Ask Your Veterinarian Presented By Equistro: Non-Musculoskeletal Causes Of Poor Performance

by | 04.10.2019 | 3:03pm

Veterinarians at Rood and Riddle Equine Hospital answer your questions about the healthcare of Thoroughbreds. Email us at [email protected] if you have a question for a veterinarian.

QUESTION: How might a trainer decide if a horse's poor performance is related to a veterinary issue when there's not an obvious lameness present?

DR. STEPHEN REED: Poor performance in horses can be caused by abnormalities in many body systems. The most common cause of poor performance is a painful musculoskeletal problem and the horse will usually show a lameness. If the lameness is obvious enough to cause a gait deficit along with a head nod the clinician will begin with a careful lameness examination to include specific nerve or joint blocks to localize the site of the lameness.  However, there are other body systems that can reduce performance.

Evaluation of poor performance begins by taking a careful history, listening to the trainer and/or rider to be certain one is investigating the problem they feel is important.  I like to start with a careful physical examination to include auscultation of the heart and lungs.  Horses with atrial fibrillation will have an irregular heart rhythm, often combined with a pulse deficit.  Horses sometimes develop the problem during exercise and will show a dramatic reduction in performance. To confirm the diagnosis an electrocardiogram is needed.  While this is the most common arrythmia seen in horses the ECG will help identify whether this or another arrythmia is present.

Dr. Stephen Reed

Other causes of poor performance include obstruction of the upper airway such as dorsal displacement of the soft palate, laryngeal hemiplegia or anomalies of the palatopharyngeal arch. Horses with upper airway abnormalities show both exercise intolerance and often make a noise during exercise. The evaluation of horses with this type of problem must include endoscopy of the nasal passages, pharynx, larynx and guttural pouches as well as passage down the trachea in search of blood or mucus, indicative of either EIPH or reactive airway obstruction. When either of the latter are identified the cause of poor performance is most likely a lower respiratory tract problem.

Poor performance can also sometimes be caused by disorders of the central nervous system.  Affected horses showing only mild to moderate neurologic deficits caused by diseases such as equine protozoal myeloencephalopathy, cervical vertebral stenotic myelopathy and equine degenerative myeloencephalopathy may perform but often at a level lower than before developing the disease.

Poor performance may be a result of primary muscle disease. The most common causes are exertional rhabdomyolysis or “tying-up” and polysaccharide storage myopathy types 1 and 2.  These disorders have different mechanisms as the underlying cause of the signs, though the common clinical manifestation is often sore painful muscles and reduced exercise tolerance.  Mild elevations of muscle specific enzymes such as creatine kinase along with increased levels of aspartate aminotransferase also be seen. Diagnosis can be by genetic testing and/or muscle biopsy.

Some horses not performing well have been diagnosed with gastric ulcers or other gastrointestinal disorders, though these are generally not presented for poor performance. Presumptive diagnosis may be made based on clinical signs. Definitive diagnosis is made by gastroscopy.

As we have discussed, poor performance may not be caused by a problem with the musculoskeletal system. Other body systems should also be considered.

Dr. Reed earned his DVM at The Ohio State University, followed by a residency at Michigan State University. He started his academic teaching career at Washington State University from 1979-1983, and then returned to The Ohio State University where he spent 26 years as a professor and mentor in the Equine Medicine department. Dr. Reed is a Diplomate in the American College of Veterinary Internal Medicine and is a noted author and editor of numerous scientific articles and textbooks. He has spoken at many state, national and international meetings. His primary research interests include Equine Neurologic Diseases. He is currently an Internal Medicine Specialist and shareholder of the Practice at Rood & Riddle Equine Hospital, Emeritus Professor of The Ohio State University, an Adjunct Professor at the University of Kentucky and is currently the Chairman of the Grayson-Jockey Club Research Advisory Committee.

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