Several classification systems have been used to describe diabetes mellitus. A human classification system revised in 1997 divides the disease into three types: type 1 (previously insulin-dependent or juvenile-onset diabetes mellitus), type 2 (previously non–insulin-dependent or adult-onset diabetes mellitus), and other specific types of diabetes mellitus (previously secondary or type 3 diabetes mellitus).
Diabetics that have an absolute deficiency of insulin are categorized as having type 1 or insulin-dependent diabetes mellitus (IDDM). Type 1 diabetics are thought to have a genetic predisposition combined with immunologic destruction of the insulin-producing beta cells of the pancreas. Generally, all canine diabetic pets fall under the type 1 (IDDM) classification.
In comparison, diabetics that have the capability to still produce some insulin, but have a relative deficiency due to insulin resistance or other dysfunction, are classified as having type 2 or non–insulin-dependent diabetes mellitus (NIDDM). Type 2 increased insulin production or increased insulin resistance is linked to genetic predisposition, obesity, and certain medications. Some diabetic cats can be managed with dietary changes and oral medication, but like dogs, the majority of diabetic cats need to receive insulin to maintain adequate regulation.
Unlike dogs, feline diabetic patients can fall under the type 1 (IDDM) or type 2 (NIDDM) classification. Generally cats initially develop type 2 diabetes mellitus, but by the time the disease is diagnosed by a veterinarian, it has progressed to type 1, and the cat is dependent on exogenous insulin.