Diseases That Occur More Frequently in Diabetic Cats
A concurrent disease (i.e., a comorbidity), is a disease that is present at the same time as another disease considered as a primary.
A concurrent disease is different from a complication that is a direct consequence of the primary disease. A diabetes concurrent disease is not a direct consequence of diabetes. Diabetes and a concurrent disease may have a common cause such as age, or obesity. However, in many cases, the relationship between both diseases is not fully elucidated, but experience has shown that these diseases are frequently associated.
This approach has some practical consequences on the management of diabetes:
- Your vet will have to search for the presence of any concurrent disease, once diabetes has been diagnosed
- If present, the concurrent disease will require specific management, in addition to diabetes treatment
- The concurrent disease treatment may interfere with the management of diabetes. Some conditions may increase insulin resistance, or require a different type of dietary management
To take care of your diabetic cat, your vet will choose to focus on hyperthyroidism, hyperadrenocorticism (Cushing’s syndrome), and chronic renal (kidney) failure.
Hyperthyroidism deserves a special attention from those who take care of cats with diabetes. Several issues concern both conditions:
- Many symptoms of the two diseases are similar
- Hyperthyroidism increases insulin resistance
- The fructosamine test for diabetes cannot be properly interpreted in case of hyperthyroidism
Hyperthyroidism is an excessive production of two hormones produced by the thyroid, the thyroxine and the triiodothyronine by the thyroid glands. The thyroid glands are located on either side of the trachea close to the larynx. The excessive hormones production comes from an enlargement of the thyroid glands. In most cases, both glands have enlarged. In some rare cases, the thyroid hypertrophy is caused by malignant tumors (cancer).
It seems that hyperthyroidism is becoming increasingly prevalent and that it is the most frequent endocrine disorder in aging cats. The cause for hyperthyroidism is not known. Experts hypothesize it may come from food, indoor behavior, genetic predisposition, or exposure to chemical substances.
Hyperthyroidism increases the need for energy, increases the blood flow in many organs (kidneys, brain…), affects neurohormones secretion, disturbs gastrointestinal motility or weakens respiratory muscles. These mechanisms lead to a variety of symptoms. Many of them, and usually the more frequent, are similar to those of diabetes:
- Slow weight loss and polyphagia (increased hunger)
- Polyuria (increased urination) and polydipsia (increased thirst)
- Lethargy and weakness
- Unkempt hair coat
- Diarrhea and vomiting (signs corresponding to severe diabetes)
Others signs cannot be confused with those of diabetes
- Restlessness and aggressive behavior
- Panting and respiratory distress
- Decreased appetite in a certain type of hyperthyroidism: apathetic hyperthyroidism
- Heat and stress intolerance
Symptoms observation, palpation of the thyroid glands, echocardiography, X rays will drive the vet to suspect hyperparathyroidism. The confirmation of the diagnosis will come from the dosage of blood thyroxine (T4).
It is important to note that hyperthyroidism affects the fructosamine test, very useful in the diagnosis of diabetes. Hyperthyroidism lowers the concentration of glycated proteins circulating in the blood. Hence, the usual fructosamine normal values are no more applicable, without valid reference range. The fructosamine test should not be used in hyperthyroid cats.
Hyperthyroidism increases insulin resistance. Increased energy needs of hyperthyroid cats make them eat more (polyphagia), absorb more sugars and secrete more insulin. Insulin resistance arises as a way, for the body cells, to limit the consequences of the increased availability of glucose. This mechanism may aggravate the condition of diabetic cats.
The goal of hyperthyroidism treatment is to reduce the secretion of thyroid hormones. It has to take into account the presence of concurrent diseases such as heart disease, renal disease or diabetes. Drugs (methimazole), surgery (surgical removal of abnormally active thyroid tissue) or radioactive iodine (radioactive iodine concentrates in the thyroid and its radiations destroy the surrounding thyroid cells) are the mainstay of cat hyperthyroidism treatment.
Prognosis: there is considerable survival variation among cats depending on their age, the nature of their tumor, their concurrent diseases and the way they are taken care of. They may survive a few years.
The Cushing’s syndrome brings together all the symptoms and signs that result from a long-term exposure of high concentrations of blood cortisol. It can originate from:
- the administration of glucocorticoids = iatrogenic Cushing’s syndrome – rare in cats
- the over secretion of endogenous cortisol = naturally occurring feline Cushing’s syndrome
Cushing’s syndrome is rare in cats compared to dogs. In most cases, the disease is naturally occurring. It mainly comes from an excessive secretion of adrenocorticotropic hormone (ACTH) from the pituitary gland that triggers the secretion of cortisol by the adrenal glands. Alternatively, tumors can cause an enlargement of the adrenal glands and lead directly to the excessive production of cortisol.
The cat is relatively insensitive to the negative side effects of long-term administration of glucocorticoids. It appears that many cats that have Cushing’s syndrome also suffer from diabetes. We have seen that persistent high concentrations of blood cortisol could lead to Cushing’s disease. Glucocorticoids and cortisol also play a role in insulin resistance and diabetes onset and progression. Glucocorticoids stimulate the production of glucose from liver and skeletal muscles glycogen. They inhibit insulin sensitivity. Glucocorticoid excess impairs insulin secretion within the pancreatic beta cells, hence helping the diabetes to progress from glucose intolerance to overt diabetes mellitus.
In addition to classic diabetes signs (polyuria, polydipsia, weight loss, weakness, lethargy), the cat may display other symptoms linked to excesses in blood cortisol:
- Redistribution of the body fat in the abdominal region: pot belly symptom
- Blood vessel friability causes bruising
- Curling of ear tips comes from weakened cartilages
- Skin disorders such as symmetrical hair loss in the trunk area, thinning of the skin, fragile skin, or skin susceptibility to infections
The diagnostic strategy includes two steps. It consists firstly of establishing the diagnosis of hyperadrenocorticism with a suppression dexamethasone test also called low-dose dexamethasone screening test (LDDST) possibly completed or preceded by a urine cortisol test.
Then, the vet will have to discriminate between pituitary-dependent hyperadrenocorticism (PDH) and adrenocortical tumor hyperadrenocorticism (ATH), with a high-dose dexamethasone suppression test (HDDST).
Treatment of pituitary dependent hyperadrenocorticism: the pituitary gland (=hypophysis) may be removed, but surgery is difficult because the gland is very close to the brain. Radiation therapy may be preferred, especially for controlling malignant tumors. In complement, some drugs such as trilostane, or mitotane may be prescribed although they are not registered in cats. They require careful monitoring and good owner information.
Treatment of adrenal tumor hyperadrenocorticism: removing one or both adrenal glands by surgery is preferred whenever possible. Sometimes, the too large size of the adrenal glands, the tissue vascularization, or the presence of a malignant tumor makes surgery not advisable. A medicinal therapy will be then provided.
Cushing’s disease is a severe disease whose prognosis is guarded or grave. Treatment brings relief to the cat and prolongs its survival time, but it is not likely to survive more than two years.
Chronic kidney disease (CKD) is a very frequent disease in middle aged to older cats. CKD is frequently associated with cat systemic hypertension and hyperthyroidism.
Kidneys are made of numerous (400 000) nephrons, functional subunits that removes from blood wastes, water and minerals. CKD is a silent disease whose symptoms develop late. Initially, a kidney inflammation, caused by complex autoimmune disorders or infections, destroys nephrons and triggers the onset of CKD. This initial injury induces the slow progression of the disease through a cascade of nephrons death.
Diabetes, by increasing urine osmolarity and kidney tissues inflammation can trigger or precipitate renal disease. However, as renal disease is much more frequent in cats than diabetes, and progresses slower, it should be considered, in most cases, as a diabetes concurrent disease rather than a complication.
If a renal disease is diagnosed in a diabetic cat, the control of blood glucose becomes part of the therapeutic strategy for slowing down the kidney disease. The cat will be fed with a renal diet, low in protein and phosphorus, but higher in carbohydrates. The insulin dose will be increased to compensate for the additional amounts of carbohydrates.