The pathogenesis of a number of common diseases of production animals, although largely related to production or management factors, 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-related 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, ketosis occurs when cows are in negative energy balance, and parturient paresis of cows (milk fever) 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. Other factors that contribute to hypocalcemia in dairy cattle include age, breed, and endocrinologic factors (eg, estrogen concentration).
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 while the animal is experiencing an ongoing high metabolic requirement for that nutrient. Examples include pregnancy toxemia in ewes, protein-energy malnutrition in beef cattle, fat cow syndrome in dairy cattle, and hyperlipemia in ponies. 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 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 long-term, steady-state conditions that can be corrected via 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.
