Primary diabetes mellitus
By the time diabetes mellitus is clinically recognized in dogs, they virtually all have type 1 or insulin-dependent disease. This is characterized by beta cell destruction (often due to autoimmune destruction of the islets of Langerhans) and hypoinsulinemia. Insulin secretagogues, such as glucose or glucagon, fail to stimulate endogenous insulin production and there is an absolute requirement for exogenous insulin injections to control blood glucose levels.
Secondary diabetes mellitus
Diabetes mellitus can be secondary to severe inflammation or neoplasia of exocrine pancreatic tissue, which leads to loss of islet function. In these cases, diabetes is also complicated by exocrine pancreatic insufficiency. Diabetes mellitus can also occur when there is either overproduction of counteracting hormones or insulin resistance. Excessive growth hormone production can be seen in intact, cycling bitches. Progesterone produced during the luteal phase of the estrous cycle induces the production of growth hormone by the mammary gland, which counteracts the action of insulin.
Dogs with hyperadrenocorticism produce excessive amounts of glucocorticosteroids that stimulate gluconeogenesis and lead to an increase in the plasma glucose concentration. This stimulates insulin synthesis, which eventually results in exhaustion of the islets of Langerhans. Only about 10% of canine Cushing’s syndrome cases are complicated by diabetes mellitus.