Production-related Metabolic Disorders
Although the development of the following diseases is largely related to production or management factors, the pathogenesis of each disease is primarily related to alterations in metabolism. In most cases, the basis of disease is not a congenital or inherited error in metabolism, but rather an increased demand for a specific nutrient that has become deficient under certain conditions. Diseases such as hypocalcemia, hypomagnesemia, and hypoglycemia are augmented by management practices directed toward improving and increasing production. They are therefore correctly considered production diseases. However, they are also metabolic diseases because management of the animal is directed at production, which at its peak is beyond the capacity of that animal’s metabolic reserves to sustain a particular nutrient at physiologic concentrations. For example, parturient paresis of cows (see Parturient Paresis in Cows) occurs when the mass of calcium in the mammary secretion is greater than the cow’s diet or its skeletal reserves can supply. Comparable situations occur with magnesium and glucose metabolism, and with phosphorus in relation to postparturient hemoglobinuria (see Postparturient Hemoglobinuria in Dairy Cows).
Most production-induced metabolic diseases result from a negative balance of a particular nutrient. In some cases, dietary intake of the nutrient is rapidly reduced because of an ongoing, high metabolic requirement for that nutrient. Examples include pregnancy toxemia in ewes (see Pregnancy Toxemia in Ewes and Does), protein-energy malnutrition in beef cattle (see Nutritional Requirements of Beef Cattle), fat cow syndrome in dairy cattle (see Fatty Liver Disease of Cattle), and hyperlipemia in ponies (see Hyperlipemia and Hepatic Lipidosis in Large Animals). Furthermore, some diseases may be precipitated when producers, primarily because of economic concerns, are compelled to not supplement the diet of animals that already have a substandard nutritional plane.
Exertional rhabdomyolysis of horses (see Exertional Myopathies in Horses) is another production-induced metabolic disease. In this case, the production activity (draft or racing) is maintained by and matched to a level of caloric intake. Management decisions not to work or race these horses without a concomitant decrease in caloric intake may result in accumulation of muscle glycogen to dangerous levels. Disease results when work is resumed and the production of lactate exceeds its metabolism.
The difference between production-related metabolic diseases and nutritional deficiencies is often subtle. Typically, nutritional deficiencies are longterm, steady-state conditions that can be corrected through dietary supplementation. Metabolic diseases are generally acute states that dramatically respond to the systemic administration of the deficient nutrient or metabolite, although affected animals may require subsequent dietary supplementation to avoid recurrence. An important aspect of dealing with production-induced metabolic diseases is accurate and rapid diagnosis. Ideally, diagnostic tests can be used to predict the occurrence of disease before its clinical onset.