Diabetes Mellitus in Dogs and Cats
Diabetes is caused by a progressive impairment of the blood sugar homeostasis that leads to persistent high glucose levels in the body.
Excessive glucose concentration is toxic to many organs. It is responsible for diabetes symptoms and its numerous complications: renal disease, heart disease, eye damages, hormonal diseases… High sugar levels are also toxic to the pancreas where they attack and destroy the β cells of the pancreas that are responsible for the production and release of insulin.
Diabetes is a silent disease. The symptoms are not seen early in the course of the blood glucose increase. Symptoms usually occur when glucose is high enough to start spilling into the urine. The stage of the disease before the apparition of symptoms is called prediabetes.
Thus, the body progressively loses its ability to bring down blood glucose: diabetes progresses. Whatever the type of diabetes, insulin plays a leading role. By analogy with human diabetes, diabetes mellitus is split in two categories: Type 1 and Type 2 diabetes.
Type 1 diabetes onset is caused by a genetic predisposition and the action of yet undefined environmental factors. Type 1 diabetes corresponds to a progressive destruction of the β cells in the pancreas. The body does not have enough insulin to lower blood glucose. Excess of glucose is also harmful for the pancreas and accelerates the progression of the disease. In dogs and cats, chronic pancreatitis (inflammation of the pancreas) leads to β cells destruction and is a frequent reason for diabetes onset. Type 1 diabetes is by far the most frequent diabetes in dogs.
Type 2 diabetes is the most frequent diabetes in human (90% of cases), and in cats. The progression of the disease is quite complex and the sequence of events is still debated between experts. Type 2 diabetes involves two main mechanisms.
- Insulin resistance: the body becomes progressively insensitive to insulin. Insulin is no more able to trigger glucose polymerization into glycogen (= hepatic resistance) nor to facilitate the utilization of glucose by the cell (= peripheral resistance)
- Impaired insulin secretion: the pancreatic beta cells compensates for the decreased sensitivity to insulin by secreting increasing amounts of insulin. The “over-secreting” β cells gradually become exhausted and die, at some point, the pancreas cannot produce enough insulin. This is the time for overt diabetes (= first clinical symptoms)
Type 2 diabetes affects cats. Diabetic cats can go into remission if they are treated soon enough.
Genetic, environmental and dietetic factors play a role in the onset and the evolution of the disease.
Genetic: The Burmese cats are three times more likely to develop diabetes compared to other breeds. Gender also plays a role: males are more exposed to the disease.
Environmental: low physical activity, indoor lifestyle, as well as uncontrolled medical condition (dental ailment, chronic diseases) are risk factors for your pet. Some drugs such as glucocorticoids or progestins may induce or aggravate resistance to insulin.
Dietetic: Cats are obligate carnivores. This means that the proportion of calories coming from carbohydrate in a healthy cat should not be more than 25%. Unfortunately, many diets contain a higher proportion of simple or complex sugars. Inadequate diet as well as obesity leads to insulin resistance and to diabetes.
Gestational or estrus diabetes
During gestation or estrus, unspayed bitches secrete progesterone, the hormone necessary for maintaining gestation. Progesterone antagonizes insulin and stimulates the growth hormone, which is also an insulin-antagonist. Progesterone thus helps increase blood glucose. This can be beneficial during the gestation: the increased glucose concentration can be used to meet the nutritional needs of the fetus. This hyperglycemia is reversible in most cases: it is transient diabetes. In some cases, the repetition of hyperglycemia episodes impairs β cells function and chronic diabetes develops. This is why it is advised to spay bitches in which gestational or estrus diabetes has been diagnosed.