Photo Credit: Tanya Randall


In humans or horses, heat stroke can be fatal.  “People get headaches, may become sick, weak, disoriented, or may lose consciousness, and then all the body systems start to fail.  The same thing can occur in horses.  In mild to moderate cases the horses are hot, with increased respiratory rate, and dehydrated and tired.   They have electrolyte imbalances, which may show up in a condition called thumps (synchronous diaphragmatic flutter).  But if they are allowed to eat and drink, and given intravenous fluids, most of these horses recover fairly quickly,” he says.

“When it becomes more severe—horses that are more dehydrated, with more severe electrolyte disturbances, etc.—it’s life threatening.  Often we see this in endurance horses.  We see acute heat strokes in racing, polo, and other disciplines. but often those horses are not so dehydrated.  They may be very hot, but they may not have been exercising long enough to have electrolyte disturbances and dehydration,” he explains.

“Severe heat stroke in horses is similar to what happens in people.  You lose the ability to control your body temperature.  You may stop sweating in spite of the fact you are very hot.  And you can develop the same effects as going into shock; the various organs start to malfunction and shut down.  One of the big risks is damage to the central nervous system and brain.”  The brain is very sensitive to heat.

The horse in danger of heat stroke may have a high temperature, but more often has a moderate temperature and has been exercised for a long time.  “Or it could be a horse at a show, that’s been out in the heat all day without sufficient water or electrolytes.  The horse may have done several competitive events and has been in and out of the multiple times.  These horses start to look a little depressed, with slightly lower head carriage.  They may not be interested in drinking or eating.  They may not be very responsive to people.  They might not be sweating but might feel hot to touch,” he says.

There can be other signs that might be misleading.  “The horse might be a bit stiff in its gait due to muscle cramps, or show signs of mild colic.  The horse might be slightly wobbly when moved,” he says.

A horse doesn’t have to be exercising to get heat stroke.  It may be standing outside in the heat with no shade (or even in a hot stall), day after day, especially if there is high humidity and no breeze, and the horse is not getting enough electrolytes.  A horse that doesn’t have access to salt may be more at risk.

“Horses can get heat stroke after intense exercise for short periods, but probably the more serious form of heat stroke follows prolonged exercise in the heat.  A very fit horse may actually be more at risk than an unfit horse, because the fit horse has a higher capacity for exercise and may keep going longer.  Sometimes the unfit horses save themselves by quitting early,” says Marlin.

Heat stroke can lead to neurologic abnormalities such as unsteadiness.  Then the horse may collapse and be unable to get up.  “The horse may then go into a horrible downward spiral, with convulsions—due to swelling and damage in the brain.  These horses may become comatose and die.  There are other things serious things that can happen, such as development of associated problems like colic or laminitis, liver failure, kidney failure, tying up, etc., so it’s best to try to prevent heat stroke,” he says.


Heather Smith Thomas has raised and trained horses for 50 years and has been writing about them nearly that long, selling more than 9000 stories and articles and publishing 20 books. She and her husband raise beef cattle and horses on their ranch near Salmon, Idaho.


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