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Introduction to Metabolic Disorders of Horses

By George M. Barrington, DVM, PhD, DACVIM, Professor, College of Veterinary Medicine, Washington State University ; David L. Evans, BVSc, PhD, Associate Professor, Faculty of Veterinary Science, University of Sydney ; Katharine F. Lunn, BVMS, MS, PhD, MRCVS, DACVIM, Associate Professor, Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University ; Donald C. Sawyer, DVM, PhD, Professor Emeritus, Michigan State University ; Ivan W. Caple, BVSc, PhD, MACVSc, MRCVS, Dean, Faculty of Veterinary Sciences, Veterinary Clinical Centre, University of Melbourne ; Sharon J. Spier, DVM, PhD, DACVIM, Professor, Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California

Metabolism refers to all processes in the body that break down and convert ingested substances to provide the energy and nutrients needed to sustain life. Foods, liquids, and drugs all generally undergo metabolic processes within the body. Many foods are complex materials that need to be broken down into simpler substances, which in turn become “building blocks” for the body to use as needed. For example, protein is broken down into amino acids, which are used in several metabolic reactions. Enzymes made by the body are needed for many metabolic processes to occur. Whenever the function of an enzyme is affected, a metabolic disorder can develop. Metabolic disorders are important because they affect energy production or damage tissues. They may be inherited or acquired. Acquired metabolic disorders are more common and significant.

Metabolic Storage Disorders

Metabolic storage disorders usually result from the body’s inability to break down some substance because of partial or complete lack of a certain enzyme. The substance can build up to a toxic level, or the body is unable to produce a substance that it needs. Although storage diseases are often widespread throughout the body, most signs are due to the effects on the central nervous system. Metabolic storage disorders can be either genetic or acquired.

Genetic (inherited) storage diseases are named according to the specific metabolic byproduct that builds up in the body. These diseases are progressive and usually fatal because specific treatments do not exist. Genetic storage diseases have not been reported in horses.

Acquired storage diseases can be caused by eating plants that contain inhibitors of specific enzymes. Eating locoweed plants (Astragalus or Oxytropis species) for a long time can result in an acquired neurologic storage disease. Several toxic components of these plants interfere with the activity of a specific enzyme. Horses are highly susceptible to intoxication (See also Poisonous Plants).

Longterm ingestion of locoweed plants (Astragalus or Oxytropis species) causes a metabolic storage disease that affects the central nervous system of horses.

Production-related Metabolic Disorders

Some metabolic disorders are caused by an increased demand for a specific element or nutrient that has become deficient under certain conditions. For example, in hypoglycemia, the animal’s metabolic reserves are unable to sustain sugar (glucose) in the blood at a level needed for normal function. Likewise, in hypocalcemia, the level of calcium in the blood is too low. In some cases, dietary intake of a nutrient, such as calcium, is rapidly used up for an ongoing, high metabolic need, such as lactation or nursing a foal. Another production-related disorder of horses is exertional rhabdomyolysis, which is also called “tying-up” or “cording-up.” In this condition, draft or race horses are forced to work or exercise after a period of rest during which feed has not been restricted. When the horses go back to work, certain compounds (such as glycogen and lactate) can build up in and damage the muscle tissue.

The difference between production-related metabolic diseases and nutritional deficiencies is often subtle. Typically, nutritional deficiencies are longterm conditions that develop gradually and can be corrected through dietary supplementation. Metabolic diseases usually begin suddenly and respond dramatically to administration of the deficient nutrient (although affected animals may need dietary supplements to avoid recurrence). Because production-related metabolic disorders are serious and develop suddenly, accurate and rapid diagnosis is essential. Ideally, diagnostic tests can be used to predict the chance of disease occurring so that it can be prevented or preparations can be made for rapid treatment.

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