Ask the Vet: Beat the heat, pt. I

Our newest columnist, Dr. Jen Johnson, pens the first in a two-part series about proper post-workout care for horses in hot weather. Today, she explains the science behind equine body temperature.

From Jen:

When the IOC granted the 1996 Olympic Games to the city of Atlanta, one of the biggest challenges was how to safely compete upper level Eventing horses in the heat and humidity of the southern United States.  Several years of quality research were conducted to fully understand the physiologic changes experienced by horses during extreme exercise in order to design the optimum cooling protocol.  The results of these studies squashed the validity of some of the old “tried and true” ways, and every competition horse came through the event without any negative effects of the heat.

Not just for upper level cross country

In the years since Atlanta, researchers have continued to investigate the effects of extreme exercise on horses.  One critical piece of information that has become abundantly clear is that aggressive cooling and proper post-workout care is not just for the elite equine athlete; temperature and electrolyte changes can occur to the same magnitude in a lower level horse.  One theory is that lower level competition horses are usually not as fit, therefore the effort needed for these horses to navigate a novice cross country course is similar to that used by a fit horse to complete an advanced course. Or, in human terms, if you sat on the couch all winter long, a 5-K would be pretty tough. But if you’re used to running a lot, ramping up the effort for a half-marathon might not seem like that big of a deal. Although the distances are very different, the effort (and physiological effects) are similar due to a difference in baseline fitness.

So, just because you ride at the lower levels, or even just for pleasure, this does not excuse you from proper post-workout care. But before you can properly design a post-workout plan, first lets look at what’s going on in your equine athlete.

How horses get hot

Horses are, without a doubt, incredibly gifted athletes. Their bodies and physiology are built for speed and endurance. They even have an extra reserve of red blood cells stored in their spleen that can be released during periods of high exercise, to maximize oxygen carrying capacity. During work, their muscles generate a lot of energy and a large portion of this energy is released as heat.  The core body temperature will rapidly rise, and at the same time, the horse’s respiratory rate will increase. A horse will move as much as 1800 liters of air (think 6 bath tubs full) over about a ½ mile distance. All this work continues to generate heat – the faster/longer they go, the more heat is generated from the muscles which in turn increases the respiratory rate to feed oxygen to those muscles, which also creates heat!

How horses get cool

A horse’s main cooling mechanism is to sweat. As the sweat evaporates from the hair coat, it takes heat with it. If the horse continues to work, the horse will continue to sweat away the heat generated from the exercise. Seems pretty straight forward, and so why all the fuss about intervening?

Enter humidity…

We know that the evaporation of the sweat is what allows the horse to cool, but when the humidity is above 75%, or when the combined temperature + humidity is over 140, the sweat does not evaporate. No evaporation, no cooling. So what’s a pony to do? Several things, none of which are terribly desirable:

• The horse continues to produce more sweat in an effort to cool
• Fluids and electrolytes are lost in the sweat
• As fluids are lost, blood vessels near the skin (which carry away heat from the body core) constrict, limiting body surface blood flow (and diminishing heat dissipation capacity)
• If the horse is still working, muscle efficiency can diminish due to electrolyte loss. This causes the horse to work harder to maintain the same level of output

In high humidity, just stopping work is not enough.  It is very common for core body temperature to continue to rise after cessation of exercise, because the cooling mechanisms are ineffective.

Stay tuned for part 2, coming your way tomorrow!