Introduction to Metabolic Disorders of Horses
Metabolism refers to all processes in the body that break down and convert ingested substances into the energy and nutrients needed to sustain life. Foods, liquids, and drugs all 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 to build new proteins and regulate key metabolic reactions. Enzymes, are proteins that stimulate biochemical reactions for many metabolic processes. Whenever the function of an enzyme is altered, a metabolic disorder can develop. Metabolic disorders are important because they can affect energy production or damage tissues critical for survival. They may be inherited or acquired. Acquired metabolic disorders are more common and significant. Metabolic disorders may result in a substance rising to toxic levels (a storage disorder), or a necessary substance may not be produced, causing a deficiency.
Metabolic storage disorders are characterized by the accumulation of a substance within cells due to the body’s inability to break it down (metabolize it) because of a partial or complete lack of a certain enzyme. The missing enzyme may result in a substance building up to a toxic level. 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 locoweed intoxication.
Some metabolic disorders are caused by an increased demand for a specific element or nutrient that has become deficient. 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 nursing a foal. Another production-related disorder of horses is exertional rhabdomyolysis, which is also called “tying-up.” This condition is a syndrome of muscle fatigue, pain, or cramping associated with exercise. There are multiple underlying causes that result in similar signs. The syndrome may be sporadic, with single or infrequent episodes, or chronic, recurring often after mild exertion. Severe episodes require intensive supportive therapy to avoid kidney damage.
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. Acquired 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 can 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.
Also see professional content regarding metabolic disorders.