Chronic Kidney Disease
Kidney Structure and Function
Like all mammals, cats have 2 kidneys located in the abdomen just behind the last rib. The kidneys are part of the upper urinary tract, while the bladder and urethra are referred to as the lower urinary tract*. The important functions of the kidneys are:
- Removal of waste products from the blood, excreted in urine
- Preserving normal electrolyte (such as sodium and potassium) levels in the blood
- Production of erythropoietin (Epo), a hormone that stimulates red blood cell production
- Regulation of blood volume, hydration, and blood pressure
Prevalence of kidney disease in cats
Kidney disease is a common disorder of cats over 10 years of age. In geriatric kidney disease, the kidneys began to degenerate as part of the natural aging process. The cause for this degeneration is unknown. All breeds and both sexes are equally affected by geriatric onset kidney disease, although certain breeds are also predisposed to earlier onset genetic renal disease.
Symptoms of kidney disease (also called renal disease)
Only 30% of normal kidney tissue is necessary for the normal kidney function, meaning that up to 70% of kidney tissue is damaged before clinical signs of dysfunction occur. Therefore, in the early stages of disease, symptoms may be too subtle to detect at home. The table below shows the common progression of kidney dysfunction to kidney failure, and the observed clinical signs:
- Loss of ability to concentrate urine resulting in excessive loss of fluid
- Inadequate clearance of waste products from the blood
- Electrolyte imbalances
- Inability to maintain normal blood volume
- Decreased activation of vitamin D, causing excess parathyroid hormone levels
- Dysregulation of normal blood pressure resulting in hypertension (high blood pressure)
- Decreased production of erythropoietin
- Increased thirst and increased urine output
- Depression, decreased appetite, vomiting, weight loss
- Muscle weakness, depression, decreased appetite
- Dehydration, constipation
- Lethargy, decreased appetite, muscle and nerve dysfunction
- Depression, blindness, seizures, stroke, bizarre behavior (e.g., night howling)
- Weakness, decreased appetite, and lethargy from anemia
* It is important to note that symptoms of bladder or lower urinary tract disease are different than those of kidney disease. Bladder disease (often caused by urinary tract infections, stones, or other irritants) is usually characterized by painful urinations, frequent trips to the litter box with only drops of urine being produced, bloody urine, vocalizing in the litter box, or abdominal pain.
What are the causes of renal dysfunction in cats?
The most common cause of progressive kidney failure in older cats is interstitial nephritis, which is a combination of inflammation, cellular degeneration, and scar tissue formation. Interstitial nephritis may be triggered by infections or other injury to the kidneys earlier in life. In most cases, the exact cause cannot be determined.
Cats may also develop pyelonephritis, a bacterial infection of the kidneys. This is more common in cats with pre-existing renal disease (such as interstitial nephritis), because resistance to bacterial colonization is often lost when renal function declines. Tumors of the kidney may also occur in middle-aged to older cats. Polycystic kidney disease (PKD) is a genetic condition which occurs more frequently in breeds such as Persians and Himalayans. Symptoms may appear early or late in life.
Acute kidney insufficiency can be caused by toxin ingestion or obstruction of urinary flow at various parts of the urinary tract. Obstruction can occur in cats of any age and is more common in males. Lilies, antifreeze (ethylene glycol), and human strength anti-inflammatory drugs can cause renal failure in cats. In some cases, depending on the cause, acute kidney disease may be reversible, although the long-term effects on the kidneys is unknown.
How do we tell if a cat has kidney disease?
Physical examination findings and laboratory testing of blood and urine samples are needed to evaluate kidney function. Because symptoms may be mild in the early stages of kidney disease, we recommend laboratory screening of all cats 10 years or older.
The laboratory values we monitor regularly include:
- Urine specific gravity—An indication of the kidneys ability to concentrate urine
- Blood urea nitrogen (BUN)—A waste product generated from protein metabolism which is filtered by the kidneys
- Creatinine—A waste product from normal breakdown of muscle tissue which is also filtered like urea, this value is less affected by diet and other variables than BUN
- Potassium—A water soluble mineral; commonly lost in excessive amounts in the urine
- Phosphorus—Normal levels are maintained by two mechanisms that occur in the kidney
- Hematocrit—A measure of the red blood cell count
- Blood pressure—Altered regulatory mechanisms in the kidney can cause high blood pressure (hypertension)
Abdominal radiographs (x-rays) may also be recommended to look for kidney stones and assess kidney size and shape. Cats with small kidneys (which can be irregular in contour) are mostly likely to have chronic renal disease characterized by interstitial nephritis. If the kidneys are enlarged and/or irregular, tumors, infection, obstruction and PKD must be considered. If one or both kidneys are enlarged, abdominal ultrasound and possible kidney biopsies may be recommended.
Once kidney disease has been diagnosed, your veterinarian will categorize the severity of disease based upon a set standard of parameters outlined by IRIS (International Renal Interest Society) in order to facilitate appropriate treatment and monitoring. The IRIS Classification system is based upon blood creatinine levels. As creatinine increases, the stage of disease will increase (Stage 1 is mildest/earliest disease while Stage 4 is advanced/late stage renal disease).
Is kidney dysfunction reversible?
Once kidney cells die, they will not regenerate. However, if a problem that is causing kidney damage is corrected (treating pyelonephritis, relieving urinary obstruction, etc) before the cells die, some function can be preserved.
If kidney dysfunction is irreversible, does that mean there is no treatment?
Absolutely not! Many cats benefit tremendously from therapy designed to help support kidney function. The treatments vary depending on the stage of the kidney disease.
Kidney dysfunction in cats is not caused by diet, nor is it worsened by diet. We do, however, often recommend diet changes in order to address some of the negative consequences of kidney disease. Our goals of dietary therapy include:
- Weight gain in thin cats
- Reduction of nitrogenous waste products in the blood
- Maintenance of normal potassium levels
- Rebuilding depleted B vitamin stores
- Avoidance of acid accumulation in the body
- Reduction of phosphorus levels
Unfortunately, we usually cannot achieve all of these with one diet. The most common obstacle in nutritional therapy of renal disease is that prescription foods which help improve metabolic imbalances often will not result in maintenance of a healthy weight, which is critical to a cat’s longevity and sense of well-being.
Because of this dilemma, the general diet recommendations we make are:
- Canned diets, which contain higher amounts of water, are preferable to dry diets.
- Reduced protein diets should not be fed to thin cats or those with poor appetites – these cats should be fed high-calorie, palatable food at all times
- Prescription kidney diets are best suited for cats that are exhibiting elevations in their blood phosphorous levels. However, these diets should only be given if the cats’ appetite is good and current weight is appropriate
Treatment plans are determined based on the individual needs of the cat but may include:
Modified protein diets/canned diets
Reduce nitrogenous waste products in the blood by providing high biologic value protein.
*Note: these diets are not recommended for cats that are underweight, hyperthyroid, or have diminished appetites!
Prevent excessive gastric acid accumulation, decreases nausea and vomiting, improves appetite. (Pepcid AC, Zantac)
Bind dietary phosphorus in the intestine to prevent absorption in the bloodstream. (Alu-Caps, Aluminum Hydroxide)
Maintains normal muscle and heart function. (Tumil-K, Renacare)
Blood pressure medication
Lowers blood pressure. (Amlodipine) High blood pressure can occur secondary to kidney disease or can be a primary condition. In either case, it may worsen or cause progression of renal disease if not treated.
Supplementing fluids reverses dehydration and reduces the concentration of waste products in the blood which can improve how your cat feels. For severe dehydration, intravenous fluids (in the hospital) are needed. For maintenance of hydration at home, we often teach owners how to give injections of a balanced electrolyte solution under the skin. Adding water to food can also help.
Needed for red blood cell production. (Ferrous sulfate, Pet-tinic)
Stimulates red blood cell production. (Epogen)
Helpful in keeping calorie intake adequate. (Cyproheptadine, Mirtazapine)
Needed any time a bacterial component is suspected to be contributing to kidney dysfunction.
B vitamin supplementation
Replenish B vitamin stores that are depleted by excessive urine output, this improves appetite and energy levels. (B complex, Pet-tinic)
Similar in function to vitamin D, this medication helps maintain normal parathyroid hormone levels in cats with secondary parathyroid dysfunction.
Monitoring cats with renal disease
We recommend that cats with renal disease be evaluated every 3 to 6 months. A physical exam is performed each time. Depending on your cat’s status, measurement of systolic blood pressure and blood and urine tests may be indicated. Based on these measurements, we may adjust our therapeutic regimen to best fit your cat’s needs.
What is the prognosis with kidney disease?
Although we cannot cure chronic renal disease, we can help support your cat by developing a therapeutic plan. With treatment, many cats may live months to years with a good quality of life. As renal disease progresses, it is important to evaluate whether or not your cat’s quality of life is declining. We are here to help guide you through this process.
Websites for more information on kidney disease: