Heat-Induced Disease & Exhaustion in Horses

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Andrew G. Lang, D.V.M.

Many factors determine a horse’s resistance to overheating and exhaustion. How accustomed is he to his environment? Horses living where it is warm most of the year can tolerate heat better than horses who live in regions with seasonal extremes. Is he healthy and fit? The heart, blood vessels and lungs of a fit horse are better equipped to handle larger heat loads. Anyone involved with equine athletes or working horses should know what to look for-and what to do-to keep a horse from overheating.

Too Hot to Trot?

Photo Credit: Thinkstock

A horse’s risk of heatstroke depends on the weather and his level of exertion. Horses working at short intervals of high intensity are less likely to become ill than horses working for a long period at a lower level of activity. The average horse needs about 10 gallons of water each day; horses working in hot weather may drink twice as much.

If a horse loses fluid (as sweat) faster than he takes in water, he will gradually become dehydrated. Extremely dangerous is heat plus high humidity. A horse gets rid of heat as his sweat evaporates, but when it is too humid, he just gets hotter. The temperature-humidity index (THI) can be determined by adding the ambient air temperature in degrees Fahrenheit to the percent of relative humidity. If the THI reaches 150, especially if the temperature is 85 degrees or higher, it is very difficult for a horse to cool down. If it hits 180, he should not be working at all.

Horses need water both during and immediately after work. It is a myth that allowing a horse to drink right away will lead to serious health problems. If he waits, he may not drink enough to replace the fluids he’s lost. He should be offered water not much cooler than the air, with electrolytes mixed in. A prepackaged mix (available at most feed stores) allows you to prepare a “sports drink” for horses.

Anyone involved with a working horse should learn to check his heart and breathing rates, gums, hydration and temperature. A veterinarian can show you how. The normal temperature for a horse is 99.5 to 100.5 degrees Fahrenheit. It can reach 103 to 104 during exercise without risk, as long as the horse cools off (shade, rest, cool water on the skin and rehydrating should do it). If his temperature reaches 105, however, a horse is seriously overheated and vigorous cooling is needed. He should be moved to shade. If there is no breeze, a fan should be turned on him. Cool water should be poured on his body, squeegeed off, and reapplied until his temperature starts to go down.

Critical Conditions
Signs of serious exhaustion and heatstroke include a body temperature between 104 and 106 degrees, and/or rapid breathing and elevated heart rates that persist after 30 minutes of rest. The horse may be depressed and unresponsive, dehydrated but not thirsty, and still sweating mildly. His muscles may visibly cramp or spasm; he may become uncoordinated. You may notice “thumps,” which look like rapid hiccups. If you suspect exhaustion-or worse-call a veterinarian immediately. Then start cooling him off.

Heatstroke most frequently occurs in horses who are overworked in dangerously warm and humid conditions, but it can also strike horses at rest in poorly ventilated stables. In heatstroke, the horse’s skin will be hot and dry. Heart and breathing rates will be fast, and body temperature will be between 106 and 110 degrees. The gums will look “muddy” instead of pink. Weakness and depression will progress to stumbling, collapse and possibly coma and death. Should this occur, a doctor had better be close by. The horse will need repeated rinses with cold water; ice water enemas, in some cases; intravenous fluids and medication. If he survives, serious illness and permanent brain damage can result.

Anyone involved with a hardworking horse must take responsibility for understanding his limits and needs. The best way to keep him safe in the heat is to be aware, mind the weather, make sure he has plenty to drink and, most importantly, watch him closely. AW

Andrew G. Lang, DVM, the manager of Animal Health for the ASPCA, is spearheading the “As” new Equine Program.

Reprinted from ASPCA Animal Watch, Summer 2004, Vol. 24, No. 2, with permission from The American Society for the Prevention of Cruelty to Animals, 424 East 92nd Street, New York, NY 10128-6804

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