Dogs typically develop Cushing’s syndrome slowly over time and signs may be easy to miss. (Note: Symptoms are indications of illness not observed by others that patients report, whereas signs are indications of illness that are noted by others. Although we may infer a particular internal experience based on outward clinical signs – such as wimpering as an indication of pain or worry – the term symptoms is not generally applied to veterinary patients). Clinical signs of Cushing’s disease are largely related to exaggeration of its normal functions, and include:
Increased urination and drinking (polyuria/polydipsia)
The body’s goal is to expel as much waste material in the urine using as little water as possible by filtering waste and water from the blood together, and then reabsorbing the water. Cortisol blocks reabsorption of water, leading to far more voluminous and dilute urine than is necessary. Increased drinking develops to keep up with demand.
Increased appetite (polyphagia) with weight loss
One of cortisol’s many functions is to reduce the ability of cells throughout the body to use blood sugar for energy, even when present in excess, by making them resistant to insulin (called peripheral insulin resistance). This means that although the liver may be creating an abnormally large amount of sugar and distributing it through the blood, the dog is effectively in a state of starvation and loses weight. Owners of dogs with Cushing’s disease may misinterpret their dog’s robust appetite as a sign of health, allowing further progression of the disease before seeking veterinary care for other signs.
Hair loss (alopecia)/Abnormal hair growth
Dogs with Cushing’s disease develop a thin hair coat, progressive hair loss and fail to regrow hair after shaving. Abnormal hair growth typically occurs on the chest and abdomen while sparing the head and limbs, and occurs evenly on the right and left (bilaterally symmetrical).
Due to cortisol’s impact on the liver and muscles, the liver enlarges (hepatomegaly) and the muscles weaken, creating a potbellied appearance.
Secondary Urinary Tract and/or Skin Infections
Excess cortisol can suppress the immune system, making infections of the urinary tract, skin, and other tissues more likely. Even if treated successfully with antibiotics in the short term, these infections are likely to recur unless the underlying Cushing’s disease is managed.
Secondary Diabetes Mellitus
The body’s natural response to high blood sugar, such as in Cushing’s disease, is to produce insulin within the pancreas. Insulin then drives sugar into cells throughout the body for use as energy. In a minority of dogs, the pancreas cannot keep up with demand and peripheral insulin resistance is severe, allowing such an abundance of blood sugar that it spills over into the urine, a condition known as diabetes mellitus (DM).
Other signs less often observed in dogs with Cushing’s disease include panting, lethargy, exercise intolerance, muscle weakness, obesity, increased skin pigmentation, failure to enter heat (estrus), and testicular shrinkage (atrophy). Dogs with Cushing’s Disease will occasionally develop areas of very thickened skin, usually along the back, that is hard to the touch and crusty. This is due to an abnormal deposition of calcium in the skin and is called Calcinosis Cutis.
It is important to remember that although most dogs develop a familiar set of signs over a predictable timeline, there may be significant variation between dogs. Furthermore, since signs usually progress slowly, they may only be apparent to those who see the dog infrequently or appear to have popped up suddenly once they become severe. It is important that dogs with these clinical signs consult their primary care veterinarian. Left untreated, this condition can result in serious illness and may even become life threatening.