Cushing’s disease in the cat is not nearly as common, well-understood, or easily treated as in the dog. In both species, the condition is characterized by an overproduction of cortisol from the adrenal glands. Cortisol is a hormone produced by the adrenal glands that is necessary for life. It controls many functions throughout the body, including maintenance of normal appetite, feeling of well being, blood pressure, and glucose regulation, among others. When cortisol is produced in excess, clinical signs are largely related to exaggeration of its normal functions.
Cause of Cushing’s disease in the Cat
Cortisol is produced by the adrenal glands in response to a messenger hormone (ACTH) produced by the pituitary gland in the brain. More ACTH means production of more cortisol, less ACTH means less production of cortisol. The pituitary gland is charged with the task of sensing when cortisol production is appropriate and either producing more or less ACTH. Cushings disease in the cat is nearly always the result of a faulty feedback mechanism where the pituitary gland does not recognize that the adrenal glands have produced adequate cortisol and continually produces ever-increasing amounts of ACTH. The adrenal glands dutifully obey and the result is excess circulating cortisol.
On very rare occasion, a primary cancer (tumor) of one of the adrenal glands produces excess cortisol despite normal signals from the pituitary gland.
Clinical signs of Cushing’s disease in the Cat
Classic signs (symptoms) of Cushing’s disease in the cat include increased water drinking, urination and appetite as well as “fragile skin syndrome”. As in the dog, hair growth can be diminished almost to the point of complete baldness of the body while sparing the head and legs. The skin also becomes very thin and weak, with occasional spontaneous, devastating tearing.
Excess cortisol reduces the ability of cells throughout the body to respond to insulin, which normally drives blood sugar into cells to be used as energy. This “peripheral insulin resistance” leads to elevated circulating blood sugar, which spills into urine and draws water along with it. This is a condition known as “secondary diabetes mellitus” (DM) and is much more common in cats than dogs with Cushing’s disease. The resulting clinical signs are increased thirst, urination, and appetite in the face of weight loss.
Diagnosis of Cushing’s disease in the Cat
The diagnosis of Cushing’s disease in cats is similar to that in dogs. A noteworthy exception includes the need for more frequent sampling during endocrine testing (LDDS and HDDS tests). Also, there may be no significant abnormalities in the “minimum database” beyond elevated blood sugar (hyperglycemia) and sugar in the urine (glucosuria). These tests are still important, however, to rule out other more common diseases and to look for concurrent illnesses.
Treatment of Cushing’s disease in the Cat
Cats with Cushing’s disease regularly develop secondary diabetes mellitus (DM), requiring treatment with insulin. The use of oral medications that lower blood sugar (glucose), like those used in human diabetes mellitus, is generally not successful in this situation. Instead, injections of insulin twice daily are needed to lower blood glucose, to allow weight gain, and to reduce excess thirst and urination. The dose of insulin required in cats with DM secondary to Cushing’s disease is typically much higher than in cats with primary DM, and is often the first diagnostic clue to the presence of underlaying Cushing’s disease in the cat.
Treatment of the primary problem of excess cortisol production from the overworked adrenal glands has been a challenging, and usually unrewarding, task in cats. Although there have been a limited number of successful case reports, medications used in dogs such as trilostane and mitotane are often poorly tolerated or produce no benefit in feline Cushing’s disease. A drug called metapyrone can be useful for a brief period before the adrenal glands adapt and the drug becomes ineffective.
Surgical removal of both adrenal glands or the malfunctioning pituitary gland can also be performed. These surgeries, however, come with significant risk of serious complications, including infection (due to immune suppression) and poor healing (fragile skin), among others. Cushing’s disease in cats, therefore, is a debilitating and challenging disease and the prognosis for long term survival is poor.