious health problems with the following management practices:
- Avoid exercising during the hottest times of day or when the heat index is above 150
- Provide constant access to cool, clean water with electrolytes
- Consider a high-fat, lower protein diet to reduce heat generated by digestion and metabolism
- Cool down your horse with hosing and fans following exercise
- Always monitor your horse for signs of heat stress including fast breathing and early exhaustion
Contact your veterinarian immediately if you suspect heat stress or heat stroke in your horse.
References
- Kang, H. et al. Heat stress in horses: a literature review. Int J Biometeorol. 2023.
- Haugen, S. M. Heat Stroke. American Association of Equine Practitioners (AAEP). 2016.
- Lindinger, M. I. When the Rider is Hot, the Horse is Hotter. Equine Guelph. 2010.
- Martinson, K. et al. Caring for horses during hot weather. University of Minnesota Extension. 2020.
- Lindinger, M. I. Sweating, dehydration and electrolyte supplementation: Challengesfortheperformancehorse. EEHNC Congress. 2008.
- Mallicote, M. Understanding Anhidrosis. American Association of Equine Practitioners.
- Ojima, Y. et al. Effect of Cooling Blanket on the Heat Stress of Horses in Hot and Humid Environments. Animals (Basel). 2022.
- Cymbaluk, N. F. & Christison, G. L. Environmental Effects on Thermoregulation and Nutrition of Horses. Vet Clin North Am Equine Pract. 1990.
- Maughan, R. J. & Lindinger, M. I. Preparing for and competing in the heat: the human perspective. Equine Vet J Suppl. 1995.
- Hintz, H. Digestive Physiology of the Horse. Journal of the South African Vet Soc. 1975.
- MacKay, R. J. et al. A review of anhidrosis in horses. Equine Vet Educ. 2015.
- Ralston, S. L. Nutritional Requirements of Horses and Other Equids. Merck Vet Manual. 2022. Accessed May 24, 2022.
- Pritchard, J.C. et al. Validity of indicators of dehydration in working horses: A longitudinal study of changes in skin tent duration, mucous membrane dryness and drinking behaviour. Equine Vet J. 2010.
- Marlin, D. J. et al. Post exercise changes in compartmental body temperature accompanying intermittent cold water cooling in the hyperthermichorse. Equine Vet J. 1998.
- Kang, H. et al. The Use of Percutaneous Thermal Sensing Microchips to Measure Body Temperature in Horses during and after Exercise Using Three Different Cool-Down Methods. Animals. 2022.
- Takahashi, Y. et al. A Comparison of Five Cooling Methods in Hot and Humid Environments in Thoroughbred Horses. J Equine Vet Sci. 2020.
- Lester, G. D. et al. Systemic, Renal and Colonic Effects of Intravenous and Enteral Rehydration in Horses. J Vet Intern Med. 2013.
- Turner, J. L. & Barraza, S. Help Your Horse Handle Heat Stress. New Mexico State University, Cooperative Extension Service. 2017.
- Morgan, K. et al. The effect of coat clipping on thermoregulation during intense exercise in trotters. Equine Vet J Suppl. 2002.
- Geor, R.J. and McCutcheon, L.J. Thermoregulatory Adaptations Associated with Training and Heat Acclimation. Vet Clin North Am Equine Pract. 1998.
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