Managing diabetes mellitus in cats: What makes it work? (Sponsored by Intervet Schering-Plough Animal Health)

Managing diabetes mellitus in cats: What makes it work? (Sponsored by Intervet Schering-Plough Animal Health)

While sometimes thought of as a disease entity, diabetes mellitus is a heterogeneous group of disorders in which insulin production is reduced or tissue cells are resistant to the effects of insulin, resulting in impaired glucose homeostasis.

Table 1. Absolute or relative insulin deficiency levels
In people, type I diabetes mellitus refers to the condition seen in individuals who are generally lean, young, and prone to ketogenesis. Type II diabetes mellitus usually occurs in older people who are often obese but less prone to the development of ketoacidosis. Type I diabetic patients require insulin therapy, whereas type II diabetics may be controlled—at least initially—with weight loss, diet, and oral hypoglycemic agents. This is because in type I diabetes, beta cell depletion results in a decreased insulin production and an absolute insulin deficiency. In type II diabetes, insulin receptor and postreceptor defects cause impaired insulin uptake by tissues. This insulin resistance and associated hyperglycemia cause the beta cells to produce more insulin; thus, this state is one of a relative insulin deficiency at the tissue level. Various levels of absolute or relative insulin deficiency exist (see Table 1).

In cats, the categorization of diabetes is not as clear. Generally, diabetes is a disorder of older, often overweight cats that are not prone to ketogenesis, more similar in signalment to type II diabetes in people. However, obese cats appear to have a defect in insulin secretion along with lower tissue sensitivity to insulin.1,2 Weight loss of even 10% to 15% results in improved tissue sensitivity, but, often by the time of diagnosis, these cats require treatment with insulin. Cats may also develop diabetes secondary to primary pancreatic disease, endocrinopathies (e.g., acromegaly, hyperadrenocorticism), or drug therapy (e.g., glucocorticoids, progestins). Risk factors for feline diabetes include weighing greater than 7 kg (15.4 lbs), not being physically active, or being over 9 years old, a neutered male, or a Burmese.1,3

Complications of unregulated diabetes mellitus

In short, diabetes mellitus results in cell starvation in the presence of hyperglycemia. While cell starvation results in polyphagia with concurrent weight loss, hyperglycemia results in polyuria and compensatory polydipsia due to glucose spilling into the urine and osmotically drawing water with it.

Table 2. Complications of unregulated diabetes mellitus
Some diabetic cats are affected by a polyneuropathy that can result in functional, structural, and biochemical defects in the peripheral motor and sensory nerves of the limbs. It generally affects the pelvic limbs more than the thoracic limbs.4 The pathology in nerve biopsies from 12 cats with spontaneously occurring diabetes included damage to Schwann cells, demyelination of nerve fibers, thin myelin sheaths, and an overall decrease in myelinated fiber density.5 See Table 2 for a list of functional changes associated with unregulated feline diabetes. Different than in people, diabetes does not predispose cats to hypertension, hypertensive retinopathy, or proteinuria.6

A recent study investigated associations among clinical signs, serum biochemical markers, and urinalysis results and found that, "Many of the cats with urinary tract infections had no clinical signs of lower urinary tract disease or changes in their laboratory values indicative of infection. Therefore, a urinalysis alone should not be used to exclude urinary tract infections in these cats."7 I recommend a urine culture for this purpose. An interesting report of pulmonary changes (such as congestion, edema, pneumonia, fibrosis, mineralization, and neoplasia) in diabetic cats without clinical signs of respiratory disease also shows the need for greater vigilance in monitoring the respiratory system.8