Diabetes mellitus (DM) in dogs and cats has many causes, just as there are multiple causes in people.
Canine diabetes mellitus is always Type 1, or insulin-dependent diabetes mellitus (IDDM). Type 1 diabetes results from congenital or acquired failure to produce insulin, requiring insulin replacement therapy by injection.
On the other hand, cats may be afflicted by Type 1 or Type 2 diabetes. Type 2 diabetes is defined as high blood glucose caused by resistance to insulin, a defect in the insulin receptor of cells. Type 2 DM can, however, be accompanied by an absolute lack of or reduced levels of insulin.
Insulin is a hormone produced by β– (beta) cells in the Islets of Langerhans in the pancreas. Insulin is required for the mammalian body to utilize glucose (sugar) for energy in cells all over the body. In the absence of insulin, glucose accumulates in the bloodstream (hyperglycemia) and many body functions fail. Long-term lack of insulin eventually results in death.
Inheritance is almost certainly a factor for dogs developing diabetes, although involved genes have not been identified. It is known that heritability of diabetes in humans involves multiple genes. Breed predisposition exists in poodles, golden retrievers, Keeshonden and Samoyeds, which lends additional evidence for genetics as a cause.
An immune-mediated (autoimmune) cause of diabetes in pets has been researched, but scientists have not yet been able to identify specific causal factors.
Endogenous and exogenous corticosteroids can result in DM, presumably by causing exhaustion of the beta cells’ ability to produce insulin as well as contributing to insulin resistance. The presence of elevated levels of glucocorticoids causes increased glucose production in the liver, gluconeogenesis, raising blood glucose levels. That increase in blood sugar results in increased production of insulin which, over time, can permanently exhaust the β-cells’ function. Endogenous cortisol most commonly results from pituitary-dependent or adrenal Cushing’s Disease. Exogenous corticosteroids are administered by the veterinarian and/or pet owner for a variety of medical conditions.
Pancreatitis is inflammation of the pancreas. Because beta cells reside in the pancreas, and nowhere else in the body, they may be damaged during inflammatory episodes. Should the pancreatitis be widespread, and enough islets are destroyed, insulin production may fall below a level commensurate with good glucose control, and diabetes mellitus results.
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