• Home	• FAQs
• Site Map	Weather
• Contacts	Weather

We Accept:

Directory Listings:

Pick of the Best!
Hillside Stable

Metabolic Diseases In Long Distance Performance Horses:

Last Modified: May 24, 1997

Patricia Dowling, DVM, MS; Diplomate ACVIM & ACVCP

Long distance performance horses such as endurance and competitive trail horses and three day event horses are different than other equine athletes in that they compete at aerobic speeds over extended distances. This type of equine activity results in unique metabolic disease syndromes. To understand the metabolic diseases seen in long distance performance horses, it is important to first understand the differences between a normal horse at pasture or light activity and the "normal" long distance horse during a competition. Then we'll compare the differences between the successful competitor and the unsuccessful horse that develops metabolic disease.

The Normal Horse versus the Successful Long Distance Competitor

Water Balance

Horses competing at aerobic speeds over long distance generate a tremendous amount of heat, which is primarily lost through extensive sweating. These horses routinely lose 5-10% of their body weight, which corresponds to a water loss of 20-40 liters in a 1000 lb horse. Human athletes produce a sweat that is mostly water, however, the horse produces sweat that is high in the electrolytes: sodium, potassium, chloride, and lower but significant amounts of calcium and magnesium. The amounts of these electrolytes lost in sweat are not proportional to their concentration in the horse's plasma. Sodium in sweat is present at concentrations equal to or slightly greater than plasma concentration. Chloride in sweat is 1.5 to 3 times greater than its concentration in plasma. And potassium in sweat is 10-20 times greater than its concentration in plasma. Therefore, it is obvious that the heavily sweating horse becomes dehydrated, with low plasma levels of chloride and potassium (hypochloremia and hypokalemia) and relatively normal plasma levels of sodium. With severe sweating, the loss of calcium and magnesium in the sweat also lowers their plasma concentrations (hypocalcemia and hypomagnesemia).

Red Cell Concentration (Packed Cell Volume)

The packed cell volume (PCV) is a ratio of the volume of red blood cells to the plasma. In the resting horse, PCV indicates the number of red blood cells. Long distance horses typically have lower resting PCVs than racing Thoroughbreds. The normal resting PCV of distance horses usually ranges from 30-40%, while the Thoroughbred normally has a PCV of 42%. Some of this is breed related, as Arabians are popular as distance horses, and they consistently have lower PCVs than other horses. However, even non-Arabian distance horses will have similarly low resting PCVs. The low PCV is an adaptation in the concentration of red blood cells in the blood to endurance exercise. The low PCV decreases blood viscosity (thickness), preventing sludging of the blood. This enables the exercising horse to minimize the workload of his heart pumping blood around the body, even in the face of dehydration.

The increase in PCV in the long distance horse during competition indicates the horse's degree of dehydration. The PCV increase of the successful distance horse reflects the horse's fitness plus the influences of environmental factors (ambient temperature, hay consumption, access to water on the trail). Some degree of dehydration is evident in all competing horses; however successful finishers readily replenish their fluid losses when water is available.

Acid-base Balance

There are many physiological processes that maintain the blood pH in a very narrow range for normal horses. Short-duration, high-intensity equine athletes exercise at levels that exceed the anaerobic threshold, resulting in accumulation of lactic acid in the muscles and development of a metabolic (systemic) acidosis. In contrast, long distance horses perform work aerobically and tend to develop a mild metabolic alkalosis. The alkalosis is a result of hyperventilation (rapid breathing in an effort to dissipate heat through the lungs), which blows off CO2- (carbon dioxide). Reducing CO2- raises the blood pH. Metabolic alkalosis is also caused by the loss of chloride (Cl-) in the sweat, in a greater proportion than the loss of sodium (Na+). The body attempts to balance the number of positive and negative ions at all times, so the kidneys conserve bicarbonate (HCO3-) in order to balance the positive charges of the sodium ions. This conservation of bicarbonate increases the blood pH.

Plasma Ion Concentrations

Compared to resting horses, exercising distance horses have low plasma concentrations of chloride (hypochloremia) and potassium (hypokalemia), and may have reductions in calcium (hypocalcemia) and magnesium (hypomagnesemia). Plasma phosphate may be increased (hyperphosphatemia), because of dehydration and because it is produced as a by-product of muscle cell activity. The metabolic alkalosis reduces the percentage of calcium in its ionized form, which is required for calcium's normal function in the transmission of nerve impulses. In addition to losses in sweat, magnesium levels are also decreased by the release of the stress hormone, cortisol. Magnesium also plays a role in normal nerve function. Low plasma magnesium worsens the negative effect of low plasma calcium on nerve impulse transmission.

Temperature, Pulse and Respiration

Most horses competing in long distance events enter checkpoints with elevated rectal temperatures, respiratory and pulse rates. Long distance horses generate enormous quantities of heat through muscle activity. Body temperature in successful competitors at high environmental temperatures and humidity can be up to 39.4° C (103° F). Body temperature should decline within 30 minutes of rest if ambient temperatures and humidity are not excessive. The primary means of cooling is through the evaporation of sweat. Sweating can be limited by dehydration and by high humidity. When sweating is insufficient to maintain normal body temperature, the horse compensates by increasing its heart rate, which increases blood flow to the skin for heat dissipation. As body temperature continues to increase, the blood is preferentially shunted to the skin while blood flow to muscles decreases.

Acceptable pulse recovery rates greatly depend on temperature, humidity, wind speed, difficulty of terrain and distance travelled, but general criteria can be made. Pulse rates should recover to 70 beats per minute (BPM) within 10 minutes, 66 BPM within 20 minutes and 60 BPM within 30 minutes. Exceptionally fit horses routinely achieve pulse rates of less than 70 BPM within 5 minutes of rest. Capillary refill time is 2 seconds and mucous membrane colour is pink.

When the cardiovascular system has reached its limits in trying to control body temperature, the horse increases its respiration rate in order to dissipate heat by moving air out of the lungs. The respiratory rate frequently exceeds the pulse rate (respiration/pulse inversion) during exercise, but should return to values equal to or less than the pulse rate with rest. In extreme heat and humidity, the high respiration rate may persist in attempt to dissipate heat.

The Horse In Trouble

Exhaustion

In the distance horse developing metabolic disease, the physiological alterations described in the successful competitor are more extreme and persist despite rest and availability of food and water. Early signs of exhaustion are identified in the exercising horse as a loss of motion elasticity and slight imprecision in the gait. When observed at rest, the horse may have slight drooping of the ears and eyelids, decreased gastrointestinal sounds, and relaxation of the anal sphincter (and penis in males). Severely exhausted horses are depressed, disoriented and oblivious to their surroundings. Dehydration is clinically evident by delayed skin fold rebound (greater than 3 seconds), dry mucous membranes and thick, scanty sweat. The PCV of successful finishers may increase to 35-45%, during exhaustion it may reach 55% or higher. Despite the dehydration, the exhausted horse may be reluctant to drink. The thirst response is controlled by an increase in sodium concentration in the plasma. With the loss of sodium ions in sweat, plasma sodium concentration remains fairly normal and the thirst receptors fail to trigger despite the dehydration.

Rectal temperatures above 39.4° C (103° F) should be rechecked every 15 minutes. Exhausted horses often will demonstrate rising body core temperatures with rest, because of the preferential shunting of blood to the skin with overheating. At body temperatures greater than 41° C (105.8° F), oxygen use exceeds supply and tissue destruction occurs (heat stroke).

Deterioration of the cardiovascular system is indicated by failure of the heart rate to recover with time. The cardiac recovery index developed by Dr. Kerry Ridgway is an easy-to-perform test that checks heart rate recovery and indicates the horse's fitness to continue. Capillary refill time (CRT) also indicates the cardiovascular status, and is measured by pressing a finger against the horse's gums and timing the return of blood to the blanched area. In exhausted horses, the CRT may exceed 3 seconds and mucous membranes may be "muddy" or dark in appearance. A wandering or thready pulse, abnormal heart rhythm and cold extremities (legs, ears) indicate imminent cardiovascular collapse.

The exhausted horse will breathe deeply and frequently, to move large quantities of air in attempt to repay the oxygen debt of muscle. In contrast, the horse that is attempting to reduce body heat via the respiratory system will breathe rapidly and shallowly. Rectal temperature and pulse recovery may help differentiate these two types of rapid respiration. If the pulse recovery is acceptable and the rectal temperature is 39.4° C (103° F) or less, then it is likely that the rapid respiration is an effort to cool. Otherwise, the rapid breathing should be considered a sign of exhaustion.

Other clinical conditions that may develop in the exhausted horse include laminitis (founder), kidney and liver failure and colitis (inflammatory diarrhea). These conditions may not appear for hours or days following the initial recovery of the exhausted horse, so follow up veterinary care is critical.

Synchronous Diaphragmatic Flutter ("Thumps")

Synchronous diaphragmatic flutter (SDF) or "thumps" are indicative of the horse's electrolyte imbalances. The condition is usually seen after the heavily sweating horse drinks plain water without electrolyte replacement. Absorption of plain water into the plasma further dilutes the decreased plasma electrolytes. Clinical signs are twitching of the horse's flank in rhythm with the heart beat. This occurs because calcium, potassium and magnesium are required for normal conduction of nerve impulses. Deficiency of these electrolytes causes increased nerve irritability. In horses, the phrenic nerve, which controls contraction of the diaphragm, passes over the heart. As the heart beats, its movement is sufficient to cause the hyper-irritable phrenic nerve to fire and cause a contraction of the diaphragm.

Exertional Rhabdomyolysis ("Tying Up")

Exertional rhabdomyolysis is a muscle disorder, also known as "tying up" because the horse's muscles become hard and sore. Rhabdomyolysis in long distance horses is initiated by poor blood flow to the large muscle groups. High body temperatures cause shunting of blood away from muscles to the skin for cooling. Low plasma potassium and increases in cortisol from stress cause constriction of blood vessels in muscle. From poor blood flow, the lack of oxygen in muscle interferes with the contraction-relaxation of muscle fibers and anaerobic metabolism in the muscle cells produces lactic acid. Despite the local acidosis in the oxygen-starved muscle, in distance horses the overall systemic balance is still a metabolic alkalosis because of the chloride loss/bicarbonate conservation. Damaged muscle cells release myoglobin into the circulation where it is removed by the kidney through the urine. Myoglobin in the urine (myoglobinuria) causes the urine to appear coffee-coloured. The myoglobin itself is damaging to the kidney, and kidney failure is a potentially fatal complication of exertional rhabdomyolysis. Horses exhibiting exertional rhabdomyolysis should not be moved except by trailer, because continued exercise increases the muscle damage, and they should receive emergency treatment.

Treatment of the Long Distance Horse with Metabolic Disease

Fluid and Electrolyte Therapy

The most important aspect of treating the exhausted horse is to correct fluid and electrolyte deficits. Fluids can be administered oral or by intravenous routes. Although the gastrointestinal blood flow is reduced, the use of oral fluids can still be effective and may be the only treatment required in mildly exhausted horses. To ensure adequate intake, oral fluids and electrolytes should initially be administer by stomach tube by a veterinarian. The electrolyte concentrations of fluids needs to be balanced. The administration of electrolyte-deficient or electrolyte-excessive oral solutions into the gastrointestinal tract causes further disturbances in plasma electrolytes.

The severely dehydrated horse may require 20 to 50 liters of fluids and they must be administered intravenously to treat the cardiovascular collapse. Intravenous fluids are given as rapidly as possible (5-10 liters per hour) until the horse responds by decreasing its heart rate, urinating, increasing gut sounds and decreasing the CRT. Because exhausted distance horses are suffering metabolic alkalosis, the choice of intravenous fluid therapy is important. Acidifying fluids arenormal saline (contains only sodium and chloride) or Ringer's solution. Lactated Ringer's solution, Multisol® or Plasmalyte® should be avoided as they are alkalinizing fluids. Either normal saline or Ringer's solution must have additional potassium added to make up the potassium deficit. If the horse is showing "thumps", calcium and magnesium must be supplemented as well, using solutions intended for treatment of milk fever in cattle.

In the horse showing signs of exertional rhabdomyolysis, aggressive fluid therapy is important to dilute the toxic concentrations of myoglobin being filtered by the kidneys. Careful attention must be paid to the amount and colour of the urine the horse is producing to monitor the development of kidney failure.

Anti-inflammatory Drugs

Muscle pain and anxiety activates the horse's sympathetic nervous system, which further reduces blood flow to the muscles and kidneys. Nonsteroidal anti-inflammatory drugs such as flunixin meglumine (Banamine®), phenylbutazone ("Bute") and ketoprofen (Anafen®) are commonly administered. However, their mechanism of action actually reduces kidney blood flow. They should never be administered to dehydrated horses without appropriate fluid therapy.

Dimethyl sulfoxide (DMSO) may help decrease muscle inflammation in the horse with exertional rhabdomyolysis. It can carefully be added to intravenous fluids at a 10% concentration after the dehydration has been corrected. In addition to its anti-inflammatory effects, DMSO acts as a diuretic (increases urine production) and increases kidney blood flow. This may help overcome the kidney toxicity of myoglobin.

Corticosteroids ("steroids") are contraindicated in the exhausted horse. The horse's own cortisol levels are already high due to stress. Additional supplementation with steroids decreases plasma calcium, increases urine excretion of potassium and increases sweating. Steroids can make exertional rhabdomyolysis worse, so they should only be used in cases of life-threatening shock.

Other Drugs

Additional therapy for horses with exertional rhabdomyolysis include tranquilizers (acepromazine, "ACE"; xylazine, Rompun®; butorphanol, Torbugesic®) and muscle relaxants (dantrolene, methocarbamol, guaifenesin) and vitamins (vitamin E, thiamine). Acepromazine is a tranquilizer that also dilates blood vessels. It may be beneficial in increasing blood flow to the muscles, but must used be carefully as it can decrease systemic blood pressure and reduce blood flow to the kidneys.

Summary

Long distance performance horses develop unique metabolic diseases. The system of veterinary checks is designed to identify horses that are not fit to continue so that appropriate treatment can be given and crisis situations prevented. However, it is ultimately the rider's responsibility to properly train and manage his/her horse and to work with the veterinary team in the best interest of the horse.

The above article was written by Dr. Patricia Dowling,
DVM, MS; Diplomate ACVIM & ACVCP who resides in
Saskatoon, Saskatchewan, Canada
It is reproduced here with her kind permission.