Dr. Brianne Henderson

As we move through the height of our summer competition season, it is not uncommon to have multiple days of severe heat and humidity - the days when you sweat while standing still.

Heat and humidity can be harder on your horse than they are on you. Most of us have worked to motivate a sluggish horse through the final jump-off or day three of a competition, but why does it happen? Was our training off the mark? Perhaps the horse needs a different energy source in his feed. Or was it caused by that energy-zapping heat and humidity? Let’s take a close look at the role sweating plays in the horses' ability to cool, and how extreme heat and humidity can affect their ability to perform.

What happens to horses while exercising in the heat?

As the horse begins to work, heat is produced as a by-product of muscle contraction at a 4:1 ratio. As the body temperature climbs and adrenaline levels increase, sweat glands respond by producing a hypertonic (highly concentrated) salt solution that coats the hair. Under normal circumstances horses cool by evaporative cooling and convection. The movement of air over their body is paramount to both of these mechanisms.

Evaporative cooling: The sweat coats the hairs and as air flows over them it pulls the moisture and the heat off the horse.
 
Convection: Blood vessels near the skin dilate and allow the transfer of heat from the blood into the air.

During intense exercise, horses can lose up to 10-15 litres per hour in sweat alone. Once their body temperature reaches greater than 42 degrees C, the respiratory system kicks in to help “blow off” some of the extra body heat (approximately 15 percent of the body heat can be dissipated via respiration).

How does heat and humidity affect the horse’s ability to perform?

Perhaps the single most important calculation when determining the risk to your horse during hot and humid conditions is the Heat Stress Index (HSI).

Photo from: Worldwide BioMedEx Inc

HSI = Ambient Temperature (degrees C) + % Humidity

In short, the HSI means that the temperature and percentage of humidity must be considered together to explain their full impact on an athlete’s ability to perform. For example, during the early morning session it may be 20 degrees C and 98 percent humidity.

HSI = 15 degrees C + 98% = 112 HSI

Based on the information in the chart above, you would need ice water to cool your horse appropriately after a period of moderate to intense exercise.

Now let’s compare that situation with a typical afternoon weather report. By the afternoon, the humidity has usually burnt off and the temperature has increased:

HSI = 26degrees C + 70% = 96 HSI

We are now safely down in the yellow zone where we would only need cold water for cooling after a hard effort.

The repeated application of cold water will effectively remove excess heat from the horse’s body

In humid conditions, the air is already saturated with moisture. This reduces evaporative cooling, allowing sweat to cling to the hair like a hot blanket, causing a rapid increase in body temperature. Horses that are struggling with overheating will often be “inverted” or be seen to pant. Inversion is when the respiratory rate is higher than the heart rate (over a one-minute count). It can look like the horse is panting.

Once fluid loss (dehydration) has reached five percent of body mass (approximately 20 – 25 litres in your average saddle horse) there is a noticeable reduction in performance. Dehydration levels above five percent cause the athlete to feel ill as organ and brain function are impaired. It is also interesting to note that dehydrated athletes sweat less than normally hydrated horses (as the body tries to conserve any fluids it can). This further reduces the cooling mechanisms of the body, starting up a vicious cycle that can only lead to an overheated and severely dehydrated horse if not corrected early.

How can we improve our athlete management under these circumstances? “An ounce of prevention is worth a pound of cure.”

1. Make a plan - discuss the day’s heat stress index with everyone at the barn so that all team members are aware of the risk and what they need to watch for over the course of the day. Early recognition is key.

2. Teaching horses at home to drink electrolyte solutions or accept syringed electrolyte supplementation is also a good practice. Electrolytes can also be supplemented in the grain rations. Remember that horse sweat is a highly concentrated salt solution - there is approximately 10 grams of electrolytes per litre of sweat. Offer water frequently in between horse show classes, and ensure buckets are topped up when the horse returns to the stall.

Not all electrolytes are created equal, and different electrolytes are designed for different types of work. A thoroughbred sprinting on the track requires different support than a sport horse competing over three or four days, in the same way a track star is managed differently from a marathon runner. Your safest bet is to discuss your options with your regular veterinarian and include them in the discussion of how to support the horses through a hot and humid competition.

3. Ensure horses (and riders) get frequent breaks out of the sun, with access to cool water and stable fans. If you notice a horse showing signs of heat stress (soaked in sweat, inverted respiratory pattern, dull eyes) get the horse into the shade and start cooling immediately while someone contacts the attending or show veterinarian.

For veterinarians and riders of endurance horses, cooling horses becomes second nature with experience and education. Start by hosing or sponging cold water down the neck over the jugular veins, along the belly and in between the hind legs (look for the big dilated veins). The repetitive application of cold water is KEY to removing excess heat from the body. Continue to apply water to the body until the water coming off is not gaining heat. The addition of air movement will enhance heat removal from the body (natural breeze or fans can be used). You can soak the horse from ears to tail with cold water if necessary. (Tying up syndrome does not result from the application of cold water; it happens inside the muscle as a result of dehydration and accumulated “cell garbage.”)

Heat stress in horses can be very serious with potentially long-lasting impact on the ability of your horse to exercise during the heat. Take care to reduce the heat load of your horse during exercise, particularly during hot and humid weather.

Stay tuned for the next article in this series which will look at common leg injuries and treatment modalities available.

Reprinted with the kind permission of Equine Guelph.

Equine Guelph is the horse owners' and care givers' Centre at the University of Guelph. It is a unique partnership dedicated to the health and well-being of horses, supported and overseen by equine industry groups. Equine Guelph is the epicentre for academia, industry and government - for the good of the equine industry as a whole. For further information, visit www.EquineGuelph.ca.

Main Photo:  Roger Branch/iStockphoto