The Exhausted Horse

CHAPTER 204 The Exhausted Horse

The exhausted horse is most commonly seen in competitions that require horses to be engaged in athletic activity for an extended period. These competitions are often held when the weather conditions of heat and humidity are extreme. Horses competing in endurance races and cross-country events are most likely to be affected, but it is possible for any horse, taken past its level of fitness and pushed to continue performing in that state, to become physiologically exhausted. This may include horses participating in hunting, marathon driving, pleasure trail riding or competitive trail riding. Horses performing in excessive heat and humidity conditions can develop clinical signs of exhaustion sooner than would be expected in more temperate climatic conditions. Factors that can contribute to the onset of exhaustion include mild lameness, heat production incurred during exercise, body water and electrolyte losses via sweating, depletion of energy stores, lactic acid production, metabolic alkalosis, fluid and electrolyte shifting, and underlying problems not detected before the onset of competition.

Lameness may contribute to the onset of exhaustion by altering the way a horse travels, leading to increased use of appropriate muscles or preferential use of alternate muscle groups, contributing to earlier onset of fatigue. Overtraining or underconditioning may contribute to depletion of energy stores. Level of fitness is another important determinant. Inexperienced riders may override fit horses, leading to exhaustion. Horses not acclimated properly to environmental conditions of heat, humidity, and altitude may succumb to exhaustion earlier than acclimated horses. It is incumbent on the rider and the veterinarian to recognize these problems early in their development and intervene before exhaustion reaches a severity that puts the horse’s life at risk. Fluid shifts occurring during such intense exercise contribute to clinical and clinicopathologic findings in these horses when they fail to replenish fluid losses during performance. During endurance races, blood flow is preferentially directed away from the intestinal tract and kidneys to the locomotor muscles for metabolic demands and to the skin for heat dissipation. The fluid shifts seen in exhausted horses lead to hypoperfusion of areas such as the kidneys, digital laminae, and gastrointestinal tract. As a consequence of decreased blood flow and increased blood viscosity, the inflammatory and clotting cascades may be triggered in these regions, contributing to problems such as renal failure, laminitis, and disseminated intravascular coagulation (DIC).


The clinical presentation of an exhausted horse is dependent on the severity and speed of onset of exhaustion and the physiologic reserve of the animal. It is important to remember that there is marked variation in the extent of exhaustion that a horse may incur, and accordingly horses may manifest a range in the degree of severity along the continuum of possible clinical and clinicopathologic signs. Clinical signs are those of dehydration, metabolic derangements, depletion of energy stores, and hyperthermia. Generally, the cardiovascular system, musculoskeletal system, and gastrointestinal tract are affected. Initially, affected horses may show only mild alterations in attitude and alertness and may develop changes in gait because of muscle soreness. More severe clinical signs include the development of mental alterations such as depression, lethargy, and dullness and may extend to more marked neurologic signs, such as ataxia, circling, seizures, or head pressing. Horses may become reluctant to continue performing and exhibit muscle soreness or stiffness, spasms or cramps, which may progress to fulminant rhabdomyolysis and eventually recumbency. Metabolic derangements may contribute to the onset of synchronous diaphragmatic flutter.

Clinical signs of dehydration are common in varying degrees of severity and include pale or congested, dry or tacky mucous membranes; prolonged capillary refill time; delayed filling of the jugular vein when it is occluded to obtain a blood sample; decreased peripheral pulses; decreased skin turgor; and high heart and respiratory rates. Horses with these signs may progress to hypovolemic shock. Decreased-to-absent gastrointestinal tract motility is often present, and some horses develop profound ileus, gastric reflux, or signs of colic. Horses commonly are uninterested in eating or drinking. High body temperature and prolonged sweating may be present. Delayed heart rate recovery is commonly found in endurance horses on presentation to the veterinary check. Arrhythmias, most commonly atrial fibrillation, may arise as a result of electrolyte imbalances. In severely affected horses, cardiovascular collapse and hypovolemic shock may lead to death.


Common clinicopathologic findings include high red blood cell (RBC) indices (packed cell volume, RBC count and hemoglobin concentration), high total plasma protein and albumin concentrations, hypochloremia, low ionized calcium concentration, and azotemia. In addition, plasma lactate concentration and serum muscle enzyme activities may be high, and there may be acid-base abnormalities. The plasma sodium concentration may be normal or low. The plasma electrolyte abnormalities measured may not accurately reflect the intracellular alterations occurring throughout exercise, as ongoing losses from sweating may be balanced by electrolyte translocation from intracellular spaces. For this reason, potassium losses, which are great in sweat, are often not reflected in laboratory values even though a whole-body potassium deficit exists.

Hemoconcentration and hypovolemia are accompanied by increased blood viscosity, which, combined with fluid shifting to preferential compartments during exercise, results in poor tissue perfusion. It is not unusual for endurance horses that are eliminated from competition because of metabolic derangements to have a hematocrit in the range of 50% to 67% and total plasma protein concentration as high as 14 g/dL (normal range, 5.8 to 7.4 g/dL). Albumin may reach 4.5 g/dL (normal range, 2.9 to 3.6 g/dL). Initial elevations in plasma protein concentration may reflect inadequate compensatory fluid shifts from the intracellular and interstitial spaces resulting from dehydration. Many fit endurance horses tend to have a low resting hematocrit compared with horses competing in other disciplines, a finding believed to be a training response to prolonged fluid loss and heat stress and indicative of an increased plasma volume. Therefore, even mild increases in hematocrit in these horses may indicate a more severe problem.

Inflammatory mediators and endothelial cell dysfunction influence microvascular permeability and increased gastrointestinal permeability may lead to translocation of bacteria from the intestine into the circulation. Leukocytes and platelets may adhere to the endothelium, leading to leukopenia, neutropenia, inversion of the neutrophil-to-lymphocyte ratio, and a left shift frequently observed within 24 hours after the onset of clinical signs. Diarrhea may develop, and plasma protein levels may subsequently decrease.

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May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on The Exhausted Horse

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