For a horse to perform well as an athlete, all body systems must be in good working order. When one or more systems "breaks down," the horse is no longer able to perform up to his potential, and the owner, rider, and trainer will likely notice a decline in performance during training and competition.
In some cases, the reason for the loss of performance is quite obvious--the horse is lame with a swollen joint, he developed exertional rhabdomyolysis (tying-up) during a recent workout, or there is an abnormal discharge from both nostrils, suggesting a respiratory problem. In these cases, a thorough clinical examination with traditional diagnostic procedures and tests will usually point to a specific diagnosis. However, many problems that result in poor performance are quite subtle and, very often, the horse appears to be fit and well when examined at rest. These kinds of problems are only evident during exercise, and some form of exercise testing is usually required to make a diagnosis.
This situation presents a particularly difficult diagnostic challenge for the veterinarian. This article discusses the approach veterinarians take to unravel the cause of poor performance problems.
Documenting Performance Loss
Poor performance refers to the inability to exercise or perform at a level previously observed or at a level that can be reasonably expected based on the horse's physical characteristics and state of training. Keeping track of your horse's accomplishments in competition is a big help to the veterinarian as he/she attempts to sort out the cause of performance problems. For the speed demons (e.g., racehorses or Quarter Horse barrel racers), an increase in running times will clearly indicate a loss of performance. For the endurance horse, an early sign of a fall-off in performance can be a higher than normal heart rate during exercise or a delayed heart rate recovery. As well, there could be a sudden or more gradual slowing of work and race times.
It is not as easy to document poor performance in some of the other disciplines. For the most part, the horse appears to be quite healthy, has a good appetite, and is in good body condition. Nonetheless, owners, trainers, and riders notice a decline in "zip" during training and a fall-off in performance during competition.
With records in hand, you will be able to provide your veterinarian with some of the information needed to develop an approach to diagnose the problem. He or she will want to know the length of time the poor performance has been observed, the time during competition when performance falls off, and whether the horse has shown a gradual decline in performance as opposed to a very sudden drop-off in form.
This questioning also might point to a problem involving a particular body system. For example, a lameness problem might be signaled by reluctance to change leads or go in one direction, gait unevenness, or a tendency to pull to one side. A problem with heart function might be suspected if the horse has become intolerant to exercise (e.g., he feels very sluggish during work or has a higher than normal heart rate during and after exercise). An abnormal noise during breathing is often caused by an obstruction in the horse's upper airway.
Examination During Exercise
The starting point for any poor performance workup is a lameness evaluation. The horse's gait should be thoroughly evaluated, either with or without a rider and both in a straight line and on both directions of a circle. If the horse appears to be lame, the veterinarian will try to localize the injury to a specific location on the limb by use of palpation, flexion tests, local nerve blocks, and imaging techniques (X rays, ultrasound, thermography, and/or scintigraphy).
Bear in mind that lameness might not be the root cause of a performance problem. Some trainers will dismiss a mild lameness as the cause of poor performance. This might be true because it is not unusual to find more than one possible explanation for sluggish performance. On the other hand, there is increasing evidence that mild to moderate lameness can contribute to poor performance, perhaps by decreasing the efficiency of movement.
Regardless of these arguments, the bottom line is that the cause of the horse's subtle lameness must be identified and treated. Then, after successful resolution of the lameness, the horse's performance can be re-evaluated. If no improvement is evident, further evaluation is necessary.
For examination of other body systems (as well as the musculoskeletal system) during exercise, there are two main options. The horse can be exercised on a track or a similar set course (a "field" exercise test), or the exercise test can be carried out on a treadmill. Each has advantages and disadvantages, and the choice will depend on the aim of the test and the type of measurements that must be collected. (For more information on fitness testing, see "Is Your Horse Fit for the Task?" in the April 2002 issue of The Horse, article Quick Find #3406 at www.TheHorse.com.) Other considerations will include the availability of treadmill testing facilities and the depth of your pocketbook!
The simplest field test will require the horse to complete exercise of a pre-determined time or distance, either under saddle or on a longe line. The intensity of exercise will depend on the horse's ability and normal athletic function, and the primary purpose of the examination.
For example, if the horse has a history of abnormal respiratory noise or refusal to perform certain gait maneuvers, it will be important for the veterinarian to see (and hear) the horse exercise under the conditions in which these problems have been recognized. On the other hand, for post-exercise evaluation of muscle damage or conditions of the lower respiratory tract, any form of moderate-to-hard exercise is helpful. For an eventer or racehorse, this might comprise a warm-up followed by a two- or three-minute gallop (or pace or trot) at a heart rate of 180-200 beats per minute.
The main limitation of field exercise tests is the limited array of diagnostic tests that can be performed while the horse is exercising. With rapid advances in technology, you can expect this situation to change in the coming years. For the moment, measurements that can be made during exercise in the field include heart rate (by use of a heart rate monitor) and electrocardiogram (ECG)--the latter allows diagnosis of heart rhythm disorders. Unfortunately, the equipment necessary for telemetric ECG is not commonly used in routine practice, so these measurements are usually made during treadmill exercise tests.
On the other hand, field tests are useful for diagnosis of a tying-up problem and some of the lower airway problems. Blood samples can be collected before and after a bout of exercise for measurement of protein markers of muscle damage (creatine kinase, or CK, and aspartate aminotransferase, or AST). The respiratory system can be examined after exercise, including endoscopic examination of the airways (nose, throat, and windpipe) and bronchoalveolar lavage (BAL). These procedures will be discussed in more detail later in this article.
A more comprehensive approach to the assessment of poor performance problems will require use of a high-speed treadmill. This type of exercise testing is available only at university veterinary teaching hospitals and some larger equine hospitals, although the number of these testing facilities is increasing each year. When your veterinarian is unable to determine the reason for your horse's poor performance, he or she might recommend that the horse be taken to one of these centers for further evaluation.
The main advantage of treadmill tests is the ability to more closely monitor the horse during exercise. More sophisticated measurements, such as oxygen consumption and gait analysis, can be made. In addition, it is possible to perform upper airway endoscopy while the horse is running--a procedure often crucial to the diagnosis of upper airway obstructions. It is also much easier to standardize the nature of the exercise test and the environmental conditions.
Several types of treadmill exercise tests have been used depending on the focus of the examination, the breed of horse, and his current state of fitness. In general, some kind of incremental test involving a stepwise increase in running speed is used. For endoscopic evaluation of the upper airways, the horse must be examined at or near his maximum capacity. This is because many of the abnormalities of the upper airway, such as laryngeal hemiplegia (roaring) and dorsal displacement of the soft palate (DDSP), only occur when the ventilation (movement of air in and out of the lung) of the horse is at its maximum.
The running speeds necessary to achieve maximum effort will vary among horses--typically, the horse will undergo a thorough warm-up after which the endoscope will be positioned in one nostril. Then, the treadmill speed is increased in a stepwise fashion--the highest speed (often 26.8-31.3 miles per hour or 12 to 14 meters per second) will be maintained for a distance of about one mile. Problems with upper airway function often occur when the horse tires, so the endoscopic examination will be continued as the treadmill speed is gradually reduced at the end of the test.
Horses readily adapt to running on a treadmill--for most horses, only one or two training (acclimation) sessions are required for the horse to appear comfortable running at different speeds on the treadmill. These training sessions can be performed the day before the actual exercise test, but in many instances are done the same day.
Another consideration for treadmill exercise tests is the tack worn by the horse. For example, Standardbred pacers that race in hobbles should wear these during treadmill exercise. If upper airway endoscopy is to be performed, the horse should be outfitted with the headgear normally worn during performance.
The type and number of diagnostics performed before, during, and after an exercise test will depend on the horse's history and the results of the resting examination. For example, when a racehorse has a history of fading at the end of a race, there might be a high degree of suspicion for a respiratory problem. Therefore, endoscopy during and after exercise and a post-exercise BAL might be recommended.
Endoscopy during exercise is particularly important because the results of an examination at rest can be misleading. For example, laryngeal hemiplegia might be evident at rest but, in many cases, the larynx appears to work fine during exercise. Conversely, conditions such as DDSP, dynamic collapse of the pharynx, and paralysis of the right side of the larynx might only be evident during vigorous exercise.
Measurement of the amount of oxygen in the blood (blood gas analysis) is another way to assess respiratory function and the significance of any abnormality observed through the endoscope. This analysis is accomplished by inserting a small catheter into a blood vessel, thus allowing blood samples to be collected while a horse is running on a treadmill. If airflow is obstructed, this will be reflected by a severe decrease in blood oxygen content. This test is also useful in assessment of lower airway problems--severe inflammatory conditions of the lung can also result in impaired oxygen transfer into the body.
To monitor heart rate and rhythm during exercise, a series of small electrodes are attached to the horse's body. These electrodes record the electrical activity of the heart and relay this information to a recording device. A printout of this information can be viewed after the test. Disturbances to heart rhythm are a reasonably common cause of poor performance, so these measurements are an important component of the overall performance evaluation. Some clinicians also recommend a pre- and post-exercise ultrasound examination of the heart (echocardiography) to detect other problems with heart function. The post-exercise examination is normally performed within five minutes of exercise completion.
About one hour after exercise, a further endoscopic exam is often performed, particularly when exercise-induced pulmonary hemorrhage (EIPH) or chronic lower airway disease is suspected. The endoscope is passed into the horse's trachea (windpipe) to check for evidence of blood and mucus. In these cases, a BAL or tracheal wash will also be performed.
For BAL, an endoscope or a specially designed flexible rubber tube is positioned in one of the lower airways (a bronchus). Then, a small volume of fluid is instilled and aspirated through the endoscope or tube. In essence, a portion of the lung is "washed." This sample is analyzed in a manner similar to that for a blood sample--the number of red and white blood cells is counted and the sample is examined under a microscope. This procedure is valuable for assessment of lower airway conditions that impair performance, and it helps in the development of treatment recommendations.
To assess muscle damage incurred during exercise, blood samples for measuring CK and AST activity are collected before and after exercise. These proteins are contained within muscle cells and are released into the blood when cells are damaged. Substantial increases in these proteins after a treadmill or field exercise test often suggest an underlying, low-grade "tying-up" problem as a potential cause of poor performance.
We have discussed only some of the diagnostic testing procedures that can be used in the evaluation of horses with poor performance. There is no set "recipe" of tests that is guaranteed to sort out these problems.
Nonetheless, it has become clear that use of treadmill exercise testing in combination with a comprehensive clinical evaluation that focuses on the musculoskeletal, respiratory, and cardiovascular systems has greatly improved our ability to diagnose problems that can impair exercise performance.
Martin, B.B.; Reef, V.B.; Parente, E.J.; Sage, A.D. Causes of poor performance of horses during training, racing, or showing: 348 cases (1992-1996). Journal of the American Veterinary Medical Association, 216, 554-558, 2000.
Morris, E.H.; Seeherman, H.J. Clinical evaluation of poor performance in the racehorse: the results of 275 evaluations. Equine Veterinary Journal, 23, 169-174, 1991.
Parente, E.J. Testing methods for exercise intolerance in horses. Veterinary Clinics of North America: Equine Practice, 12, 421-433, 1996.
About the Author
Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University