Your Horse: Heat Stress and Hydration

If you feel suppressed by repeated days of hot and humid weather, you can be sure that your horse feels worse. 

Horses have a much more difficult time maintaining their ideal core body temperature during hot humid conditions than humans do. This is due in part to their very high amount of contracting muscle in relation to the surface area that is available to get rid of the excess heat. Core body temperature of horses can increase at a rate of approximately one degree Celsius (C) per minute when exercising in hot, humid conditions. The major mechanism for cooling involves diverting blood flow away from internal organs — especially the gastrointestinal tract (GIT) — towards the skin, and production of large amounts of sweat on the skin, which cools the diverted blood via evaporation. When exercising in temperatures less than 28 degrees C (as measured by “wet-bulb globe temperature” which takes into account air temperature, humidity, wind velocity, and solar radiation), this is sufficient to lower core body temperature back down to the “ideal” level. However, at temperatures greater than 28 degrees C blood flow to the GIT is progressively reduced, which can lead to intestinal ischemia and development of leaky gut syndrome, colic, and/or death. 

As the core body temperature begins to climb, there is also a decline in blood flow to the brain, which increases permeability of the blood-brain barrier, resulting in incoordination, irritability, and sometimes dangerous erratic behaviour. The core body temperature at which these changes occur is debatable, but studies have suggested 42.5 degrees C in the blood and 45 degrees C in muscle. Note that changes in rectal temperature (i.e., the way most of us monitor core body temperature in horses) is the last place to increase in temperature, so your horse’s core body temperature is likely one or two degrees C higher than that measured on a rectal thermometer.

The most effective method for cooling an overheated horse is by repeated applications of cold (ideally ICE cold!) water in a shaded area. Water can conduct heat away from the skin at a rate 20 times greater than that of air at the same temperature and can hold up to 3500