Diabetes mellitus in cats and dogs

What is diabetes?

Diabetes mellitus is a disease resulting from a primary deficiency of insulin. Insulin is a hormone produced by the pancreas, a gland located in the abdomen near the duodenum (small intestine).
Inside the pancreas, small groups of cells called islets produce hormones, while the outside produces digestive enzymes. Insulin enables the body's tissues to use glucose from the blood. Insulin therefore lowers blood glucose levels. In diabetics, the lack of insulin produced by the pancreas, or the hormone's reduced action on body tissues, leads to a rise in blood glucose levels. The clinical signs that follow may vary, but there is often an increase in urine output. Over time, weight loss may occur, although diabetics may be overweight at the time of diagnosis, as obesity reduces the effectiveness of insulin on tissues.
Diabetes can also cause medical emergencies. This may be due to exceptionally low blood glucose levels, which can lead to diabetic coma. On the contrary, too high a blood glucose level can lead to ketosis, dehydration, collapse and death. Despite the risk of serious complications, diabetic dogs and cats often show very few signs when presented to the vet.
Long-term complications associated with high blood sugar levels include kidney failure, loss of nerve function in the extremities, hypertension, blindness and (in dogs) cataracts. Given their shorter life expectancy, dogs and cats develop fewer complications than humans, and if properly treated, can lead normal, fairly long lives.
Diabetes mellitus should not be confused with diabetes insipidus, which involves other hormones.

What causes diabetes?

Diabetes can be a primary disease, or appear secondary to another illness. Here's a brief summary of the types of diabetes.

Type 1 diabetes

This type of diabetes occurs when the immune system destroys the insulin-producing cells of the pancreas (beta cells). It's the most common primary diabetes in dogs, but rare in cats. There may be genetic factors in dogs. Because this type of diabetes results in a complete absence of insulin production, patients must be treated with daily insulin injections. In human medicine, this type is known as insulin-dependent. It is recommended that females with the disease be sterilized, as the hormones secreted during heat and gestation make it difficult to control blood sugar levels.

Type 2

This type of diabetes is not very well-defined in terms of causes, but involves obesity, loss of tissue sensitivity to insulin and accumulation of a substance called amyloid in the pancreatic islets. The pancreas of type 2 diabetics may produce none, little, enough or too much insulin, but this insulin cannot produce its normal effect. Type 2 is the most common form of diabetes in cats, but is rare in dogs. In human medicine, it's known as non-insulin-dependent diabetes. Most affected cats require insulin. In cats, diabetes may be transient and disappear over time. Whether diabetes will be permanent or temporary cannot be predicted, as the pathological basis of this form of the disease is not well understood.

Type 3

Diabetes is secondary to another disease or medication. Diabetes may not need to be treated if the other condition is resolved. Diseases that can lead to diabetes in dogs and cats include Cushing's disease (hyperadrenocorticism), hyperthyroidism, acromegaly and others. Frequently used drugs that can cause diabetes are progestogens and corticosteroids. If these drugs are used long-term, diabetes may become permanent and persist even after the medication has been discontinued. Generalized disease of the pancreas (tumor, pancreatitis) can lead to diabetes if a sufficient number of beta cells are destroyed.

Insulin treatment

Insulin injections are necessary for the treatment of most dogs and many cats diagnosed with diabetes. Human insulin preparations are often prescribed, although a veterinary insulin called Caninsulin is also available. Insulin pharmacology is complex and involves differences at the molecular level between different species, resulting in variations in action from one species to another. There are few detailed studies on the pharmacological effects of insulin in dogs and cats, but there are many clinical trials with some types of insulin.
Intensive treatments, as in human diabetes, are not practiced on animals. These treatments involve multiple daily measurements of blood glucose levels at home, and multiple injections of a calculated dose of insulin. The aim of treatment in animals is to achieve reasonable control of blood glucose and clinical signs of diabetes using injections of long-acting, medium-acting insulin once or twice a day.
As there is considerable variation between individuals receiving insulin, an initial trial period is instituted in the veterinary hospital with frequent blood glucose monitoring. Once stabilized, the patient can return home with a fixed treatment plan, or one that varies according to urine glucose measurement with a dipstick. As insulin requirements tend to change over time, periodic reassessments are essential.

Diet

The importance of diet in diabetic humans is well known. Meals are planned to coincide with maximum insulin action, and specific quantities and types of food are recommended. High-fiber diets are used to reduce the blood glucose spike that follows meals. For animals, high-fiber diets should be chosen for their consistent calorie content and nutritional quality. Meal timing and diet are important parts of diabetes management in dogs. Meal timing depends on the type of insulin used and the dog's specific response as established by clinical blood glucose testing. It may be impossible to feed a cat a meal, as they like to nibble throughout the day. However, high-fiber diets are beneficial for diabetic cats.
It is very important that the animal reaches its optimum weight, especially in obese animals, as this will reduce the demand for insulin. In cats, weight loss can even eliminate the need for insulin treatment altogether. Reduced-calorie, high-fiber diets are ideal for these cases.
In animals that are too thin, weight gain is desirable, and the animal should return to its normal weight. High-calorie diets should be avoided. Anorexia is particularly dangerous in insulin-treated animals, as blood glucose can become too low if no food is consumed to balance the insulin dose. It is very important that the insulin-treated patient does not fast, even if a less-than-ideal food is given. The diet must be adjusted over a long period, and abrupt changes should be avoided.

Other medications

Drugs that stimulate insulin secretion or modify its action are used in human diabetics, either alone or in combination with insulin. Few of these drugs have been evaluated for safety or efficacy in dogs and cats. Sulphonylureas, which stimulate insulin secretion, have been evaluated in dogs and cats. They have no effect in dogs, but a certain proportion of cats with mild diabetes can be controlled with this drug alone.

Vivianne Venisse MDV