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Diabetes Mellitus in Cats and Dogs
by Dr Greg Martin BVSc MACVSc

What is Diabetes?

Diabetes mellitus is a disease syndrome that results primarily from a deficiency of insulin. Insulin is hormone that is produced by the pancreas, a gland located in the abdomen next to the duodenum (small intestine). Within the pancreas, small groups of cells called "islets" produce hormones, while the bulk of the gland produces the digestive enzymes. Insulin is the major hormone that allows body tissues (such as the liver and muscles) to use the glucose in the blood, and therefore insulin lowers blood glucose levels. Insulin also has a variety of other effects on various tissues. In diabetics, either lack of insulin from the pancreas or reduced action of insulin in body tissues leads to an increase in blood glucose levels. Accompanying clinical signs can vary, but increased urination is common. With time, weight loss often occurs; however diabetics may be overweight at diagnosis as obesity reduces the response of body tissues to insulin.

Diabetes may also cause medical emergencies. This may be due to exceptionally low blood glucose, which can lead to "diabetic coma". Alternatively, the disorder "diabetic ketoacidosis" is associated with high blood glucose, dehydration, and increased ketones in the blood and may lead to collapse and fatalities. Despite the potential for serious complications, diabetic cats and dogs may have only mild clinical signs when the disease is diagnosed.

Some of the long term complications associated with chronically elevated blood sugar that occur in diabetic people may also occur in diabetic pets, such as kidney failure, loss of nerve function in extremities, hypertension, blindness, and (in dogs) cataracts. Due to their shorter life span, pets develop fewer long-term complications from diabetes than people, and diabetic cats and dogs have a good chance of a normal life span if properly treated.

Diabetes mellitus should not be confused with diabetes insipidus, which is a different hormonal disease.

What causes diabetes?

Diabetes may occur as a primary disease, or may occur secondarily to another disease. Here is a brief summary of the types of diabetes:

Type 1
This type of diabetes occurs due to immune system destruction of the insulin-producing (beta) cells of the pancreas. This is the predominant form of primary diabetes in dogs, but is rare in cats. There may be some genetic component to the cause of diabetes in dogs. Because this type of diabetes results in total absence of insulin production, patients with this form of diabetes must be treated with insulin injections. In human medicine this has lead to type 1 diabetes being referred to as "insulin dependent diabetes mellitus" or IDDM. It is generally recommended that female dogs with diabetes be spayed as the hormonal release associated ith estrus or pregnancy tends to destabilise control of the diabetes.

Type 2
This type of diabetes is not as well defined in its cause, but is characterised by obesity, loss of tissue sensitivity to insulin, and deposition of a precipitate called amyloid into the pancreatic islets. This is the type of diabetes often developed by people later in life. The pancreas of type 2 diabetics may be producing no, too little, normal amounts, or even excessive amounts of insulin, but this insulin is failing to achieve its normal action. Type 2 diabetes is the most common form of diabetes in cats, but is rare in dogs. Humans with type 2 diabetes can often be treated without using insulin, therefore type 2 diabetes in people is often referred to as "non-insulin dependent diabetes" or NIDDM. Most cats with type 2 diabetes do require insulin for successful treatment, therefore the term NIDDM is not appropriate as a synonym for type 2 diabetes in this species. A particular feature of diabetes in cats is that it may be transient in nature, the condition resolving with time. The pathological basis of this "transient diabetes" is yet to be fully determined, and there is currently no way of predicting whether diabetes will be transient or permanent at the time the condition is diagnosed.

Type 3
This is where diabetes is occurring secondarily, as a result of another disease, disorder, or drug treatment. This may or may not necessitate treatment for the diabetes as well as the associated condition. Diseases that may result in secondary diabetes in cats and dogs include: Cushing's disease (hyperadrenocorticism), hyperthyroidism, acromegaly, and others. Commonly used drugs that may cause diabetes in cats and dogs include progestens and corticosteroids. With extended use of these drugs the diabetes may be permanent, with diabetes persisting after drug treatment is ceased. Generalised disease of the pancreas, such as that caused by an invasive tumour or pancreatitis may result in diabetes if a sufficient proportion of beta cells are destroyed.

Insulin treatment

Insulin injections are necessary for the treatment of most dogs and many cats in which diabetes has been diagnosed. Preparations of insulin intended for human use are often prescribed, although there is a veterinary insulin preparation called "Caninsulin" available in some countries. The pharmacology of insulin is complex, involving differences in the insulin molecule between the different species, a multitude of different insulin drug formulations, and differences in the way the various injectables behave when used in different species. There have been few detailed studies into the pharmacological effects of insulin in dogs and cats, although there is much clinical experience with some insulin types.

It is generally regarded that the intensive treatment regimes, as used by many human diabetics, are not practical for use in cats and dogs. The intensive regimes involve multiple daily blood glucose measurements at home, and multiple daily injections of precisely calculated dosages of insulin. The aim of insulin therapy in pets is to attain reasonable control of blood glucose levels and the clinical signs of diabetes by using once or twice daily injections of medium to long acting insulin compounds.

As there is considerable variation between individual responses to a given insulin, an initial trial period is usually undertaken with the pet staying at the veterinary hospital for blood glucose monitoring. Some cats and dogs will be able to be managed with once daily injections, others will require twice daily treatment. Once stabilised on a dose schedule, management at home may be done either by a fixed dose plan or by measuring the glucose in the urine using dipsticks and adjusting the insulin dose accordingly. As the patient requirements for insulin tend to change over time, periodic rechecks are essential. Rechecks usually will involve a clinical examination, plus in-patient testing of blood glucose over one or more days.

Diet

The importance of dietary management of diabetes in humans is well known. Meals are timed to coincide with maximum insulin action, and specific quantities and types of food are advised. High fibre diets are used to reduce the peak elevation of blood glucose that follows eating. For pets, high fibre diets should be especially formulated or chosen from available commercial preparations to ensure consistency of calorie content, and nutritional completeness. Timing of meals and the use of high fibre diets is an important component of diabetes therapy in dogs. The timing of the meal is dependent on the type of insulin being used and the response of the individual dog as established by serial blood glucose determination. Meal feeding of cats may not be possible as many cats prefer to browse, eating many small snacks throughout the day and night. Cats may also refuse to eat high fibre diets. However, preliminary studies show that high fibre diets are beneficial to feline diabetics.

Of greater importance is that the animal attains the optimum weight, especially in obese animals as this will reduce the insulin demand, and in cats may eliminate the need to give insulin at all. Reduced calorie high fibre diets are particularly suitable for this purpose.

In animals that are underweight, it is desirable that weight is gained and that the animal attains normal weight. Very calorie dense diets should be avoided, especially those that are high in soluble carbohydrates. Anorexia is particularly dangerous in the insulin treated diabetic animal, as the blood glucose may become dangerously low if food is not eaten to balance the insulin dose. It is more important that the insulin treated diabetic eat something rather than nothing, even if it is not the most suitable food. Efforts should be made to adjust the diet gradually over a period of days or weeks rather than making sudden changes.

Other Drugs

Drugs that promote insulin secretion or modify insulin action are used in human diabetics either alone or in combination with insulin treatment. Few of these drugs have been evaluated for safety or effectiveness in dogs and cats. The sulphonylurea drugs that promote insulin secretion have been evaluated in dogs and cats. They are usually unsuccessful in dogs, but a proportion of cats with relatively mild diabetes may be able to be managed on the drug glipizide alone. However, the tablets need to be given twice daily by mouth, and some owners find that once daily insulin injections are more convenient as a treatment. Other drugs with similar properties require further evaluation before recommendations for use can be made. Drugs that are given in people as adjunctive therapy, such as anti-hypertensives and cholesterol-lowering drugs are not currently used in pets.

Further information on diabetes in cats and dogs:

Veterinary Clinics of North America "Diabetes Mellitus" Volume 25, number 3. May 1995. Editors: D.S. Greco and M.E. Peterson. W.B. Saunders & Co, Philadelphia, USA.

Textbook of Veterinary Internal Medicine - Diseases of the Dog and Cat. 4th Edition (2 volumes), 1995. Editors: S.J. Ettinger and E.C Feldman. W.B. Saunders & Co, Philadelphia, USA.

The Waltham Book of Clinical Nutrition of the Dog and Cat. 1st Edition, 1994. Editors: J.M. Wills and K.W. Simpson. Pergamon Press, Oxford, UK.

The Cat: Diseases and Clinical Management. 2nd Edition, 1994. Editor: R.G. Sherding. Churchill Livingston, New York, USA.

Dr Greg Martin BVSc MACVSc
Diabetes Research Group, Department of Companion Animal Medicine and Surgery, Faculty of Veterinary Science, University of Queensland, Australia