During prolonged exercise lasting many hours, heat generated in the course of mainly aerobic ATP resynthesis imposes a thermoregulatory demand on the animal. Responses include sweating and/or panting to remove heat from the body. The result is dehydration and acid-base and electrolyte disturbances. These factors are usually implicated in the fatigue, exhaustion, and even the possibility of shock and death that can occur after such exercise in horses. Fatigue during prolonged exercise has also been associated with depletion of muscle and liver glycogen stores and hypoglycemia. Studies of fatigue during prolonged exercise are difficult in horses, because horses will continue to perform treadmill exercise in the presence of severe, life-threatening hyperthermia and dehydration.

Exhausted Horse Syndrome

Horses occasionally develop severe clinical signs of fatigue at endurance events, despite current practices of evaluation of recovery at rest stops. Horses that have competed in 3-day events or endurance rides may present with exhaustion, a life-threatening condition. Horses may lose fluids at 10–15 L/hr by sweating during prolonged exercise, and urgent treatment of fluid and electrolyte deficits and hyperthermia (rectal temperatures >40.5°C) may be required. Affected horses may lose ≥10–20 L of body water; many have body fluid deficits of 40 L. Sodium and potassium deficits may be 4,000 and 1,600 mmol, respectively. Affected horses show signs of depression, fatigue, dehydration, elevated heart and respiratory rates, and hyperthermia. Hyperthermic horses should be continuously hosed with cold water, stood in the shade and, if possible, placed in a cooling breeze. Misting fans have been used at high level equestrian events in hot climates. Washing the horse with water that is extremely cold (an “ice slurry”) is a more aggressive and more effective form of cooling a hyperthermic horse. Isotonic balanced electrolyte solutions can be administered PO and IV for dehydration. Horses can be given 8 L, PO, initially, with subsequent administration of 4–8 L every 1–2 hr if needed. Numerous commercial electrolyte mixtures are available for oral administration. Hypertonic, hypotonic, and alkaline solutions should not be used. In severe cases, IV treatment is preferred. Up to 50 L may be required, which can be given at 5–10 L/hr. About 30 L of Ringer's solution is required to replace a sodium deficit of 4,000 mmol.

Environmental temperature and humidity have a major impact on the degree of disturbance to body fluids during prolonged exercise. It is particularly important to ensure adequate hydration before the event and to provide access to fluids during and after the event to reduce the likelihood of exhaustion. Administration of supplementary water, electrolytes, and glucose to horses before and during competition may reduce the incidence of exhausted horse syndrome.

Overtraining Syndrome

Highly intense exercise training over many weeks can result in a form of chronic fatigue referred to as overtraining syndrome. Racehorse trainers have long used the terms “overtraining,” “staleness,” or “sourness” to describe a syndrome of poor performance, failure to recover from exercise, and prolonged fatigue that does not resolve for weeks or months. By definition, signs of overtraining syndrome should persist after ≥2 wk rest or reduced activity. A less severe form of overtraining syndrome is termed overreaching, which is also a syndrome of poor performance and fatigue, but recovery occurs within 1–2 wk after a reduced workload.

Overtraining syndrome was first reported in Swedish Standardbred trotters based on observations of horses with signs of fatigue and poor performance combined with weight loss, inappetence, and signs of psychic stress, including tachycardia, nervousness, muscle tremor, sweating, and diarrhea. The horses appeared to develop an extreme form of overtraining associated with red cell hypervolemia and adrenal exhaustion that may be similar to parasympathetic overtraining reported in people.

In experimental induction studies, a milder form of overtraining syndrome has been reproduced, without any evidence of red cell hypervolemia or adrenal gland exhaustion. However, the syndrome was associated with a decrease in the plasma cortisol response to intense exercise, suggesting that overtraining is associated with dysfunction of the hypothalamic-pituitary-adrenal axis. Recent research has shown that overtrained horses have altered growth hormone activities, with an increase in the normal pulsatile growth hormone activity overnight, plus altered glucose metabolism.

Overtraining syndrome should be suspected in horses with evidence of sustained decreased performance in association with one or more physiologic or psychological (behavioral) signs. While no single physiologic marker is able to identify the syndrome, signs in horses may include decreased body weight, increased heart rate during exercise, decreased plasma cortisol response to exercise, and increased muscle enzymes or γ−glutamyl transferase concentrations. Behavioral signs are a consistent and early marker of overtraining syndrome, and development of behavioral scores to assist in early detection of overtraining syndrome in horses is warranted.

Last full review/revision July 2011 by Catherine McGowan, BVSc, MACVSc, DEIM, DECEIM, PhD, FHEA, MRCVS