Equine Heat Index Tool

Heat Index Guidelines for Horses

Keeping Horses Cool in the Summer Heat: When is it Too Hot?

Summer is one of the most exciting times of year for many horse owners and riders. Longer daylight hours mean more time spent in the saddle, with young riders off from school and show season in full swing.

While summer is a busy time for equestrians, it is crucial to prioritize the well-being and safety of both horse and rider, especially when facing severe heat and extreme weather conditions.

One factor that demands careful consideration is the heat index, a measure that takes into account both temperature and relative humidity to determine how hot it feels outside. Checking the heat index is important when determining whether it is safe to ride your horse or whether any special precautions are required.

In this article, we will discuss how to use the heat index to keep your horse safe and share key considerations for the care of your equine partner during hot summer weather.

Understanding the Heat Index

The heat index, also referred to as the “apparent temperature,” measures how hot it feels (perceived heat) by factoring relative humidity into the actual air temperature. This measure provides a better representation of risk of heat-related illness because humidity makes it harder for the body to cool itself effectively.

Humidity levels, or the amount of moisture in the air, directly impact the body’s natural cooling process, including sweat which is the horse’s primary mechanism for dissipating body heat. When the air is saturated with moisture, sweat doesn’t evaporate as quickly or efficiently from the skin, making it feel hotter than the actual air temperature and reducing the body’s ability to regulate its temperature.

Equines are more susceptible to overheating than humans due to their higher proportion of active muscle mass, which produces a lot of body heat. Horses also have a lower body surface-to-mass ratio, meaning they have less surface area through which to dissipate heat via sweat. ^[9][5]

High heat index values can lead to heat stress and heat-related illnesses for both horse and rider, especially when exercising. Always check the heat index before exercising your horse and modify activities or take precautions accordingly.

How is the Heat Index Calculated?

The heat index is calculated using a combination of air temperature and relative humidity, which measures the amount of moisture present in the air. The formula for calculating the heat index slightly varies across different countries, and it also depends on whether the temperatures are expressed in Fahrenheit (°F) or Celsius (°C).

A commonly used method to compute the heat index in Fahrenheit (°F) is the formula devised by the National Weather Service (NWS) in the United States. ^[13] This formula requires the following inputs:

• Air temperature (T): The ambient temperature in Fahrenheit (°F)
• Relative humidity (R): The percentage of moisture in the air, indicating how saturated the air is with water vapor

If the air temperature is given in Celsius (°C), convert this value to Fahrenheit (°F) first to perform the calculation. Note that this formula is designed for temperatures of 80°F or more, and relative humidity values of 40% or more. The heat index is not accurate for lower values.

Also, remember that the heat index is designed for shady, light wind conditions. Exposure to direct sunlight can increase heat index values by up to 15°F.

Figure 1: National Weather Service Heat Index Chart for Humans in Shady Locations ^[13]

Equine Heat Index Chart

Heat index guidelines for exercising horses are similar to those for human athletes, albeit with slightly different temperature thresholds, recommended precautions and activity modifications.

The following heat index chart provides equine-specific protocols for determining when it is safe to exercise your horse as well as appropriate cool-down routines to protect your horse from heat stress.

Staying Safe When Riding in Hot Weather

In hotter climates or during periods of extreme heat, temperature management, hydration, and proper care are vital to ensure the safety and welfare of both the rider and the horse. Before jumping into the saddle, consider the following: ^[11]

1. Monitor the heat index: Before you head out for a ride, check the heat index for your area. Avoid or modify riding when the heat index reaches potentially dangerous levels, usually above 90°F (32°C).
2. Hydration is key: Ensure that both you and your horse are adequately hydrated before, during, and after riding. Offer your horse water regularly, provide free-choice loose salt and feed electrolytes to replenish those lost in sweat. Take breaks during your ride to allow your horse to rest and drink water.
3. Time your rides appropriately: Plan your rides during the coolest parts of the day, usually early morning or late evening when temperatures are lower and the heat index is more favorable.
4. Adjust the intensity and duration of the ride: When the heat index is high, shorten the duration and reduce the intensity of rides to prevent overexertion and excessive sweating. Listen to your horse and be mindful of any signs of fatigue or distress.
5. Provide shade and ventilation: If possible, choose routes that provide shade, such as wooded trails or areas with buildings. Proper airflow helps horses regulate their body temperature. Ensure that your horse’s stable and turnout areas have sufficient ventilation.
6. Watch for signs of heat stress: Be vigilant for signs of heat stress in your horse, including excessive sweating, rapid breathing, elevated heart rate, lethargy (fatigue), stumbling, or muscle weakness. If you notice any of these signs, dismount immediately, find shade, and cool your horse down with water.

By carefully monitoring the heat index, modifying your riding routine, and being attentive to signs of heat stress, you can ensure your horse’s comfort and protect them from the risks associated with excessive heat. Ultimately, adopting an informed approach to riding in hot weather will keep both you and your horse safe and healthy.

What to Do if Your Horse is Overheating

If you suspect your horse is experiencing heat exhaustion or heatstroke, it is vital to take immediate action. Keep in mind the five principles of SHADE: ^[7][11]

Heat-Related Illness in Horses

Heat stress, heat exhaustion and heatstroke are often used interchangeably, but they each represent distinct stages of heat-related illnesses. Below, we will delve into the definitions and differences between these concepts.

• Heat stress is the physiological strain on the body when exposed to high temperatures or a hot environment. It typically occurs when the body’s ability to cool down through sweating and other cooling mechanisms is challenged.
• Heat exhaustion is a more serious heat-related condition when the body cannot adequately cool and maintain normal core temperature. It is usually a result of prolonged exposure to high temperatures, inadequate hydration, and insufficient electrolyte balance.
• Heatstroke is the most severe and life-threatening heat-related condition. It occurs when the body’s core temperature rises significantly due to prolonged exposure to high temperatures or strenuous physical activity in hot conditions. Heatstroke is characterized by a breakdown in the body’s ability to regulate its temperature, leading to multiple organ dysfunction.

Effects of Overheating in Horses

Overheating can have a profound impact on your horse’s health and well-being, potentially leading to central nervous system dysfunction and, in severe instances, even mortality. Consequences of heat exhaustion and heatstroke include dehydration, muscle damage, and organ dysfunction. ^[9]

Horses with heat-related illnesses often present with excessive fluid loss, resulting in electrolyte imbalances. Disruption of electrolyte levels can lead to the impairment of normal bodily functions (including muscle and nerve function), and give rise to additional complications. ^[9][7]

Heatstroke also stresses the horse’s vital organs due to increased body temperature, dehydration, and reduced blood flow. These factors can contribute to organ dysfunction and potentially lead to long-term tissue damage if not properly addressed. ^[9][7]

If your horse is showing signs of overheating, it is important to seek immediate veterinary attention and provide proper supportive care, including cooling measures, rehydration, and close monitoring. Prompt intervention is crucial to minimize negative consequences and support your horse’s recovery.

Signs of Heat-Related Illness in Horses

In hot and humid conditions, it is important to be vigilant for signs of heat stress and heatstroke in your horse and respond promptly. Exercising your horse in hot weather demands additional vigilance.

Here are some of the most common signs of heat-related illness in horses: ^[9][7]

1. Excessive sweating or lack of sweating: Heatstroke can manifest in horses as either excessive sweating or, in severe instances, a complete absence of sweating (anhidrosis). If your horse stops sweating in hot weather, this is a serious indication of heatstroke.
2. Rapid and shallow breathing: Horses with heatstroke often exhibit rapid, shallow breathing, panting, or increased respiratory rate. Your horse may show signs of respiratory distress.
3. Elevated heart rate: During heatstroke, a horse’s pulse can be significantly increased.
4. Elevated body temperature: Horses experiencing heatstroke will exhibit a noticeably elevated body temperature, typically measured as a rectal temperature above 105°F or 41°C. However, rectal temperature alone is insufficient to diagnose heatstroke, as other factors such as routine exercise and stress can temporarily increase body temperature.
5. Neurological abnormalities: Heatstroke can cause your horse to become weak, lethargic, and uncoordinated. They may appear dull, listless, or disoriented. Their performance and response to stimuli may also be noticeably decreased. In severe cases, this can progress to recumbency (lying down and unable to rise) and seizures.
6. Dark urine: Dehydration resulting from heatstroke may lead to concentrated urine, which appears darker in color than usual. Dark urine may indicate compromised kidney function due to severe fluid loss.
7. Red or congested mucous membranes: The mucous membranes inside your horse’s mouth, gums, and eyes may appear reddened or congested due to poor circulation caused by heat stress.

Normal Cooling Mechanisms in Horses

Horses use a combination of cooling mechanisms to dissipate heat and maintain normal body temperature (thermoregulation). These mechanisms include thermal radiation, convection, conduction, and sweat evaporation. ^[9] When these mechanisms are disrupted, horses become susceptible to heat stress.

Thermal Radiation:

Thermal radiation describes the process by which heat is exchanged between the skin (or hair surface) and the surrounding environment via electromagnetic waves. ^[6] Thermal radiation can be visualized with an infrared camera.

Convection:

Convective heat transfer occurs when the movement of a gas or liquid facilitates the transfer of heat, such as wind passing over the skin or cool air being inhaled into the lungs.

• Airflow around the horse’s body enhances convective heat exchange by displacing hot air close to the skin’s surface and replacing it with cooler air. ^[16]
• When blood flow increases, it helps dissipate heat through convection by transporting the heat away from the working muscles and toward the peripheral areas of the body. ^[2]
• An increased respiratory rate also helps with heat dissipation during inhalation by exchanging heated air in the lungs with cooler external air. ^[12]

Conduction:

Conduction occurs when heat is transferred between surfaces through a temperature gradient, such as when a horse is sponged or hosed down with cool water. Conductive heat transfer is one of the most effective methods to cool the horse’s down, so long as the water applied to the skin is cooler than the horse’s body temperature. ^[15]

Evaporation:

Equines, bovines, and primates (including humans) are the only species that utilize sweat production and evaporation as their primary form of thermoregulation. ^[2] In horses, approximately 70% of heat loss during exercise occurs through evaporative cooling, so long as the humidity is low. ^[3]

However, the efficiency of sweat evaporation is dependent upon relative humidity. Evaporative cooling is significantly less effective in humid conditions. ^[2]

Horses dripping in sweat are not effectively cooling themselves because the heat loss from sweat running off the body is only 5-10% compared to the heat loss that occurs through the evaporation of sweat from the skin. Additionally, excessive sweat production can lead to dehydration and electrolyte imbalance. ^[3][2]

Evaporative cooling also takes place through respiration as water evaporates from the nasal passages, mouth, and lungs. When horses exhale, the air they breathe out is warm (at body temperature) and filled with moisture (100% humidity), which facilitates evaporative cooling.

Although horses cannot pant like certain animals, their breathing rate and the volume of air they inhale can increase tenfold or even more during intense exercise, helping them eliminate more heat. ^[1]

Which Horses have the Highest Risk of Heat Stress?

Horses of any age, breed, sex, and fitness level are at risk for heat stress if they are living and/or exercising in extreme heat or humidity. However, some horses may be more susceptible to overheating, including those that are poorly conditioned or overweight.

Additionally, horses with underlying health conditions, such as respiratory disease, metabolic disorders, or anhidrosis, may have reduced heat tolerance and increased susceptibility to heat stress.

Respiratory Compromise

Horses with respiratory diseases, including equine asthma, heaves and pneumonia, have a harder time compensating in hot temperatures because they are less effective at dissipating heat through respiration.

Metabolic Disorders

Metabolic disorders include Pars Pituitary Intermedia Disease (PPID; previously referred to as Equine Cushing’s Disease) or Equine Metabolic Syndrome (EMS).

Horses with undiagnosed or unmanaged PPID often have long, thick hair coats and/or delayed shedding in the spring. This makes them more susceptible to overheating, even in moderate summertime conditions. Body clipping may be required for these horses.

Horses with EMS are often overweight or obese, making them less tolerant to hot and humid weather.

Anhidrosis

Anhidrosis describes a condition in which horses produce little to no sweat. The exact cause of anhidrosis is unknown, but it is thought to be related to a decreased sensitivity of sweat glands to the hormone epinephrine, which plays a role in thermoregulation. ^[8]

Unsurprisingly, horses with a reduced ability to sweat have a significantly increased risk of overheating because they cannot effectively dissipate heat through evaporative cooling.

Other Risks for Heat Stress

Other factors that increase the risk of overheating in horses include: ^[9]

1. Shipping long distances in hot, poorly-ventilated trailers without adequate breaks
2. Insufficient water intake or inadequate consumption of salt and electrolytes
3. Recent transportation from a cool climate to a hot climate without adequate time to adjust; research shows it can take horses 21 days to fully acclimate to hot conditions ^[2]

Alternative Heat Indexes

The Heat Index published by the National Weather Service is widely available to riders in the United States. However, horse owners and equine industry professionals may use alternative heat indexes that are either more readily available or provide a more comprehensive assessment of the risk factors associated with heat stress.

Wet Bulb Globe Temperature Index

The Wet Bulb Globe Temperature (WBGT) is another measure of apparent temperature that takes into account ambient temperature, humidity, wind speed, sun angle and cloud cover (solar radiation). This differs from the NWS heat index, which is calculated for shady areas and considers temperature and humidity only.

WBGT is used by athletes, military personnel and various industries to determine appropriate exposure levels to high temperatures. WBGT is a particularly important metric for activities requiring physical exertion, as it provides a more comprehensive understanding of the body’s ability to dissipate excess heat under different environmental conditions.

The WBGT is derived from three temperature readings:

• Dry Bulb Temperature (DBT): The standard air temperature.
• Wet Bulb Temperature (WBT): Measured by a thermometer covered in a water-soaked cloth, over which air is passed.
• Black Globe Temperature (BGT): Measured by a thermometer placed inside a black globe, which absorbs radiant heat.

These temperatures are then used to calculate the Wet Bulb Globe Temperature with the following formula:

WBGT for Horses

The Wet Bulb Globe Temperature (WBGT) Index is the only validated heat index for use in equestrian sports. It was first employed during the cross-country phase of the three-day eventing competition at the 1996 Olympic Games held in Atlanta. ^[14]

Today, the Fédération Équestre Internationale (FEI) utilizes the WBGT Index to assess heat stress risk and manage equine sporting events. Many other equestrian organizations also follow FEI guidelines. ^[10]

However, equestrians may only become aware of the WBGT Index if participating in FEI-sanctioned or other equine sporting events in which the competition is impacted by weather conditions. This is because WBGT values are not commonly broadcasted for public use.

The WBGT calculation is performed in real-time on-site and is not a widely available index for the general public. Obtaining WBGT values also requires specific equipment, which is typically not readily available to individual equestrians.

While some weather services provide general WBGT values for a given region, these values may not accurately reflect the specific conditions experienced by an individual horse or rider in a particular microclimate or localized area.

Table: Recommendations for the Cross-Country Day of Eventing (FEI Eventing Memorandum) ^[10]

Simplified Heat Index Guidelines

Adding together the temperature in degrees Fahrenheit (°F) and the percentage of relative humidity (RH%) provides a simplified representation of the heat index without requiring complex mathematical calculations.

This approach, while not as precise as the complex formula used by meteorological organizations, is popular because it is easy to understand and uses readily available data. This makes the simplified heat index a practical tool for the general public to make quick assessments about the potential risk of heat-related illnesses in their horses.

Keep in mind that this simplified heat index should be used as an approximately and not a definitive measure, as the actual heat index algorithm involves additional variables and calculations. Furthermore, this formula is not accurate in all conditions.

Table: Simplified Heat Index Guidelines for Horse and Rider (Adapted from the University of Minnesota Extension) ^[11]