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Heat Stress in Horses: Prevention and Management

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Horses are more susceptible to high temperatures and humidity than humans. When temperatures exceed 28°C, coupled with humidity levels above 50%, horses can struggle significantly to maintain their body temperature. To prevent overheating and other heat-related issues, it’s crucial to help your horse cool down effectively.

Understanding Heat Stress in Horses

Heat stress occurs when a horse’s body temperature rises to levels that compromise cellular function and overall health. This can cause significant damage to the horse’s body. A horse can overheat due to hot environmental conditions, intense exercise, or a combination of both. Because of their large muscle mass and in comparison with their surface area, they warm up faster and cool down less efficiently than humans. This makes them particularly vulnerable to heat-related health issues.

Horses vs. Humans: A Comparative Perspective

Horses and humans have different mechanisms for regulating their body temperature. Horses are less tolerant of high temperatures (>28°C) and humidity (>50%) compared to humans (Lindinger & Marlin, 1995). The following table highlights some key differences.

Differences in temperature regulation between horses and humans (Marlin & Nankervis 2002; Sloet 2014)

                                    Horse Human
Normal body temperature at rest 37,4 – 38 °C 37 – 37,5 °C
Body temperature at hard work or hot weather Up to +/- 41 °C and more Up to +/- 41 °C and more
Thermoneutral zone -5 – +15 °C  25 – 30 °C
Amount of sweat 10 – 15 litre/hour  3 litre/hour

Thermoneutral Zone & WBGT

A horse’s thermoneutral zone ranges from -5°C to +15°C (Sloet, 2014). This is the temperature range in which a horse can easily maintain its body temperature. The thermoneutral zone can vary depending on the horse’s environment when temperatures approach 28°C or higher, especially with high humidity, horses struggle to dissipate heat effectively. This increases the risk of dangerously high body temperatures(Lindinger & Marlin, 1995).

The Wet Bulb Globe Temperature (WBGT) index is a good tool for assess heat stress accurately. Unlike regular temperature measurements, WBGT considers humidity, sunlight, and wind to provide a comprehensive measure of thermal stress (Marlin et al., 2018; Sloet, 2014).

  • WBGT under 28: Generally safe, no additional measures are required.
  • WBGT above 28: Implement cooling measures to prevent overheating.
  • WBGT above 33: It’s not safe to work horses in these conditions (Marlin et al, 2018; Sloet, 2014). 

Dangerously High Temperatures

If a horse’s rectal temperature approaches 39.5 -40°C, it indicates a dangerous level of overheating. Immediate cooling with cold or ice water (5-10 °C)  is crucial to prevent serious health issues, because such high temperatures can damage body cells (Lindinger & Marlin, 1995).

When taking the horse’s temperature, ensure the thermometer is inserted deeply enough into the rectum to avoid inaccurate readings (Green et al., 2008; Sloet, 2014).

Overheating is a serious danger for horses in hot weather with fatal consequences

Symptoms of Overheating

A high body temperature can lead to various heat-related illnesses, such as overheating, heat damage, and heatstroke. Clinical signs include:

  • Lethargy, weakness, and ataxia (loss of coordination)
  • Lack of interest in food, water, or surroundings
  • Reduced sweating ability
  • Metabolic issues like muscle stiffness, colic, and kidney failure
  • Reddened mucous membranes (Bongers et al., 2017; Sloet, 2014)

Additionally, high core temperatures can lead to fatigue, as the central nervous system decreases muscle strength to prevent further heat production (Bongers et al., 2017). If you notice one or more of these symptoms in your horse, it’s critical to act quickly.

What Can You Do to Prevent Overheating?

Preventing heat stress in horses requires proactive management, especially during hot weather:

  • Monitor environmental conditions: Use the WBGT index to assess when it’s safe to train or compete (Marlin et al, 2018; Sloet, 2014). 
  • When not needed to compete in hot environments OR when ambient temperature > 30 °C –> Limit exercise: Reduce the intensity and duration of training sessions during hot weather and avoid the heat by training in the early morning or late evening.
  • Ambient temperature < 30 °C & need to compete maybe under warm circumstances –>Acclimate your horse: Expose your horse to warm conditions and train during the warm periods of the day for at least 14 days in a row  to help them adapt physically (Marlin et al., 2018, Munsters et al. 2024). Monitor your horse closely and seek expert advice when doing this.
  • Cooling strategies: After exercise, cool your horse thoroughly with cold or ice water. Make sure to target the large muscle groups and areas with high blood flow, such as the neck and chest (Lindinger & Marlin, 1995).
Actively cooling horses using cold water is essential to prevent heat related injuries

Conclusion

Managing heat stress in horses is vital for their health and performance, especially in high temperatures and humidity. Horses are more prone to overheating than and are less able to cool down efficiently. By understanding the risks and signs of heat stress, monitoring environmental conditions with tools like the WBGT index, and implementing effective cooling strategies, you can help prevent serious health issues and keep your horse safe and comfortable.

    Sources

  • Bongers C.W.G, Hopman M.T.E & Eijsvogels T.M.H. (2017). Cooling interventions for athletes: An overview of effectiviness, pshysiological mechanisms, and practical considerations. Temperature, 4(1), pp. 60-78.
  • Green A.R, Gates R.S, Lawrence L.M & Wheeler E.F (2008). Continuous recording reliability analysis of three monitoring systems for horse core body temperature. Computers and electronics in agriculture 61, 88-95.
  • Lindinger M.I & Marlin D.J (1995), Heat stress and acclimation in the performance hors: where we are and where we are going.Equine vet. Educ, 7(5), pp. 256-262.
  • Marlin D., Misheff M & Whitehead P. (2018). Optimising performance in a challenging climate: Preparation for and management of horses and athletes during equestrian events held in thermally challenging environments. FEI Sports Forum
  • Sloet M., (2014). Hittestress bij het paard. Dier en Arts
  • Sloet M., (2014). Oververhitting. Paard&Sport
  • Munsters C., (2024) Effect of a 14-Day Period of Heat Acclimation on Horses Using Heated Indoor Arenas in Preparation for Tokyo Olympic Games. Animals, 14, 546
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