Diabetes
mellitus is a metabolic disorder that comes from the reduction of insulin
available for normal function of many cells in the body. The unavailability of insulin
can be a result of degenerative changes in Beta cells of the pancreatic islets,
reduced effectiveness of the hormone because of the formation of antiinsulin
antibodies or inactive complexes, damage to Beta cells from immune-mediated
islet cytotoxicity, or inappropriate secretion of hormones by neoplasms in
other endocrine organs. The most used classification scheme to human diabetes
mellitus is Type 1 and Type 2. Type 1 DM is characterized by a pancreatic
Beta-cell autoimmunity, insulin deficiency and disease onset in childhood. On
the other hand, the Type 2 DM is associated with obesity and is characterized
by insulin resistance and onset during adulthood.
As the
canine DM is a disease of insulin deficiency, it is regarded as most similar to
human Type 1 DM. The spontaneous cases of diabetes are most likely to occur in
mature dogs and in females approximately twice as often as males.
In dogs,
the Langerhans islets are often destroyed because of inflammation of the
exocrine pancreas. The replacement of both endocrine and exocrine pancreatic
cells by fibrous connective tissue due to a chronic relapsing pancreatitis is a
frequent cause of diabetes mellitus. In these dogs, the pancreas becomes firm,
multinodular, and has scattered areas of hemorrhage and necrosis after a recent
relapse (picture 1).
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Picture 1. Pancreas of a diabetic dog at necropsy
Department of Veterinary Pathology - UFRGS |
In the
animal with DM, hepatomegaly occurs as a result of fatty degeneration. The
lipids accumulate in the liver as the result of increased lipid mobilization,
and the hepatocytes injured by ketonemia have decreased usage of lipids (picture 2).
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Picture 2. Necropsy of a dog with diabetes mellitus
Department of Veterinary Pathology - UFRGS |
In poorly
controlled diabetic dogs, cataracts are often seen. The formation of cataract
in the diabetic patient is related to the unique sorbitol pathway by which
glucose is metabolized in the lens. Glucose is converted, by the enzyme aldose
reductase, to sorbitol and subsequently to fructose by sorbitol dehydrogenase.
These sugar alcohols accumulate within the lens and result in an intracellular
accumulation of solute and hypertonicity. This accumulation causes a hydropic
degeneration of the lenticular fibers. Later, macromolecular aggregation or precipitation
of normally translucent lenticular proteins develops, accompanied by disruption
of lenticular fibers and formation of interfibrillar clefts. The result is
diffuse, often bilateral lens opacity observed in animals with chronic DM (picture 3).
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Picture 3. Cataracts
Department of Veterinary Pathology - UFRGS |
Other extra pancreatic lesions found in animals with a poorly controlled DM,
such as chronic renal disease, blindness, and gangrene, are the result of
microangiophaty characterized by thickening of the capillary basement membrane.
These dogs develop nodular or diffuse glomeruloesclerosis (picture 4) characterized by
PAS-positive fibrillar deposits of glycoprotein, which occasionally form
spherical nodules in the glomerular capillary tufts. Other renal lesions
include accumulation of glycogen within cells of the loop of Henle and distal
convoluted tubule.
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Picture 4. Diffuse glomeruloesclerosis in the kidneys
Department of Veterinary Pathology - UFRGS |
