Produced by glands in the skin in response to adrenergic stimulation sweat spreads over the skin surface aided by the surfactant protein Latherin to facilitate cooling by evaporation. Sweating is the main cooling mechanism used by horses and horses which are unable to sweat properly (anhidrosis or dry coat) are at risk of dangerous, potentially lethal overheating.
Sweat contains electrolytes and some amino acids as well as water. It may appear to foam due to the soapy material Latherin. Sweat losses in exercised horses working at moderate pace in mild conditions average 4.5 L per hour rising steeply to 10L – 20l per hour for those working hard in high ambient temperatures and may reach 40L per day representing significant loss of electrolyte ions.
Electrolyte depletion as well as dehydration may result from sweat losses causing reduced appetite, loss of skin elasticity (pinch test) and a ‘’tucked up’’ appearance in the belly due to loss of water from the gut reserves.
Severely depleted horses may refuse to drink initially. In endurance racing, horses provided with electrolyte supplements immediately before the start and during the race replaced sweat losses better than those provided with plain water only. Similarly, horses subjected to extended treadmill exercise simulating endurance rides of 30-45km drank well when offered water with 0.9% salt but refused plain water in the first few minutes. Lowered sodium in the body fluid from sweat losses tends to reduce the thirst reflex by reducing plasma osmolarity. In the humid tropics of S.E. Asia electrolytes are normally administered in the drinking water, not the feed, to encourage drinking following the heavy sweat losses routinely encountered there. However, to avoid exacerbating dehydration by overdosing salts, plain water should preferably be offered also.