Fluid and electrolyte loss may lead to fatigue, muscle spasms and cramps, thumps (spasm of the diaphragm muscle, triggered by a decrease in calcium and potassium), dehydration, or heat stroke in horses that work hard in hot weather. Dehydration from sweating can interfere with the body’s ability to cool itself. A dehydrated horse develops obvious symptoms, according to Dr. Mike Foss at Alpine Veterinary Hospital (Hood River, Oregon). The horse’s temperature rises because the body can no longer sweat enough to cool itself. Skin becomes less elastic due to fluid loss from underlying tissues. The skin pinch test can help you estimate how dehydrated the horse is.
“If the horse has been sweating he’s been robbing fluid from the blood to do it,” says Foss.
There’s also less fluid in skin tissues. If he’s dehydrated, a pinch of skin pulled out from neck or shoulder does not spring right back into place, but stays pinched up for several seconds.
“If it takes 2 or 3 seconds for the skin to sink back into place, the horse is mildly dehydrated. He’s lost at least 6% to 7% of his body weight. Assuming the horse weighs 1,000 pounds (and many ranch horses weigh more) this means he’s lost 60 to 70 pounds of fluid (about 7 to 9 gallons),” says Foss.
“If the pinch of skin stays elevated for 6 to 10 seconds or longer, the horse is severely dehydrated. He’s probably lost 10% of his body weight, which means 100 pounds of fluid or 12-plus gallons. This horse needs immediate veterinary help.”
Mucous membranes in the mouth become dry and discolored, turning brick red instead of pink. Heart rate increases as the body tries to pump more blood to the surface for cooling, but has less fluid to do it with. The horse’s eyes seem sunken; eyelids and tissues around the eyes appear wrinkled, due to loss of fluid. Sweating is diminished, and what little sweat the horse produces is often thick, sticky and short on fluid. Other symptoms of dehydration are dry mucous membranes and poor capillary refill time. If you press your finger into the horse’s gum, the blood does not rush right back afterward; when you remove your finger, that spot stays white and pale for a few seconds.
The fastest way to cool an overheated horse is with cool water, applying it over the body, especially on areas where blood is near the skin surface such as neck, chest, belly and legs.
“You can’t just put cold water on; you must scrape it back off because it heats up immediately, creating a layer of insulation that slows the cooling. It works best to put it on, scrape it off — taking the heat with it — and put on more,” says Foss.
“Signs of heat stroke may begin as stumbling, fatigue, and increased respiratory rate. The horse may become restless, anxious, then show erratic or irrational behavior, depression or excitability, disorientation, rapid pulse and respiration. With mild heat stress, respiration may be as high as 40, and pulse 60. With severe heat stress, respiration may be above 60 and pulse above 80, even at rest. Some horses show muscle tremor,” Foss says.
Rectal temperature may reach 106 to 110 degrees. At first the horse is drenched with sweat, but in severe cases sweating stops because the horse has run out of water. Hot, dry skin is a clue to impending heat stroke. The horse won’t eat. He may be oblivious to his surroundings, and has difficulty moving. If he is hauled, he may go down in the trailer.
If left untreated, the horse may collapse, go into convulsions or coma and die within a few hours.
“Treatment for heat stroke includes cooling the horse as quickly as possible, and IV fluids to reverse severe dehydration,” says Foss.
Prevention of heat stroke is better than having to treat it. In most instances it can be prevented by gradually getting the horse in shape with short rides, paying close attention to vital signs and sweating, and avoiding situations that might trigger heat stress. Fat horses and heavily muscled horses that don’t dissipate heat well should be watched closely in hot, humid weather.