Diabetes mellitus is a common endocrinopathy in dogs in cats. Dogs usually get type 1 or the insulin-dependent form. This occurs from loss of pancreatic beta cells such that adequate amounts of insulin are not produced and secreted. Cats usually get type 2 or the non-insulin dependent form of diabetes mellitus. This occurs as a result of insulin-resistance, often from obesity. The pancreatic beta cells are forced to produce increasing amounts of insulin to overcome the resistance, resulting in ultimate loss of function in the cells. Additional loss results from direct glucose toxicity of the beta cells. Cats can go into diabetic remission a syndrome in which insulin requirements diminish or cease to be necessary after initial treatment. This is probably due to correction of a condition causing insulin resistance.

Diagnosis

Diabetes mellitus is relatively easy to diagnose. Clinical signs include polyuria/polydipsia, weight loss, persistent or recurrent urinary tract infections, weakness and muscle wasting, cataracts (usually dogs), and peripheral neuropathies (usually cats). Diagnosis can be made by recognition of appropriate clinical signs, and demonstration of persistent hyperglycemia and glucosuria. One confounding factor to this diagnosis is stress. Stress, alone, can cause hyperglycemia that can be high enough to be cause spill over into the urine and glucosuria. Should the clinician have any doubt of whether hyperglycemia and glucosuria are due to diabetes mellitus, s/he should check a serum fructosamine level. This value gives the average of the blood glucose over the preceding 2–3 weeks. If elevated, then diabetes mellitus can be diagnosed. If not elevated, then the hyperglycemia/glucosuria was probably due to stress.

Initial evaluation will decide how intensively the patient should be managed as diabetic treatment is begun. If the animal is eating and drinking normally and is well hydrated, there is no reason to hospitalize him/her while insulin therapy is initiated. If the animal is ketotic, acidotic, hyperosmolar, or dehydrated, s/he should be admitted to the hospital and stabilized before long-term insulin therapy is instituted. The most common concurrent diseases seen with diabetes mellitus include: urinary tract infections, concurrent endocrinopathies such as Cushing’s disease, hypothyroidism, and hyperthyroidism, pancreatitis, infections, and pregnancy. Initial evaluation of the diabetic animal should include a complete physical examination, CBC, chemistry profile, urinalysis, and T₄ (cats). A urine culture should be considered even if urinalysis and sediment parameters are normal since up to 35% of urinary tract infections can be clinically silent in animals with dilute urine. Abdominal imaging may be pursued if clinically indicated. Concurrent medical conditions should be addressed aggressively so that the diabetes mellitus can be more easily controlled.

Treatment

Diabetes mellitus can be frustrating, expensive, and time-consuming for owners to treat. Our goals of therapy should be correction of clinical signs, control of concurrent diseases, and avoidance of emergency situations such as hypoglycemia, ketosis, and hyperosmolality. It is beneficial to have an in depth discussion with the owner as to the time and effort s/he can realistically commit for diabetic control for the pet. It is important to establish a good rapport with diabetic owners since they will be asked to provide invasive (injections) and time-consuming (glucose monitoring) care for their pets.

Stable cats with blood glucoses less than 400 mg/dl may be treated initially with a diet change. Cats are very carbohydrate intolerant, and a low carbohydrate diet can result in euglycemia in some cats. Prescription diets are recommended; however, if the owners can’t or won’t commit to these diets, commercial diets can be used. Canned diets tend to be lower in carbohydrates and some websites include carbohydrate data on specific diets. Because of the deleterious effects of a hyperglycemic environment on the cat pancreas, diet change alone should not be tried for more than 2 weeks. If the cat has not become euglycemic over that time period, then insulin therapy should be instituted.

Insulins recommended for cats include: ProZinc™, glargine (Lantus™), and Vetsulin™. ProZinc™ insulin has an advantage in that it is labeled for veterinary use. Detemir™ may also be used, although there is less experience with this insulin in cats. NPH insulin may be used but it does not have the duration of action in cats to allow BID treatment. Optimally NPH needs to be given TID-QID in cats. The starting dose of insulin (regardless of type) in cats is: 0.25 U/kg BID. The optimal diet for diabetic dogs is one high in insoluble fiber. This diet slows glucose absorption from the gut and postprandial hyperglycemia.

Unlike cats, dogs will not usually revert to euglycemia as a result of a food change. Newly diagnosed diabetic dogs should be started on insulin therapy. Appropriate insulins for use in the dog are: Vetsulin™, NPH, Detemir™, ProZinc™, and glargine (Lantus™). Dogs are initially dosed at 0.5 U/kg BID except for Determir™ that should be started at 0.25 U/kg.

Dogs and cats should be fed twice a day when insulin is given. Many owners give insulin while the animal is eating. This makes the insulin injection a pleasant experience for the pets and easier for owners to treat the animal. Some cats prefer to nibble food throughout the day. These grazers can often be well managed by allowing them free choice eating with insulin injections twice per day. Care should be taken to ensure that the cats are not receiving more than their caloric needs since extra weight should be avoided.

Exercise is beneficial to diabetics and serves to lower insulin requirements and provide better glycemic control. Daily walking for dogs and cat play can be effective ancillary treatments for diabetes mellitus. Average time for initial diabetic control is 4–6 weeks.

Monitoring

After insulin is started, the veterinarian should wait 7–14 days to monitor any effects since it takes this long for the animal to adjust to insulin therapy. During that time period owners may measure urine glucose and ketones. They should call if there are more than 2 negative urine glucose readings or if the ketones are positive. If urine glucose is negative, one doesn’t know if the blood glucose is 40 or 200 mg/dl. Alternatively, the owner can use a portable glucometer and measure blood glucose directly. Insulin doses should not be changes as a result of readings, but owners should call if the animal is ketotic or hypoglycemic.

At the initial recheck, the veterinarian should question the owner about resolution of clinical signs. A physical exam and weight measurement should be completed. In this way the veterinarian can evaluate the clinical response to insulin. Serum fructosamine levels have been advocated for use in monitoring insulin response. For fructosamine to be interpretable, the pet should have been on a stable insulin dose for at least 3 weeks before the fructosamine is taken. Fructosamine levels can be useful; however, it is inappropriate to use in unstable animals or those in which a hypoglycemic-hyperglycemic (Somogyi) response is suspected. For these patients, a glucose curve must be completed.

Glucose Curves

A glucose curve is the only way to truly evaluate the body’s response to insulin. Important information obtained from glucose curves includes the onset of action of the insulin, the duration of action of the insulin, the time of peak activity of the insulin, and how low the glucose goes (nadir). The first three parameters indicate whether the right type of insulin is being used; the last parameter gives information about the dose of insulin used.

The traditional glucose curve has many limitations, including disruption of the patient’s normal activity and eating routine, the introduction of stress-related hyperglycemia, and labor intensiveness of the procedure. Furthermore, both diabetic dogs and cats have been shown to have significant variations in their day-to-day glycemic control. Intermittent blood sampling over only a 12-hour period may grossly over or underestimate a patient’s glycemic control, and glucose peaks and nadirs may be missed if they occur between samplings. Continuous glucose monitoring systems (CGMS) provide a minimally invasive method for continuously evaluating glycemic control for up to 72 hours. Interstitial glucose has been shown to correlate well with plasma glucose levels. They are comprised of an external sensor with a flexible electrode that reacts with glucose when it is inserted into the subcutaneous tissue. This sensor then communicates to a small monitor that records the glucose data. MiniMed iPro2 and Abbott Freestyle Libre systems have been used successfully in veterinary medicine and can be sent home with animals.