The leading cause of both illness and death in cats over eight years of age is kidney disease, unlike older dogs and humans, in whom the heart is the most common organ to fail with age.

Cats are more susceptible to chronic kidney (renal) disease in part because cats are born with fewer individual functional kidney units (called nephrons) than other species. Cats have just 200,000 nephrons compared to about 1 million in the dog and 3 million in the human.

Nephrons are the tiny tubules within the kidneys that filter the blood, removing waste products while forming urine. Once these nephrons are badly injured, they cannot regenerate.

It is not until cats lose 70 to 80 percent of their total functioning kidney mass that we begin to observe signs of kidney disease in the home environment or see biochemical changes on lab tests consistent with kidney failure.

Causes of chronic kidney disease include "wear and tear" or age-related degeneration, "smoldering" kidney infections (for instance, bacteria climbing up to the kidney from the bladder, or viral, including feline infectious peritonitis), kidney stones, toxins and congenital anatomical disorders, such as malformations or cysts. Certain vaccines and dental disease have also been implicated as controversial potential sources of injury. And, recently, contaminated pet food has played a tragic role.

With advances in veterinary care in recent years, including more routine pre-anesthesia screening and more frequent older or "senior" cat laboratory profiling, we can and do recognize cats with chronic kidney disease much earlier in the course of their disease. Blood and urine testing recommended by your veterinarian represents an important opportunity to benefit cats found to have early signs of kidney insufficiency, potentially improving their quality and quantity of life.

Since the kidney is responsible for filtering the blood of waste products - such as blood urea nitrogen (BUN) and creatinine - that are then eliminated in urine, levels of these substances may increase in the blood if the kidney function is inadequate. Because another major function of the kidney is to conserve body water by concentrating urine, persistently more diluted urine than normal can be an early sign of kidney insufficiency.

Depending upon any such abnormal findings, patient history and other factors, your veterinarian might recommend additional tests, such as a urine culture to identify occult bacteria in the urine, a blood pressure screening for possible hypertension, X-rays to screen for stones in the urinary tract and/or an ultrasound to visualize the architecture of the kidneys and urinary tract. All of these diagnostic tests are designed to pinpoint the cause and severity of disease and guide an optimum course of initial therapy.

Because older cats demonstrate less thirst than younger cats, encouraging water intake or regularly administering a balanced electrolyte solution under the skin (subcutaneously) are common early treatments for long-term kidney support. Depending upon the stage of the kidney insufficiency, your doctor might suggest additional therapies, including special or prescription diets, oral potassium supplementation, hormone therapy (calcitriol for elevated blood phosphorous levels or Epogen to stimulate increased red blood cell formation to treat anemia) or medications for hypertension, if present. Since chronic renal failure is an inexorable process, follow-up evaluations are critical in determining which of these medications or therapies are appropriate or require adjustment over time.

Dr. Steve Dana is the medical director at Pet Emergency Specialty Center of Marin and writes for the Marin Humane Society.

Tails of Marin appears every Saturday in the Home & Garden section of the Marin Independent Journal