Heat stress and heat stroke refer to dangerous conditions that may occur when horses are working hard in hot weather—such as ranch horses gathering cattle all day.

Extremely hot, humid weather is always a risk, but most likely to be life-threatening when a horse is exerting (creating more body heat) or hauled in an enclosed trailer with inadequate ventilation.

Dr. Mike Foss of Alpine Veterinary Hospital, Hood River, Ore., says horses sweat to cool themselves, so when they are working hard, riders need to make sure they don’t run out of fluid and electrolytes and become dehydrated. On a long or hard ride, horses should be given a chance to drink at every opportunity. If a horse becomes dehydrated he can’t sweat enough to cool himself and may suffer heat stress.

“In the arid West we are fortunate because horses can sweat to cool themselves,” says Foss. “Dry air aids evaporation of sweat, taking heat with it. About 80% of excess body heat can be dissipated via sweating. A small percent is dissipated via air exchange in the lungs. Horses breathe faster, to facilitate that exchange. If they lose ability to sweat, either through dehydration or increased humidity (which hinders evaporation) they must utilize other ways to get rid of excess body heat, which are not as efficient,” he says.

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Fast respiration in a horse is the body’s attempt to cool off. 

“You must slow down and let the horse rest,” says Foss.

Avoiding heat stress

When a horse is too hot and unable to cool down he may be lethargic and depressed.

“After hard work he may be breathing fast (almost panting) and not recovering quickly. This might be the first thing you notice, along with not wanting to eat,” says Foss.

“You can take rectal temperature.  Most veterinarians don’t consider high temperature serious until it’s over about 105 degrees,” he says.  If it’s that high, check it again periodically to see if it starts dropping.  If it doesn’t, the horse has a problem. 

Transporting horses in hot weather can put them at risk. “I know of a case in California in which two horses were hauled to the vet on a hot day in an air-tight trailer, and they died in the trailer on the way,” says Foss.

“It’s much warmer in a trailer compared to outside. You need good ventilation and air flow, and it’s best if you don’t travel during the heat of the day,” he says.

“Put a thermometer in your trailer to know how hot it gets inside. It gets hotter in a trailer because of heat radiating up off the asphalt.  Some horse vans and trailers have air conditioning. If they don’t, they need lots of air flow. When hauling horses, try to do it during the night or early morning when it’s cooler, and stop and check them regularly. Don’t stop too long. During your leisurely lunch in an air-conditioned building, the horses are in the ‘oven’.”

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You might pull into a gas station periodically and use a hose with a spray/mister to dampen the horses with water, or use a sprayer bottle to spray water on horses if they’ll tolerate it. This can help keep them cool and give their sweating mechanism a break; they’ll dehydrate less.

Cooling a hot horse

After exertion, cool the horse quickly, especially if he’s extremely overheated. Air movement with fans is helpful, but if he is dangerously overheated--his temperature above 105--the fastest way to cool him is with cold water applied over the body, especially areas where blood vessels are near the skin surface (neck, chest, belly, legs) bringing overheated blood to the surface for cooling.

“You can’t just put cold water on the horse; you must scrape it back off because it heats up immediately, creating a layer of insulation that slows the cooling process,” says Foss. “It works best to put cold water on, scrape it off, and put on more. Keep scraping it off, taking the heat with it,” he says.

“In hotter climates people often add a pint of alcohol to a gallon of cold water, to enhance evaporation, or use ice water. Take his temperature every 5 to 10 minutes, to know whether you are bringing his temperature down. Stop cooling the horse when his temperature gets down to about 103, because you don’t want to chill him.” Muscles may cramp if he starts to get cold.

Walk the horse slowly in a circle, between water applications, to facilitate more air flow around the body--and allow any breeze to hit both sides of the horse. Gentle exercise helps blood circulation bring overheated blood to the skin surface for cooling.

“Try to have him in the shade, take off all tack, and use lots of water. If your water supply is limited, use a sponge to put it on and wipe it off. An overheated horse should not return to work that day even if he’s cooled out,” says Foss. If he was severely overheated he may need several days to recover.

Check with your veterinarian. “Heat stress can trigger other problems, including laminitis. We don’t always know how hot the horse actually got (before we checked his temperature) or how stressed he was, or how much damage might have been done.  Sustained temperature of 108 can cause serious problems. If the brain gets too hot the horse has seizures, and there may be damage that might be fatal.”