Bit & Bridle

Equine Heatstroke

Other Names

  • Exertional Heat Illness (EHI)
  • Hyperthermia
  • Heat exhaustion
  • Anhidrosis-related hyperthermia (in horses that cannot sweat)

What is Equine Heatstroke?

Equine heatstroke — also called hyperthermia or exertional heat illness — is a potentially life-threatening condition that occurs when a horse’s core body temperature rises to dangerous levels and the animal can no longer dissipate heat effectively. Horses generate substantial internal heat during exercise, and under normal conditions they rely primarily on sweating to cool down. When heat production exceeds the body’s ability to lose it — whether due to high ambient temperature, humidity, heavy exercise, inadequate water intake, or impaired sweating — core temperature rises rapidly and can reach levels that damage cells, organs, and the central nervous system.

The normal resting temperature for a horse is approximately 37.5–38.5°C. Temperatures above 40°C are cause for serious concern, and those exceeding 41–42°C can lead to organ failure, brain damage, and death if not addressed immediately. Heatstroke can develop in working horses, horses in transit, and even horses standing in hot, poorly ventilated conditions without access to water or shade.

Causes

Heatstroke arises from an imbalance between heat production and heat dissipation. Contributing factors include:

  • High environmental temperature and humidity: The combination of heat and humidity is particularly dangerous. High humidity reduces the effectiveness of evaporative cooling through sweating — the primary mechanism by which horses cool themselves.
  • Intense exercise: Working horses generate heat through muscle activity. Sustained galloping, competition, endurance riding, or hard schooling in hot weather significantly increases core temperature.
  • Poor ventilation: Horses in poorly ventilated boxes, trailers, or lorries during hot weather are at risk even without strenuous exercise.
  • Anhidrosis: A condition in which horses are unable to sweat normally (particularly common in horses moved to tropical climates), leaving them without their primary cooling mechanism. See equine anhidrosis for more detail.
  • Dehydration: Inadequate water intake reduces plasma volume and compromises thermoregulation.
  • Obesity: Excessive body condition increases metabolic heat production and reduces the surface-area-to-body-mass ratio, impairing heat dissipation.
  • Certain medications: Some drugs, including certain sedatives and phenothiazines, can impair thermoregulatory mechanisms.

Symptoms and Signs

Early recognition is critical. Signs range from mild heat stress to life-threatening collapse:

  • Elevated rectal temperature: Above 39.5°C post-exercise is concerning; above 40.5–41°C is a veterinary emergency
  • Excessive sweating or, paradoxically, absence of sweating (in anhidrotic horses)
  • Rapid respiratory rate: Horses breathe faster to try to dissipate heat through the respiratory tract
  • Elevated heart rate that fails to return to normal after exercise ceases
  • Weakness and muscle tremors
  • Incoordination and stumbling
  • Reduced or absent gut sounds
  • Colic-like discomfort
  • Dark, concentrated urine or reduced urine output
  • Collapse
  • Seizures or coma in severe cases

Diagnosis

Diagnosis is primarily clinical, based on the history, environmental conditions, and physical findings:

  • Rectal temperature: This is the single most important measurement. Temperatures above 41°C in the context of exercise, heat exposure, or inadequate cooling confirm hyperthermia.
  • Heart rate and respiratory rate: Sustained elevation out of proportion to the level of exertion confirms physiological stress
  • Blood work: Haematocrit and total protein elevated (haemoconcentration), elevated creatinine and urea (kidney stress), elevated muscle enzymes (CK and AST if muscle damage — rhabdomyolysis — has occurred), electrolyte imbalances (sodium, potassium, chloride)
  • Urinalysis: Dark, concentrated urine; myoglobinuria (red-brown discolouration) if rhabdomyolysis is present

Rhabdomyolysis (tying-up) frequently occurs alongside heatstroke during intense exercise and must be identified and treated concurrently.

Treatment

Immediate cooling is the priority — do not wait for the vet before starting emergency cooling measures:

  • Move to shade and cool air immediately
  • Apply cold water to the large vessels: Continuous application of cold water to the jugular grooves, inner thighs, and neck is highly effective. Contrary to older advice, applying cold water over the large muscle masses and scraping it off repeatedly is now the accepted gold standard — there is no evidence that cold water causes muscle cramping or vasoconstriction in the skin that would reduce cooling efficiency.
  • Ice packs: Applied to the jugular grooves, femoral arteries (inner thigh), and axillae (armpits)
  • Fan the horse: Moving air accelerates evaporative cooling
  • Oral or intravenous fluids: For dehydration and electrolyte replacement. IV fluid therapy is required in moderate to severe cases.
  • Monitor temperature every 5 minutes: Continue aggressive cooling until temperature drops below 38.5°C, then cease to avoid hypothermia
  • Veterinary treatment: NSAIDs to reduce fever, IV fluids for rehydration, treatment of concurrent rhabdomyolysis, cardiac monitoring in severe cases

Prevention

  • Acclimatise horses gradually to hot weather — allow at least 2–3 weeks when moving to a warmer climate
  • Schedule exercise in the coolest parts of the day (early morning or evening)
  • Monitor the Heat Index (temperature combined with humidity) — when combined values exceed a threshold of approximately 150–160°F (65°C), competitive events should be reconsidered
  • Ensure unlimited access to fresh water at all times
  • Provide shade and ventilation — particularly during transport
  • Cool horses down properly after exercise: walk to reduce heart rate, then apply water and allow to dry in shade before stabling
  • Monitor horses with anhidrosis particularly carefully in hot and humid conditions

Prognosis

Horses with mild heatstroke that are cooled promptly typically recover fully with no long-term consequences. Horses that develop severe hyperthermia (above 41.5–42°C), organ damage, or rhabdomyolysis have a more guarded prognosis. Central nervous system damage from prolonged hyperthermia may have lasting consequences. Death can occur in cases of extreme hyperthermia or where treatment is delayed.

When to Call the Vet

  • Any horse with a rectal temperature above 40.5°C that does not drop quickly with initial cooling measures
  • Collapse, incoordination, muscle tremors, or altered mentation
  • Failure of heart rate and respiratory rate to normalise after 30 minutes of rest and cooling
  • Suspected rhabdomyolysis (muscle stiffness, reluctance to move, dark urine)
  • Any horse exposed to extreme heat that becomes suddenly quiet, weak, or unresponsive

Frequently Asked Questions

Is cold water safe to put on a horse with heatstroke?

Yes. Modern evidence strongly supports the use of cold water applied repeatedly to the skin surface as the most effective first-aid cooling method. Older recommendations about avoiding cold water causing vasospasm or muscle cramping are not supported by current research. Aggressive cold water application combined with scraping and fanning is the recommended emergency response.

Can horses get heatstroke just from being left in a trailer?

Absolutely. A poorly ventilated horse trailer or lorry in direct sunlight can reach dangerously high temperatures within minutes, particularly in summer. Always ensure proper ventilation when transporting horses in warm weather, provide water at rest stops, and never leave horses unattended in a vehicle in the heat.

How does the humidity affect heatstroke risk in horses?

Horses cool themselves primarily through sweating, and evaporation of sweat from the skin is how heat is dissipated. High ambient humidity reduces the rate of evaporation, making sweating far less effective as a cooling mechanism. A horse working at 28°C and 80% humidity may be at greater risk than one working at 35°C and 20% humidity. The combined temperature-humidity index is a better guide to risk than temperature alone.

What is the difference between heat stress and heatstroke?

Heat stress describes a state of physiological strain where the body is working hard to maintain normal temperature — the horse is hot, sweating heavily, and showing elevated vital signs, but temperature is not yet dangerously elevated. Heatstroke describes the more severe state where thermoregulatory mechanisms are overwhelmed, core temperature rises to dangerous levels, and organ damage begins to occur. Both warrant intervention, but heatstroke is an emergency requiring immediate veterinary involvement.