Cushing's Disease: A Threat to Older Dogs
By: Jennifer Stone
University of Illinois College of Veterinary Medicine
Cortisol is an essential hormone for normal body function and is released in times of illness, stress, pain, and injury to help the body cope with these stressful events. When an excessive amount of cortisol is produced, however, the effects can be harmful and even fatal.
Dr. Jana Gordon, a former resident in internal medicine at the University of Illinois Veterinary Teaching Hospital in Urbana, says, "Cortisol is a steroid hormone released by the adrenal glands. In older animals, especially dogs, the adrenal glands secrete excess cortisol due to either a tumor in the pituitary gland of the brain or a tumor in the adrenal glands themselves. The tumor results in the production of large amounts of cortisol in excess of what the body needs." This condition is called Cushing's disease.
"The incidence of the two types of Cushing's disease can vary with dog breed and body size," says Dr. Gordon. "Smaller-breed dogs tend to get tumors of the pituitary gland, whereas larger-breed dogs are more likely to develop tumors of the adrenal gland.
In both cases, an excess amount of cortisol is released into the body, causing many problems. Cortisol can reduce the ability of the kidneys to reabsorb water, which causes the loss of large amounts of water in the urine. This means that a dog with Cushing's disease may urinate much more than usual and may drink large amounts of water in an attempt to replace the water lost by the kidneys. Some dogs also become ravenously hungry as a result of excess cortisol on the appetite centers of the brain.
Having excess steroids in the system over a long period can also cause muscle wasting, redistribution of body fat, and an enlarged liver resulting in the classic "pot-bellied" appearance seen in dogs with hyperadrenocorticism. Symmetrical hair loss in the trunk area can also occur as a result of the effects of excess cortisol on the hair follicles. Other skin problems such as thinning of the skin, fragile skin, and infections of the skin are due to the effect of cortisol on immune function.
Complications of Cushing's disease include increased risk of infection, high blood pressure, congestive heart failure, pancreatitis, diabetes mellitus, and blood clotting abnormalities. If the disease is left untreated, the animal will most likely succumb to the disease and its complications.
Many treatments are available for animals that develop Cushing's disease. If the disease is caused by a tumor of the adrenal glands, the best treatment is surgical removal of the tumor. If this is not possible, for example, if the tumor has spread to other tissues (metastasized), then oral medications are available that can temporarily decrease the amount of cortisol produced.
Tumors of the pituitary gland can be either large (macroadenomas) or small (microadenomas). The large tumors are rare and treated with radiation therapy followed by oral medication. The microadenomas are typically treated with oral medication alone. Because the pituitary gland is located in the brain, surgery is not generally recommended.
"Regardless of breed, treatment can often help a beloved pet live longer, extending its lifespan by an average of about 18 months," says Dr. Gordon.
If you have a pet that may be displaying signs of Cushing's disease, please contact your local veterinarian. Treatment for any disease usually works best when the problem is spotted early.