Reported by Joshua Hall, DVM
Heatstroke is a potentially deadly condition that can come on suddenly with little warning. Heatstroke occurs when heat production outpaces heat loss. A horse’s normal temperature is 100 to 100.5 degrees Fahrenheit. Temperatures beyond 103.5 challenge their metabolic processes. Beyond 105 degrees, horses may show signs of subtle incoordination or lack precision and regularity in their paces. The horse may be fretful and irritable, less attentive and sluggish in its work. If its temperature stays over 105 for longer than a few minutes, the horse’s sweating mechanism starts to shut down, and it loses interest in eating and drinking. All that internal heat dulls its brain’s cognitive functions, leading to disorientation. If the temperature is not brought down within five to 10 minutes, or if it rises to 108 or beyond, the horse likely will collapse, suffer convulsions and die. At temperatures over 111.2 degrees, tissues can literally break down.
Heat production even during gentle exercise can increase by 10 to 20 times over resting values. Sprinting results in an increase of 40 to 50 times over resting values. At work levels of 150 heartbeats per minute, a horse’s temperature will go up 1.8 degrees Fahrenheit every three minutes.
Working your horses in a steamy climate significantly increases their susceptibility to heatstroke. High humidity compromises the evaporative process because up to two-thirds of the heat-releasing sweat will roll off of the horse’s body before it can evaporate and cool the horse. This means efficient sweating is not always synonymous with efficient cooling.
The following rule of thumb has been devised as a guide. If the sum of the temperature and humidity is:
· Less than 130 – no problem;
· Greater than 150 – use caution, especially if the humidity is greater than half of the total;
· Greater than 180 – use extreme caution, since normal cooling is almost ineffectual and horses may resort to panting.