Older cats are especially at risk for chronic renal failure. Here’s what you need to know about this deadly threat.
More than 30% of cats will get kidney disease at some point in their lives, and as they get older, the likelihood they will develop kidney disease increases. By the time they reach the age of 15, more than half of cats over are afflicted with some form of kidney disease.
Your cat’s kidneys play a central role in many important processes. They help to control the blood pressure and regulate the amount and chemical composition of fluid in the bloodstream. They produce a variety of essential hormones and enzymes, and they contribute to the production of red blood cells. They also remove metabolic waste products from the bloodstream.
Normally functioning kidneys
Each kidney contains hundreds of thousands of tiny filtration units called nephrons. When waste-laden blood enters the kidneys through the renal artery, it moves through progressively smaller vessels until it reaches these nephrons, where it is filtered through microscopically minute structures called glomeruli. The cleansed blood—about 95 percent of the total fluid volume that originally entered the kidneys—then circulates back to the heart for yet another voyage through the body. The remaining fluid, containing the waste products, is passed along as urine from the kidneys to the bladder through thin tubes called ureters, and is eventually excreted.
Kidney Disease in Cats
When the kidneys and their complex filtering system break down, toxic waste products can start to accumulate in the bloodstream. If a proper balance of waste, minerals, and electrolytes (such as sodium and potassium) is not maintained, severe complications may affect other organs.
Unfortunately, feline kidneys are susceptible to a wide range of conditions that may diminish kidney function, and which, if severe enough, may cause illness and even death. Chronic kidney disease is a common cause of severe illness in cats, particularly older cats.
A risk for kidney disease may be inherited, and there is evidence that some breeds such as Persians, Himalayans, and British Shorthairs are more genetically predisposed to this condition. The great majority of cases, however, are acquired, and they fall into either of two broad categories: acute kidney failure (or injury) or chronic kidney disease. The difference between the two is that acute kidney injury is a severe condition with a relatively sudden onset with clinical signs that become apparent over a period of a week or a month, while chronic kidney disease is a disease that has been present for a long time.
Common causes of acute kidney injury include:
If diagnosed at its earliest stage and treated immediately, kidney damage resulting from acute kidney injury is potentially reversible, although the prognosis for all cats with acute kidney failure is generally guarded. In some cases, depending upon the cause of the acute injury, the prognosis may be poor, but in others, the prognosis may be more favorable and, with appropriate therapy, kidney function may return to normal.
Chronic kidney disease, in contrast, is an incurable condition with a gradual onset (months to years) that mainly afflicts middle-aged and older cats. Until recently, veterinarians could not detect chronic kidney disease until it was fairly advanced, when the kidneys had lost most of their function permanently, but testing is now available that can make the diagnosis earlier in the course of disease (See Signs/Diagnosis of Kidney Disease).
Often the exact causes of chronic kidney disease are unknown, although the loss of healthy kidney tissue may be a natural endpoint for several conditions, including advanced dental disease, kidney infections, inflammation, and obstruction of the urinary tract. Irrespective of the cause, however, by the time a cat is showing signs of chronic kidney disease, the degree of loss of healthy kidney tissue will often limit options for effective treatment. In some cases, little can be done to prolong the life of a cat diagnosed with late-stage chronic kidney disease. For this reason, early diagnosis and monitoring of this serious illness is critically important for cats.
Signs/Diagnosis of Kidney Disease
Cats with chronic kidney disease often show no visible signs early on, which is why early diagnosis can be challenging. In later stages, signs may include increased thirst, increased urination, weight loss, decreased appetite, lethargy, and, in some cases, vomiting. If these signs are observed, the owner should seek veterinary consultation at once.
If kidney disease is suspected, a veterinarian will usually first perform a biochemistry panel on a cat’s blood and a urinalysis (physical, chemical, and microscopic evaluation of urine).
A biochemistry panel measures the concentration of a number of compounds in the blood, including waste products (such as creatinine) that would normally be shed in the urine. Increased levels of creatinine indicate that the kidneys are not adequately filtering metabolic wastes from the blood stream. However, measuring creatinine alone typically does not detect kidney disease until it has progressed to later stages, when the kidneys have lost 75% of their function. In addition, creatinine testing alone may not work well for thin pets or in those without much muscle mass, so it can sometimes miss even advanced disease.
A recently developed blood test that measures the concentration of symmetric dimethyl arginine (SDMA), a waste product of protein metabolism, is capable of diagnosing a decrease in kidney function earlier than previous tests, but it is not yet clear whether treating cats at these earlier stages based on SDMA is always helpful. Preliminary studies suggest that the SDMA kidney test can help detect kidney disease an average of nine months earlier than traditional testing, when kidney function may be more intact, allowing a veterinarian to administer appropriate therapies earlier in the disease process. Further studies are required to determine whether early intervention based upon SDMA testing will translate into improved outcomes for cats with chronic kidney disease.
Finally, a urinalysis will provide additional information on the extent of kidney damage and determine whether an infection might be responsible for the diminished kidney function. Further tests, including X-rays, ultrasound, urine culture, and/or a kidney tissue biopsy, may be required to confirm a tentative diagnosis of kidney disease and to determine its cause.
The treatment of kidney disease is determined by the type of disorder causing the condition (if this can be determined) and the extent of kidney damage that has been sustained. Acute kidney failure is an emergency, and cats with this condition are usually hospitalized and treated aggressively. If caused by any type of urinary tract blockage, for example, the obstruction must be removed immediately. The use of any suspect medications must cease, and intravenous solution infusions are administered without delay in order to correct fluid and electrolyte imbalances in the blood.
Treatment of chronic kidney disease, especially early disease, is focused on attempting to slow disease progression by identifying the underlying cause, where possible, and prescribing specific treatment. High blood pressure and urinary protein leakage (proteinuria) are complications of kidney disease that may accelerate kidney disease progression, so they should also be recognized and treated when possible.
Dietary changes may also help. There is evidence that cats with chronic kidney disease that consume a therapeutic kidney diet containing conservative amounts of high quality protein, adequate energy from non-protein sources, restricted amounts of phosphorus, and adequate supplementation with b vitamins, vitamin D, and omega-3-fatty acids may live longer and have fewer episodes of severe illness attributed to their kidney disease.
Patients with advanced chronic kidney disease will often receive conservative palliative treatment. This typically involves support for adequate hydration with supplemental fluids and nutritional support with the use of palatable therapeutic diets and medications to help stimulate appetite and control nausea. Other cats might also be treated for secondary anemia; receive supplements for low potassium; or be treated with medications to help reduce blood phosphorus. Renal transplantation is available for aggressive treatment of feline kidney disease at selected centers, but the emotional, physical, and financial costs of this surgery can be high. Dialysis is also offered at selected veterinary centers, but is usually reserved for cats with acute renal injury while their kidneys recover from insult or in preparation for kidney transplant. In rare cases, surgical removal of a malfunctioning kidney may be recommended to eliminate a tumor or an abscess, for example, but only if the remaining organ is healthy enough to carry out the many essential kidney functions by itself.
While the range of treatment options for feline kidney disease has increased in the last decade, successful treatment depends on a strong partnership between care givers and veterinarians, and is aided by early diagnosis and diligent patient monitoring.