Horses occasionally develop severe clinical signs of fatigue at endurance events, despite current preventive practices, including evaluation of recovery at rest stops. Horses that compete in sports that include 3-day eventing, endurance rides, or combined driving are at risk of presenting with life-threatening exhaustion, which is known as exhausted horse syndrome.
Etiology of Exhausted Horse Syndrome
In hot conditions, horses may lose body fluids at a rate of 10–15 L/hr through sweat during prolonged exercise. Urgent treatment of fluid and electrolyte deficits and hyperthermia (rectal temperatures >40.5°C [104.9°F]) may be required. Exhausted horses may lose up to 10% of their body weight in water, with some having body fluid deficits of up to 40 L, depending on their size. Water lost as sweat is mainly lost from the extracellular fluid and circulating plasma. Decreased blood volume reduces perfusion to vital organs and hampers thermoregulation. In severe cases, cardiovascular compromise may result in multiorgan failure, including damage to the kidneys, GI mucosa, and lamina of the hoof.
Pathophysiology of Exhausted Horse Syndrome
Sweating is associated with the loss of electrolytes, mainly sodium, potassium, and chloride, as well as calcium and magnesium. Alterations in muscle electrolytes can contribute directly to signs of fatigue. The most common acid-base alteration resulting from electrolyte imbalances is metabolic alkalosis (see video). Aerobic energy metabolism during endurance events produces minimal amounts of lactic acid; therefore, alkalosis caused by alterations in strong ions (hypochloremia and hypokalemia) predominates. Depletion of magnesium and calcium may contribute further to neuromuscular dysfunction, causing ileus, cardiac arrhythmias, and synchronous diaphragmatic flutter.
Unlike endurance riding, anaerobic metabolism predominates at the time of competition in events such as 3-day eventing and combined driving, resulting in metabolic acidosis (see video) during the event itself. After recovery, which can range from 30 minutes to 2 hours after the event, lactate is oxidized and the acidosis resolves. Metabolic alkalosis will then predominate. It is important to recognize that horses in these events, with multiple intermittent periods of maximal intensity exertion, may show clinical signs of both exertional heat illness and exhausted horse syndrome.
Clinical Signs of Exhausted Horse Syndrome
Horses affected by exhausted horse syndrome demonstrate a range of symptoms, including:
perfusion abnormalities and signs of dehydration
sweating, which may be inappropriate or absent
mentation changes, depression
alterations in gait, ataxia
clinical signs of myopathy (hard muscle bellies, pain on palpation)
persistent tachycardia and tachypnea, despite adequate rest
elevated rectal temperature (≥42°C [107.6° F])
Treatment of Exhausted Horse Syndrome
External and internal cooling therapies
Restoration of the circulating fluid volume
Cooling should be initiated by moving hyperthermic horses to shade and treating with cool or cold water sponge baths, cold hosing, or misting fans. Water should be removed from the hair coat with a sweat scraper and the process repeated, to prevent an insulating layer of warm water from forming on the skin. Application of ice water should be avoided, as well as alcohol baths or direct ice application, to prevent tissue damage. Cool-water enemas, or peritoneal and gastric lavage, may help to reduce body temperature in severe cases.
Isotonic balanced electrolyte solutions may be provided for dehydration by nasogastric intubation if the horse has normal borborygmi. Horses may be given up to 8 L initially, with subsequent administration of 4–8 L every 1–2 hours, as needed. Commercial electrolyte mixtures for horses are suitable, but hypertonic, hypotonic, and alkaline solutions should not be used. In severe cases, IV fluid therapy is preferred. A shock dose of a balanced electrolyte solution should be provided initially (20–40 mL/kg bolus) with addition of 100 mL 23% calcium gluconate per 5 L and 5% dextrose. Additional fluids and additives should be based on reevaluation of serum chemistries and hydration status. Diuresis may be needed in cases with renal insufficiency or concurrent myoglobinuria.
Additional treatments may include NSAIDs for muscle pain and colic, administered simultaneously with fluid therapy to prevent renal injury, and phenothiazines for anxiety caused by myopathies. Anticonvulsant medications may be required, and dexamethasone may help to reduce cerebral edema. Dimethyl sulfoxide administered IV may be useful to further reduce inflammation, and low-molecular-weight heparin can be administered SC to treat coagulopathies.
Prevention of Exhausted Horse Syndrome
Environmental temperature and humidity have a major impact on the severity of disturbance of the horse's fluid balance during prolonged exercise. It is important to ensure adequate hydration before an event, especially after long trailer rides to the competition, and to provide access to fluids during and after exercise to reduce the likelihood of dehydration. Administration of supplementary fluids, electrolytes, and glucose before and during competition, when allowed by doping regulations, may reduce the incidence of exhausted horse syndrome.