The Exhausted Horse Syndrome

The Exhausted Horse Syndrome

This is a collection of four articles (edited for length and content) which deal with different aspects of the Exhausted Horse Syndrome - Muscle changes, heat regulation and respiratory and cardiovascular effects, fluid and electrolyte aspects, and, field treatment. These papers were prepared by four specialists in the field of equine sports medicine and were distributed at a veterinary convention in 1986. The articles were printed because the exhausted horse is being seen more and more often these days, and we would like to make all OCTRA members and riders more aware of the problem and the systems to watch for. Although the articles mention endurance horses, this condition can just as easily be found in the over-stressed competitive horse and should be a concern to us all.

The Exhausted Horse - Muscle Changes
by David R. Hodgson, BVSc. PhD, Washington State University
Prolonged submaximal exercise, for example, that required during endurance rides, is the usual predisposing factor associated with the exhausted horse syndrome. Horses afflicted with this syndrome frequently demonstrate profound muscular fatigue, which is often manifest as an inability of the horse to perform any further exercise. A common element in this complex clinical phenomenon is depletion of the intramuscular glycogen store. During the exercise there is a selective depletion of glycogen from the different muscle fiber types as a function of the intensity and/or duration of the exercise. Depletion of glycogen from within muscle fibers is associated with a decreased force producing capacity by those fibers. Therefore, although muscle fibers are selectively recruited during submaximal exercise (with new ones being added as others become exhausted) when enough fibers are depleted of their glycogen stores, the overall force producing capacity of the muscle is reduced, resulting in the onset of fatigue, which may in turn contribute to the exhausted horse syndrome.

A major metabolic adaptation to endurance training is the body's ability to better utilize the fat stores in the muscles and the free floating fat cells in the blood. Greater care should be taken in conditioning the horse for the work planned to avoid over-stressing the animal.

Another muscular disorder which may accompany this syndrome is exertional rhabdomyolysis (tying up). Clinically this disease may produce a stiff, stilted gait, with hind-limb and back muscles most commonly affected, and some animals displaying marked muscle dysfunction sufficient to result in a reluctance of inability to move. In extreme cases, horses may become recumbent and incapable of rising. The affected muscles feel hard, and deep palpitation of these structures may produce obvious signs of pain. A similar disorder, although not as serious, produces mild to moderate gait abnormalities. This 'cramping' usually occurs in endurance horses when ambient temperature is high and the animals are dehydrated.

As the pathophysiology of this disease is poorly understood, treatment is usually aimed at controlling symptoms and involves replacing fluid and electrolyte deficits to ensure adequate tissue perfusion and renal function, analgesia and anti-inflammatory therapy, tranquilization and the use of drugs such as dantrolene sodium in an attempt to reduce ongoing muscle damage.

Exhausted Horse Syndrome: Thermoregulation; Respiratory & Cardiovascular Effects
by Reuben J. Rose, BVSc, PhD. University of Sydney, Australia
The generation of heat is an inevitable consequence of prolonged exercise, such as endurance riding. Because much of the energy released during exercise is converted into heat, hyperthermia can be a real problem unless this excessive heat is dissipated. The main means of accomplishing this hear loss are:

•Conduction - transfer of heat to and from the skin by direct contact i.e.. immersion in cold water
•Convection - forced transfer of heat by movement of gas of fluid i.e.. wind
•Radiation - transfer of heat as wave motion i.e.. gain of heat when the horse is in the sun
•Evaporation - the evaporation of sweat.

Evaporation is the main means of heat loss and its efficacy is dependent on skin temperature, local and general humidity and air movement. This heat stress is not just a result of high ambient temperatures but also the relative humidity, the wind velocity and the radiant heat.

Cardiovascular responses - Normally a great increase in muscle blood flow during exercise. The main cardiovascular response to heat stress is to divert some of this blood flow to the skin. This blood flow facilitates the dissipation of heat, but diverts blood flow from the visceral circulation (digestive system). This is why we often see colic problems in exhausted endurance horses. Also a decreased blood volume due to fluid loss in sweat and transudation of fluid into tissues. Therefore, a higher heart rate is needed for maintain cardiac output. The practical aspect of this is in the recovery period after exercise, when a poor recovery heart rate often indicates dehydration.

Respiratory responses - During prolonged exercise, especially in hot humid environments, sweating rate cannot be maintained. The respiratory system then serves an important role in thermoregulation and thus exhausted horses will often pant. Even under relatively normal conditions, the total minute ventilation will be increased and is associated with an increased ventilation. An endurance horse with an elevated post exercise respiratory rate should be watched very carefully for signs of exhaustion.

Fluid & Electrolyte Aspects of the Exhausted Horse Syndrome
by Gary P. Carison, DVM, PhD. University of California
Protracted exercise in endurance horses results in a massive metabolically generated heat load. Dissipation of this heat load is largely accomplished by the evaporative processes. With exercise in warm to hot weather, all horses develop significant fluid and electrolyte deficits. Why some horse develop signs of exhaustion and other horses are able to compensate and continue are not fully understood. It is known that the incidence and severity of exhaustion are much greater on rides conducted during periods of high environmental temperatures and/or humidity. Larger sweat losses would be anticipated under these conditions. Horses pushed at too fast a pace, that are not fully fit or heat-acclimatized or that are unable of unwilling to replace fluid deficits by voluntary consumption of feed and water appear to be at greater risk. The exhaustive disease syndrome is a multisystemic disorder involving dehydration, electrolyte imbalance, altered energy metabolism, and impaired heat dissipation.

As the horse's body fluid levels decrease through dehydration, his blood volume decreases and the heart must work harder to circulate the remaining blood throughout the body. Delayed recovery of pulse and respiratory rates may largely be related to changes in plasma volume.

While sweat losses of water and electrolytes could be partially replaced by water consumption, the desire to drink is dulled or eliminated by a change in the volume of blood and its concentration of salts and minerals. The horse will also refuse to eat and will look very depressed. Gut sounds will decrease, and muscle cramps and spasms including - synchronous diaphragmatic flutter (thumps) may be seen.

The combined effects of these interrelated factors alters function in the exhausted endurance horse such that he is unable to replace his fluid, electrolyte and energy deficits by voluntary consumption. Prevention of exhaustion is certainly preferred to treatment. However, once exhaustion is recognized, prompt and vigorous fluid therapy plays a central role in successful management of these horses.

Field Treatment of the Exhaustion/Shock Syndrome
by Mortimer Cohen, VMD. Acherman Creek Large Animal Clinic, California
Prevention is the most effective treatment of the exhaustion/shock syndrome. Recognition of unusual or extreme weather conditions should result in the modification of criteria or riding time, or even cancellation of the ride. These unusual conditions should also put the veterinarians and the auxiliary crew (ride workers - P& R people) on full alert.

Early recognition of the exhausted horse can be greatly facilitated by the utilization of the auxiliary crew. These people should be instructed to report any horse that either appears abnormal or stands for an inordinately long time (longer than 20 - 30 minutes) prior to presentation for pulse and respiration.

A persistently elevated pulse is often the first, and sometimes the only, sign of an exhausted animal. Other signs, which are variable include depression, lethargy, and obviousness to surroundings. Body temperature is generally elevated, although a diminished anal sphincter tone may result in inaccurate temperature readings.

Respiration is generally shallow and inefficient. Additional, thumps, arrhythmias, loss of skin torpor, dry mucous membranes, and inefficient sweating may be noted. These animals often have markedly decreased gut motility. Muscle fasciculation is also common. Treatment should be prompt and aggressive. Owners should be advised to bring horses to the attention of veterinarians if they suspect problems. It should be mandated that all horses be examined post-ride, even animals that were pulled during the competition. The time factor may be critical. Catch-up medicine is frustrating, often unrewarding, quite time consuming, and usually preventable. With aggressive early treatment, most horses will turn around quickly.

The aims of treatment are to restore the fluid volume, correct the electrolyte deficit, provide a readily metabolizable energy source, and reduce hyperthermia.

If an animal demonstrates no desire for food or water, has a pulse greater than 76 after 60 minutes post-competition, but is not manifesting severe signs, oral fluids are used. Approximately 2 gallons are pumped into the horse using a stomach tube. This regime may be repeated hourly 2 or 3 more times. If there is no improvement within 1 or 2 hours following oral fluids, or a deterioration is noted, then IV fluids should be instituted. Fluid volume restoration and replacement of sodium and chloride are the most important aspects of fluid therapy. Large volumes of fluids must be infused - as much as 40 liters in some horses - and both jugular veins may need to be cauterized.

The primary objective of fluid therapy is to restore enough fluid, electrolytes and glucose to enable the horse to replenish the defects by its own voluntary consumption. No steroids should be given as they are known to cause founder. Non-steroidal anti-inflammatory agents such as Banamine and Bute should be used with caution. They cause gastrointestinal and renal upset. Dosages should be low and only given after some fluid restoration has been completed.

Animals displaying 'thumps' should be treated with milk fever preparations. This should be done slowly, and discontinued if cardiac irregularities occur.

Indications of success include a decreased heart rate, improved attitude, development of appetite, appearance of gut sounds, decreased capillary time, improved colour of mucous membranes, and passage of urine.

Hyperthermia (high temperature) is treated by spraying the animal with cold water in an area with free air movement. Cold water enemas may also be used. Additionally, one may stand the animal in a creek and apply cold water to the area of the jugular furrow. Rapid cooling is essential if abnormal central nervous system function is noted.

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