Metabolic diseases may be inherited or acquired, the latter case being more common and important. Metabolic diseases are clinically important because they affect energy production or damage tissues critical for survival.
Congenital erythropoietic porphyria (CEP) is a rare, hereditary disease of cattle, pigs, cats, and people that results from a significant yet variable decrease in uroporphyrinogen III synthase (URO-synthase) activity. URO-synthetase is the fourth enzyme in the heme biosynthesis pathway, and it normally converts hydroxymethylbilane to uroporphyrinogen III. With decreased URO-synthase activity, hydroxymethylbilane accumulates primarily in erythrons and is nonenzymatically converted to uroporphyrinogen I. Further decarboxylation of uroporphyrinogen I leads to the formation of various porphyrinogen I isomers, with coproporphyrinogen I occurring as the final product. Coproporphyrinogen I cannot be further metabolized to heme and is thus nonphysiologic.
Hypocalcemic tetany is an uncommon condition in horses caused by exertion, transport, or lactation. Signs vary in type and severity. Diagnosis is based on signs, serum calcium levels, and response to treatment. Slow intravenous administration of calcium usually leads to full recovery. Reducing stress and fasting during transport can help prevent the disorder. Pregnant mares should be fed adequate amounts and ratios of calcium and phosphorus throughout pregnancy to prevent lactation tetany.
Blood and intracellular magnesium concentrations are tightly regulated, with chronic dietary deficiencies resulting in whole-body depletion as assessed by decreased urinary excretion or low intracellular ionized magnesium concentrations. Low blood concentrations can occur with acute illness and are exacerbated by anorexia, lactation, and stress, such as transportation. Severe hypomagnesemia can lead to muscle weakness, neurological signs, belligerent behavior, and sudden death. Lactating beef cattle on winter pastures are most at risk.
Potassium homeostasis is mainly determined by the balance between absorption of potassium from the GI tract and subsequent excretion by the kidneys (all animals) and saliva (in adult ruminants). Transport of potassium is passive in the small intestine and active in the colon under the influence of aldosterone. The most important hormone affecting renal and salivary potassium excretion is aldosterone, which is released from the zona glomerulosa of the adrenal gland in response to hyperkalemia and other factors. One of aldosterone’s primary actions is to enhance the secretion of potassium ions in the distal renal tubules and collecting ducts.
Equine metabolic syndrome is a collection of clinical signs and clinicopathologic changes that are due to the inability of some equids to respond normally to dietary carbohydrates (insulin dysregulation). Laminitis is the most important morbidity; however, preputial and mammary swelling, obesity, and infertility also occur. Diagnosis is based on measuring increased insulin secretion after a carbohydrate challenge. The condition is best controlled by limiting carbohydrate intake. If changes to diet and exercise are not sufficient, pharmacologic agents may be needed to decrease insulin concentrations.
Muscular fatigue during exercise is the decline in the ability of a muscle to generate force of contraction, resulting in the inability of the animal to continue to perform at the same level of intensity. Alterations in cardiovascular parameters, serum electrolytes, and muscle tissue may be observed if physiologic compensatory mechanisms are exhausted. Treatment involves rest, rehydration, restoration of normal serum electrolyte concentrations, and cooling strategies.
Fever of unknown origin refers generally to any persistent fever (ie, that does not resolve spontaneously or with empirical antimicrobial treatment) for which the underlying cause is not revealed from the case history, physical examination, and initial diagnostic testing. A variety of causes are possible, including infectious, immune-mediated, and neoplastic disease. A systematic, staged approach can assist with diagnosis. In the absence of a diagnosis, trial treatment may be attempted.
Fatty liver disease is a disorder of highly productive dairy cows resulting from an excessive negative energy balance at the onset of lactation. Mobilization of large amounts of body fat reserves in response to insufficient dietary energy supply results in a transfer of fatty acids to the liver. Excessive amounts are deposited in the hepatocyte as trigylcerides and can result in disturbed liver function and liver cell injury. The condition is associated with pronounced ketosis, feed intake depression, and decreased productivity; severe cases lead to liver failure and a fatal outcome. Diagnosis can be made directly by determining the liver fat content but is most commonly done indirectly by assessing severity and duration of negative energy balance. Treatment is similar to the treatment of ketosis, with supportive care.
Ketosis is an elevated concentration of ketone bodies (acetone, acetoacetate, beta-hydroxybutyrate) in all body fluids. Key clinical signs of ketosis are vague but include anorexia, decreased milk production, noticeable loss of body condition, firm dry feces, and, occasionally, neurologic signs (nervous ketosis); however, ketosis may be subclinical or go unrecognized. Cows with ketosis can be identified via routine testing using appropriate cowside blood, milk, or urine tests. The most efficacious treatment for ketosis is oral drenching of propylene glycol.
Malignant hyperthermia (MH) is a potentially fatal pharmacogenetic disorder of skeletal muscle calcium regulation. Triggered by exposure to certain drugs or stressors, clinical signs include sudden and dramatic rise in body temperature, muscle fasciculation, muscle rigidity, tachypnea, tachycardia, arrhythmia, myoglobinuria, metabolic acidosis, renal failure, and death. An in vitro contracture test or DNA analysis are used for diagnosis; minimizing stress and appropriate anesthetic selection are key for prevention. Dantrolene is the treatment of choice, with other therapy being symptomatic and supportive.
Transport tetany is more appropriately designated a "management syndrome" versus a specific disease, with multiple management risk factors leading to tetany due to hypocalcemia, hypomagnesemia, or both. Transport tetany typically occurs in cows and ewes in late pregnancy, after the stress of prolonged transport. It also occurs in lambs transported to feedlots and in cattle and sheep transported to slaughter. Crowded, hot, poorly ventilated transport vehicles (railroad cars or trailers) with minimal or no access to feed or water seem to predispose animals to the condition; however, prolonged travel by foot is also a risk factor. The disease is characterized by recumbency, gastrointestinal stasis, and coma, and is generally fatal.