Diabetes mellitus is caused by inadequate synthesis or secretion of insulin.
The endocrine pancreas may not produce and secrete sufficient insulin.
Vacuolar
degeneration of islets of Langerhans in a dog, HE staining, 10x
(Dept.
of Pathology, Faculty of Veterinary Medicine, University of Utrecht).
Severe inflammation (e.g. pancreatitis) or neoplasia of the exocrine pancreatic tissue can also lead to loss of islet function. In these cases canine diabetes is usually complicated by exocrine pancreatic insufficiency (EPI).
The main cause of diabetes in dogs is immune-mediated destruction of the islets of Langerhans.