July 2025 NL

Heat Stress in Horses

It takes horses (and humans) 2-3 weeks to acclimate to the hot weather. This year we went from a cool spring, rain in April and May, to instant heat and humidity in July.   When the total of the outside temperature and humidity (temp 86-93 + humidity 75-85%) is greater than 150, the horse’s ability to cool itself is greatly affected, which can result in heat stress. We have seen several heat related cases at the clinic already this year.  

Signs and Diagnosis of Heat Stress and Dehydration

1. Increased respiration and temperature of 103-105+ (normal for the horse is 99- 101.5).
2. Dry, tacky gums. Do a capillary refill time test by pressing your finger on the horse’s gums and counting how long it takes to return to its pink color.  If it is more than 2 seconds, the horse is dehydrated.
3. Decreased neck skin elasticity (skin tenting).  A skin pinch test is done by pulling up a fold of skin on the horse’s neck.  If it takes more than 2 seconds to flatten out, suspect dehydration.
4. Disorientation, incoordination, or down.

Heat stress also affects the intestinal system because during exercise the blood flow is directed away from the gut to the muscles, legs and skin.  The GI tract weakens, the GI immune system is compromised, and the GI tract is inflamed which can lead to colic, diarrhea and GI toxicity.

Factors Affecting Heat Tolerance

1. Age:  older horses sweat less and take more time to cool down.
2. Body Condition:  overweight horses have more fat insulating their body so more heat is retained.
3. Breed: Arabians are more heat tolerant.  Draft breeds are heat susceptible.
4. Health Condition:  Non sweater (anhidrosis), metabolic disorders, respiratory & heart issues.

Treatment of Heat Stress

Cold hose the horse and use a sweat scraper to remove the water so evaporative cooling can take effect, especially under a fan. Ice cold salt water (5lbs of salt in a 5 gallon bucket of ice water) sponged on will cool faster, like ice cream makers.  Cold hose the large muscle groups of the neck, shoulder, back, hindquarters until the horse’s temperature drops (usually 9 minutes). If you think that the horse is dehydrated, call the vet and seek care.

Prevention

Decrease the intensity and duration of exercise on hot humid days.  Preferably ride early in the morning, or after sunset when temperatures are lower.  If the old horse has not shed his hair or has a thick coat, clip him.  Add probiotics, salt and electrolytes to the feed. Give the horse electrolyte paste 1 hour before intense exercise, competition, or hauling.  Concentrate on the cool down of your horse post exercise.  Horses doing quick sprints often have a peak body temperature 30 minutes post exercise so keep periodic hosing and scraping for 1 hour.  At the end of a long ride, the body temperature is highest so hose right away and for 1 hour.