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]

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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]