Racehorse Heat

June 7, 2008, 7:45 am
It’s Hot Out Here for a Horse
By SID GUSTAFSON
Horses don’t like heat. They evolved in cool, even sub-arctic climates and are generally poorly-suited to deal with hot, humid weather. Heat makes horses sweat. Horses dissipate 75 percent of excess heat by sweating. The remainder of the heat is blown off by respiration. High humidity reduces the horse’s ability to dissipate heat by sweating, making it more difficult to keep the body temperature normal.
Hydration and electrolyte balance are critical in the racing thoroughbred. Muscle, nervous, pulmonary, cardiac, and joint function are vulnerable to electrolyte imbalances. Most electrolyte imbalances in thoroughbreds are caused by excessive pre-race anxiety and perspiration (washing out), which can be exacerbated by the use of race-day Lasix.
Potassium is one of the critical electrolytes depleted by washing out, as are sodium and chloride. Lasix depletes calcium and magnesium. These electrolytes are all essential for proper nerve, muscle, and circulatory function, and they all must be balanced in relation to one another.
When electrolyte dysfunction begins, wobbliness and weakness ensue, stressing the musculoskeletal system. After electrolyte imbalance becomes marked, the syndrome can move into thermoregulatory dysfunction, and the core temperature of the horse becomes elevated, causing further and more serious consequences. Although, high temperatures cause exercising horses to sweat heavily to dissipate the internal heat, susceptibility to heat stress is not solely influenced by ambient temperature alone. Excitable temperaments are the biggest culprit. Calm horses can generally maintain a normal body temperature and minimize sweating utilizing their ability to remain quiet and relaxed. In hot weather, anxiety-riddled horses can become electrolyte imbalanced before the race begins.
Other factors that may make horses vulnerable to heat include failure to be acclimated to hot temperatures and high humidity, tendency to sweat, and withdrawal of drinking water before racing. Racehorses may lose to 10-20L of sweat in a one-mile race. Fluid loss thickens the blood, making it flow more slowly, delivering less needed oxygen as the race perseveres. Additionally, hot horses redistribute blood flow to the skin in attempt to cool the blood off. This combination results in less blood being available for critical racing muscles, resulting in muscle weakness and cramping, weakness that may become especially noticeable in the last half mile of a one and a half mile race.
Sid Gustafson is a novelist, social commentator, and former thoroughbred attending and examining veterinarian licensed in New York, Washington, and Montana, where he has had significant experience in the regulation of racehorses, especially as it pertains to soundness and breakdowns.