Equine Sports Medicine

Common Causes of Poor Performance in a Horse

Section: Overview

  • Within the animal kingdom, horses are considered to be elite athletes because of their unique physiology.
  • Since they perform at such a phenomenally high level, even the smallest change in their health can knock down their performance.
  • These minute, often subtle effects on their health that effect performance are really challenging to detect, sometimes requiring special diagnostic tests.
  • It is important to investigate a decline in performance right away, since it will only get worse with further athletic activities.
  • Common causes of poor performance are usually categorized by the body system they affect:
    1. The musculoskeletal system (bones, joints, and muscle)
    2. The respiratory system (nose, upper airways, trachea, lungs)
    3. The cardiovascular system (heart, blood vessels, blood)
    4. The nervous system (brain, spinal cord, nerves)
    5. The gastrointestinal system (stomach, intestines)
  • Clearly visible symptoms that can be pinpointed to a specific body system are frequently not evident during the early stages of diseases that effect performance.
  • Diagnosis during the early stages rests on taking a careful and precise history, detailed physical examination, and a "high-tech" evaluation of suspicious body systems.

What to Watch For

  • The symptoms that clue you into poor performance depend on the horse's discipline. For example:
  • Dressage horses may have trouble coming onto the bit, or may refuse to perform a maneuver that they have had no trouble with in the past.
  • Racehorses may suddenly quit during the race, slow down at the 3/4 pole, or have a general loss of form.
  • Jumpers may start to refuse, or may take down rails.
  • Event horses may show poor recovery - such as prolonged high heart rate, respiratory rate, and temperature - from the more strenuous phases of the event, or may have trouble making times.
  • Pleasure horses may stumble, or show signs of irritation (tail swishing, head bobbing).
  • Endurance horses, like event horses, may show poor recovery, or inability to finish the ride.
  • Overt signs of disease, such as lameness or difficulty in breathing may arise in any type of horse.
  • Other signs may develop, such as going off feed, decreased manure production, 'crabby attitudes', or general loss of bloom.

Veterinary Care

  • Veterinary care should include diagnostic tests so that the specific cause(s) of poor performance can be identified and treated successfully.
  • Because there are so many possible causes of poor performance, it is important to be logical and methodical in choosing the following examinations or diagnostic tests.

Diagnosis

  • Complete physical examination and history. This may include watching the horse performing its accustomed sport, so that the veterinarian can appreciate what the owner has noted.
  • Musculoskeletal / Lameness examination.
  • Respiratory examination.
  • Cardiovascular examination.
  • Neurological examination.
  • Gastrointestinal and dental examinations.
  • Routine blood tests (complete blood count and serum chemistry panel)
  • The results of these tests will determine which of many other, more extensive diagnostic tests should be done.

Treatment

  • Treatment of poor performance is completely dependent upon the underlying cause.
  • Much time and money can often be saved by pursuing the cause of poor performance aggressively and early.
  • The problems generally worsen, so watching and waiting is not a good policy. The game is early detection!
  • Trial (empirical) treatments given without diagnostic support often fail and cost more money in the long run.

Home Care

  • Home care is dependent upon the cause of poor performance.

Common Causes of Poor Performance

Musculoskeletal - Degenerative Joint Disease

  • One of the most common causes of lameness in sport horses is degenerative joint disease (DJD), otherwise known as osteoarthritis. Areas that are often affected include the hocks (where it is known as bone spavin), the fetlocks, the pastern joints (where it is known as ringbone), coffin joints, and, less frequently, the carpal joints (usually called the knees by horsemen).
  • Degenerative joint disease, as the name implies, is a disease of wear and tear. The joints are lined by specialized tissue, called hyaline cartilage that is vital to smooth joint function. This cartilage can become frayed and damaged due to the mechanical wear associated with exercise. Nasty inflammation results, which furthers the damage in a vicious cycle - damage, inflammation, damage from inflammation, more damage, etc. The normally smooth glistening cartilage becomes eroded, exposing bone and causing pain. Horses show lameness that may respond to rest initially, but usually worsens with time. Anyone with a knee problem can attest to the pain and recurrence of DJD

Musculoskeletal - Navicular Disease

  • Disease associated with the navicular bone, which resides within the hoof capsule, has shortened the athletic career of many horses. Researchers are still working hard at determining the ultimate cause of navicular disease. The accepted causes included increased pressure within the navicular bone itself, and arthritis involving the navicular bone and surrounding structures such as tendons and the coffin joint surfaces. Although navicular disease has long been associated with Quarterhorses (possessing large bodies and small feet), in reality, it is seen in most large sport horses, including racehorses and warmbloods.
  • Because many horses have navicular disease in both front feet, many owners do not realize that their horse is lame. Rather, they may report that the horse has developed a very short, choppy gait, a "shoulder lameness", or seem reluctant to go forward. These horses are really sore when turned on hard surfaces, and actually start to limp.

Musculoskeletal - Tying up Syndrome (RER)

  • Veterinarians know this disease as recurrent exertional rhabdomyolysis (RER), which literally means disintegration of muscle. It causes a painful breakdown of muscle, which is often accompanied by a high heart rate and respiratory rate, sweating, and anxiety.
  • RER was traditionally called 'Monday Morning Disease', because it was seen in draft horses who were given Sunday as a day of rest but fed the full grain ration anyway. It is probably not the same disease that we see in chronic RER (see EPSM below).
  • We now know that RER affects all types of horses, but is most common in young fillies in training. It is seen in horses that are intermittently rested as well as in horses that are in continual work. Theories as to the cause of rhabdomyolysis include genetic defects in muscle function, carbohydrate (starch) overloading, thyroid imbalances, vitamin E and selenium deficiencies, and hormonal imbalances. The truth is that most cases are not associated with any of these causes, and the reason for the tying up is unclear.
  • Recently, an inherited disease, equine polysaccharide storage myopathy (EPSM), has been shown to be a cause of tying up in some Quarterhorses, draft horses, and warmbloods (See previous PetPlace articles on Tying-up -- Rhabdomyolysis, and PSSM).
  • Horses with RER may show vague symptoms such as stiffness after work, a shortened stride, an odd lameness which comes and goes, stiffness and weight loss. In rare cases, this condition can cause the horses to collapse, unable to get up on their own. Of course, this condition should not be confused with HYPP (Hyperkalemic Periodic Paralysis -- see PetPlace article on HYPP), whereby horses collapse and show weakness rather than stiffness after work.

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Respiratory - Inflammatory Airway Disease (IAD)

  • Horses in addition to being great athletes, are greatly allergic! Who doesn't know a horse with hives or with heaves? An early form of respiratory disease causing poor performance stems from an allergic condition as well. In certain horses that are predisposed, allergens set up inflammation in the small airways (hence the name, inflammatory airway disease, IAD). Some of the allergens we think are particularly instrumental in causing IAD include fungal spores, bacteria and their toxins, and air pollutants. The fungal spores can come from the cleanest looking hay, so proving to someone that hay was the source is a hard sell. This inflammation smolders for a long time and is generally invisible to the even the keenest observer, until the horse slows down or can't work as hard. At this time, further diagnostic tests are pursued that reveal the problem.
  • In horses with IAD, there are more profound changes than just inflammation. Inflamed airways transform, becoming hyperreactive, or 'twitchy.' Essentially, they constrict more readily and to a greater degree than do the airways in normal horses. The symptoms of constriction (bronchoconstriction) are coughing and exercise intolerance.
  • Because horses with allergies (IAD) have endured bouts of inflammation and constriction over and over again, their airways are also thicker due to development of excessive tissue and inflammatory secretions, and thus have a narrower lumen (passageway). The narrower airways cause a bottleneck in airflow, especially during hard work, which in turn decreases the available oxygen that is needed for work.
  • Because horses have an enormous respiratory reserve (i.e. the amount of excess lung not called upon during rest), the effects of IAD are often not noted until the horse is asked to exercise strenuously, breath deeply, or work at higher temperatures. This is why IAD was first recognized in racehorses. However, by this time IAD takes hold and causes the horse to slow down or a long-term cough to develop, the damage done to the lungs, while reversible, may be profound. It is absolutely imperative that IAD is recognized as early as possible, or the condition could go on to cause heaves.

Respiratory - Left Recurrent Laryngeal Neuropathy (LRLN, laryngeal paralysis, roaring)

  • The larynx (voicebox) forms the passageway to the trachea (windpipe). For effective function, the larynx must be able to close while the animal is swallowing, and it must become very wide during maximal exercise, to afford the greatest airflow.
  • Laryngeal paralysis, which is most common in horses taller than 16H, primarily affects the left side. For reasons that are not well understood, the nerves (the left recurrent laryngeal nerve) that serve the muscle that opens the larynx, begin to die. When nerve function is lost, the muscle is no longer stimulated, and so it atrophies. The end result is that the left side of the larynx becomes paralyzed and can no longer open normally. This prevents the horse from breathing freely during intense exercise.
  • The degree of laryngeal paralysis is very minor in a lot of horses (up to 80%), and severe in up to 10% of horses. It's a problem that worsens with age, so the percentage goes up in the higher age brackets. This is one reason why older horses can't work as hard as young racehorses.
  • Owners may suspect that their horses have LRLN because the collapse of the larynx during inspiration causes them to make a distinctive roaring noise, somewhat akin to a St Bernard panting or the sound of sawing timber. In some cases, the sound is more like whistling.
  • LRLN is more of an aesthetic problem in horses that do not compete at maximum capacity. It is not until horses are competing in races greater than one mile, high goal polo, or upper level three-day events, that laryngeal paralysis will actually interfere with performance.
  • In rare cases, the right side can get bad in addition to the left, and the larynx collapse. These horses can no longer perform without surgical intervention.

Respiratory - Exercise Induced Pulmonary Hemorrhage (EIPH, or 'bleeding')

Endoscopic view of blood in the trachea of a horse with EIPH
  • EIPH is very common in racing Thoroughbreds and Standardbreds (80-90% of all horses in this category). However, very few horses bleed visibly (less than 5%).
  • The causes of EIPH are widely debated. One of the most accepted theories is that the pressure in the vessels of the lungs becomes so great in racing horses, that capillaries (very small blood vessels) in the lungs actually rupture. Another theory is that horses have a low-grade blockage somewhere in their respiratory tract (nose, throat, lungs) and they need to pull air in harder. This creates a huge vacuum in the lungs with furthers explodes vessels. Still another theory contends that a "shock wave" travels up from the weight-bearing forefeet through the chest wall to the lungs, which shake so violently that there is rupture of vessels. Of all these theories, only the first theory has a lot of support form research studies, and from the fact that Lasix™, which decreases vessel pressure, seems to reduce bleeding in some studies. But the debate about the cause of EIPH and the effects of Lasix™ go on.
  • Although EIPH has caused great concern to many trainers, owners, and spectators, and poor performance is often ascribed to EIPH, it likely does not cause poor performance except in the few horses that bleed extensively on race day. We don't yet know how extensive the bleeding must be to affect performance. (See Figure 1)

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Cardiovascular - Atrial Fibrillation

  • Horses have enormous hearts in comparison to other species, and with training, they become even larger.
  • Intrinsic neurologic (vagal) input to the horse's heart also ensures that the horse has a low resting heart rate (32-44 beats per minute). This is known as having high vagal tone.
  • Both the size of the horses heart and the high vagal tone contribute to the development of atrial fibrillation. In horses, atrial fibrillation is usually benign, meaning that there is no underlying cardiac disease (this is different from cats, dogs, and humans). Occasionally, horses may develop atrial fibrillation because of problems such as leaky heart valves.
  • With atrial fibrillation, the electrical signals that ordinarily pass from the atria, the first set of filling and pumping chambers of the heart, to the ventricles, the second set of chambers, becomes disorganized. This causes the atria to beat in a very erratic fashion.
  • Because the ventricles still beat normally despite the erratic signals sent to them, the heart is able to pump adequate amount of blood at rest. But the extra little blood pumped in by the atria, while insignificant during rest, becomes important for the horse to perform strenuous exercise. For this reason, atrial fibrillation may go undetected for a very long time in horses that do light work, such as pleasure horses, trail horses, and show hunters. In a race horse, A fib would be picked up very soon. (See Figure 2)

Electrocardiogram of a horse with atrial fibrillation

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Neurologic - Equine Protozoal Myeloencephalitis (EPM)

  • EPM is caused by a protozoal parasite that invades the neural tissues of the horse. The spinal cord is most frequently affected, but the brain may also be involved (see PetPlace.com article on EPM).
  • The parasite, Sarcocystis neurona, is thought to be passed to horses when they accidentally ingest the feces of opossums. Horses do not pass on the infection to other horses or the environment. That's because S. neurona cannot complete its life cycle in the horse, thus the horse is an accidental (incomplete), "dead end" host, in technical terms.
  • EPM causes a wide array of neurologic symptoms. The most commonly seen abnormalities include ataxia (lack of coordination), muscle atrophy (especially of specific muscles in the head and tongue, as well as the gluteal and quadriceps muscles).
  • EPM often initially appears to be a lameness that can't be localized. This is where is becomes a cause of poor performance. With time, the disease worsens, and it is recognized as a major neurological dysfunction. Most cases of EPM are bad enough that they don't allow competition, so the symptoms are picked up at rest, but some are much more subtle. In these latter cases, it can be difficult to really implicate EPM as the cause, since there are many lamenesses related to the muscles and bones that are more likely, and yet equally frustrating to pinpoint. Keep an open mind.

Neurological - Cervical Vertebral Myelopathy (CVM, or Wobblers)

  • CVM causes neurologic symptoms due to problems in the neck. The bones in the neck have a deformity or instability that pinches the cord. This can happen either slowly over time due to unequal growth of the spinal cord and surrounding bones (vertebrae), or suddenly due to an up & down movement of a neck which has unstable connections, that pinches the cord while slipping from it's stable position. Alternatively, osteochondrosis dessicans (OCD) can affect the joints in the neck bones just as it can any joint such as a knee or hock. In this case, the OCD causes inflammation and huge calluses to build up around the bones, which later get so big that they actually pinch the spinal cord.
  • Horses with CVM have difficulty transmitting signals from the brain to the nerves controlling the legs, since they need to travel through the cord within the neck, which is narrowed or pinching it off.
  • Horses may show general incoordination, especially in the hind limbs. They may drag or "catch" their toes, stumble, completely knuckle, misplace their feet, sway from side to side unknowingly, or pitch out (circumduct) a leg when circled. They may also have a very stiff gait, and the owner or veterinarian may note that the toes are chipped and worn due to the abnormally stiff gait.
  • Although owners often report an initial traumatic incident, such as falling in the field, the fall was actually due to CVM, and the signs (which were subtle) now become severe enough to be clearly recognized.
  • CVM is a developmental disease, and is usually recognized in young horses, especially large, rapidly growing male horses. It gets worse with development since spinal cord is growing, but the bony canal is deformed.

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Gastrointestinal - Gastric Ulceration

Endoscopic view of gastric ulceration in a horse
  • Many of us know what gastric ulcers feel like. Gastric ulceration refers to the pitting or deep erosion of the stomach lining. It's painful, and some horses don't perform well when they have ulcers.
  • In nature, horses were meant to eat relatively poor quality roughage continually. Under domestication, horses are generally fed large amounts of high quality concentrates at infrequent intervals (for a horse, even 4 feedings a day constitutes infrequent intervals, unless he always has hay in front of him!)
  • Because horses are by nature continual eaters, they also secrete gastric acid continually. When they stop eating, the acidity in their stomachs rises dramatically. It is important to note that horses that are turned out to pasture, and have continual access to roughage, do not develop gastric ulceration.
  • Although the discovery of the bacterium, H. pylori, has been a great advance in the understanding and treatment of gastric ulceration in humans, no evidence of this bacterium has been found in horses. (See Figure 3)

Horses with gastric ulceration may show varying signs, such as poor appetite, chronic colic, poor performance, 'crabby attitude', and teeth grinding.

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Veterinary Care In-depth

Horse undergoing a stress test
  • Your veterinarian may perform one or many of the following tests:
  • For respiratory disorders:
    • Endoscopy of the upper airways will reveal laryngeal paralysis and may show signs of bleeding due to EIPH or excessive mucous due to IAD. In order to determine if the laryngeal paralysis is actually impeding performance, it may be necessary to carry out endoscopy while the horse is working on a treadmill.
    • Lung function testing or bronchoalveolar lavage ("lung wash") is imperative in the detection and treatment of IAD. Lung function testing reveals if there is small airway blockages, and how bad it is. Bronchoalveolar lavage will demonstrate the composition of inflammatory cells in horses with IAD, and red blood cells or their break-down products in the horse with EIPH.
    • X-rays of the chest may show areas indicating inflammation and thickened lung tissue in horses with IAD and EIPH.
  • For musculoskeletal disorders:
    • Your veterinarian will flex your horse's legs to see if he can pinpoint the site of lameness
    • Nerve and joint blocks are necessary to narrow down the exact site of lameness.
    • X-rays of suspected sites of lameness will help to determine if there is evidence of DJD.
    • Ultrasound of tendons, ligaments, and joints will let your veterinarian determine if there are any structural abnormalities involving these joints.
    • Nuclear scintigraphy will help to diagnose DJD and navicular disease when specific lesions do not show up on x-rays. It can also be useful in diagnosing RER.
    • Specific blood chemistry tests can help to diagnose RER. In particular, creatine phosphokinase (CPK), an enzyme that leaks from damaged muscle cells, will be elevated in horses with RER.
    • Urinalysis will detect evidence of damaged muscle cells in horses with RER. The most common finding is a discolored, brown-red urine that contains byproducts of muscle breakdown such as myoglobin.
    • Muscle biopsy may be necessary to determine the extent of damage that has been caused by recurrent bouts of RER, and is the only way to definitively diagnose EPSM.
  • For neurological disorders
    • X-rays of the neck may show areas of narrowing or damage due to unstable vertebrae. Often, it is necessary to perform a contrast study, called a myelogram, in order to determine if there is any compression of the spinal cord.
    • Cerebrospinal fluid (CSF) tap is the best way to diagnose EPM. Horses who are positive to testing for S. neurona are more likely to actually have the disease. Blood testing alone will only tell you if your horse has been exposed to the disease (in many areas, greater than 50% of horses have been exposed to S. neurona without developing neurologic disease). However, even the CSF test is fraught with many interpretation dilemmas.
  • For cardiovascular disorders
    • An ECG (electrocardiogram) will show the characteristic very irregular rhythm of atrial fibrillation.
    • An echocardiogram is often recommended to determine if there is any underlying cardiac disease in horses with atrial fibrillation.
    • A treadmill stress test with an ECG and echocardiogram may be useful to determine the extent to which the cardiovascular system is contributing to poor performance.
    • A stress test can be performed in some institutions to see if a heart problem is effecting performance. (See Figure 4)
  • For gastrointestinal disorders
    • Gastroscopy (endoscopic examination of the stomach) will reveal areas of reddening and erosion in the otherwise smooth, glistening surface of the stomach.

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Treatment In-depth:

Horse receiving aerosolized treatment for IAD
  • There are many potential treatments for all of the causes of poor performance we have discussed. Some of the most common are detailed below:
  • Inflammatory Airway Disease (IAD) is best treated with a combination of environmental management, anti-inflammatories (corticosteroids), and judicious use of bronchodilator drugs. This often means taking the horse off hay, and replacing hay with special hypoallergenic hay, hay or alfalfa cubes, or a complete pelleted ration. Alternatively, you can soak the hay, but it doesn't always stave off the breathing in of fungal spores by the horses. Today, the most effective method to treat the airways of horses with IAD is to give aerosols, just like the puffers used by asthmatics. (See Figure 5)
  • Left Recurrent Laryngeal Neuropathy is treated with surgery. However, unless the noise is truly bothersome (or prohibited, such as in hunters), it is important to determine if the LRLN is actually affecting performance. Your veterinarian can size up the problem to determine if surgery will be effective or not. The most commonly performed surgical procedure is called a tieback, in which the left side of the larynx is fixed in the semi-open position. In addition there are procedures that remove the vocal folds, and this is why horses after tieback make a hoarse, ineffective, whinny sound.
  • Exercise Induced Pulmonary Hemorrhage is commonly treated with Lasix, which is a diuretic. Studies have shown that Lasix may improve performance due to its effect to cause the horse to urinate, and thus shed 15-30 lbs of body weight right off the start. Otherswise, there is no known reason to think that Lasix will improve performance, and there is only minimal evidence that Lasix effects the course of EIPH. Some horses appear to respond to rest, as well.
  • Atrial fibrillation is treated with a drug called quinidine. Because quinidine is actually quite a toxic substance, it is necessary for this treatment to be administered under constant veterinary supervision. Most horses with benign atrial fibrillation will respond favorably to quinidine administration.
  • Equine Protozoal Myeloencephalopathy must be treated with anti-protozoal drugs. The most commonly used combination is pyrimethamine (Daraprim) and sulfa drugs. Other anti-protozoal drugs are currently still in the experimental stages. Treatment for EPM ordinarily must be continued for a minimum of 6-8 weeks, but some horses have required treatment for 3-6 months. Full resolution may not be achieved.
  • Cervical Vertebral Malformation - there is no real cure for CVM. Surgery to stabilize the vertebral column has been attempted, but full return to athletic function should not necessarily be expected in any case.
  • Gastric ulceration should be treated with a combination of management changes (more roughage, less concentrate in the diet, more turnout), and drugs that decrease acid production in the stomach. Some of the most commonly used drugs include cimetidine (Tagamet), ranitidine (Zantac), and omeprazole (Gastrogard).
  • Degenerative Joint Disease can be treated with a combination of training changes (usually, decreasing the concussive workload), anti-inflammatories such as phenylbutazone and corticosteroids (corticosteroids should be reserved for low motion joints such as the lower hock joints), and physical therapy (warm water soaks before work, cold water after, massage, range of motion exercises). Cartilage protectants, such as glycosaminoglycans (psGAGs), hyaluronic acid (HA), and chondroitin sulfate can be very helpful to restore the damaged joint surface and probably provide pain relief if they are anything like humans that attest to its effects.
  • Navicular disease is initially treated with shoeing changes, which may include egg bar shoes and degree pads. Judicious use of phenylbutazone and drugs that are thought to increase blood supply to the area may also be recommended. In chronic cases that do not respond to conservative treatment, a neurectomy (cutting the nerves that supply sensation to the heel) may be recommended. Isoxpurine, a vasodilator, also seems to reduce pain in some horses.
  • Recurrent Exertional Rhabdomyolysis is often an emergency in the initial stages. The horse should usually be transported home, and not be asked to walk. Depending on the severity of the signs, the horse may require intravenous fluids, anti-inflammatories, and sedation. Preventative treatment of tying up is controversial. Most veterinarians will advocate steady work without prolonged stall rest, low carbohydrate diets, supplementation with minerals and vitamins, and adequate salt in the diet. Other therapies that have been promoted include vitamin E and selenium supplements to the diet, dimethylglycine (DMG), methylsulfonylmethane (MSM), acepromazine prior to work, thyoid supplements, and dantrolene. None of the above treatments is dependable in warding off RER. The most important thing you can do is to reduce carbohydrate intake and maintain a dependable exercise regimen for your horse.

Follow-up

  • This will depend on what your veterinarian diagnoses as the cause of poor performance.
  • Regardless of the cause of poor performance, it is important to schedule re-evaluations as recommended by your veterinarian. A good example is to re-measure muscle enzymes in horses that have previously tied up.
  • It is also important to follow treatment recommendations. Do not expect overnight success. Often, specific treatments may take weeks to months to see real improvement. For example, treatment of IAD can take several weeks to months before the horse has returned to full form, and in many cases, the recognition and proper treatment can result in new lifetime marks and top performances in these horses.

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