by Dr. Derek Duval, VMD
I received a number of letters requesting information about kidney disease in cats and how diet may be important in its cause and treatment. Many cats will develop renal (kidney) insufficiency as they age. With age the kidneys will loose nephrons (the functional unit of the kidney) and renal failure will result. The kidney (like the liver) has a large built in reserve capacity. By the time we see signs are renal insufficiency 60-75% of the renal mass is lost. The first signs are usually loss of the ability to concentrate the urine which results in increased drinking and urinating. With more damage the toxins that are normally filtered by the kidney begin to build up and can cause depression, decreased appetite, a foul odor to the breath, oral ulceration, and vomiting. In the end stage of renal failure, a low body temperature, seizures, and severe depression and coma can result.
Diagnosis: The most common clinical signs are vomiting, increased urination and increased thirst. If you see these signs have your veterinarian evaluate your cat. (Other disease share these same signs; such as diabetes, hyperthyrioidism ...) The veterinarian should do blood and urine tests. Blood tests that are important are the BUN, Creatinine and phosphorous. BUN or blood urea nitrogen is a chemical that the liver makes from ammonia. Ammonia is absorbed in the gut from the breakdown of dietary protein. After production in the liver, BUN is excreted by the kidneys. BUN increases in other disease states as well including dehydration and urethral obstruction. The creatinine is a breakdown product of muscle and is excreted by the kidney at a constant rate. Although we do not believe it causes problems when it is high, the creatinine serves as an important marker for kidney function. It is also effected less by dehydration and may help us determine if an elevated BUN is due to renal causes or not. In renal failure the phosphorous may increase and lead to mineralization of various sites in the body.
In the urine we look for signs of infection, ability to concentrate, and loss of protein. When cats with healthy kidneys become dehydrated the urine will become concentrated. Cats with renal insufficiency cannot concentrate their urine despite being dehydrated. Therefore, a urine sample is very important to the evaluation of renal disease.
The normal blood and urine values vary from lab to lab, but BUN <30 mg/dl, Crea <2.2 mg/dl, Phos <5 mEq/l, Urine specific gravity >1.018 are relatively normal. Obviously the interpretation of blood and urine values can be quite complicated (that's probably why then send us to school for so long) so seek veterinary advice.
Other modes of evaluating the kidneys include radiography (x-rays), ultrasound examination, and palpation. The kidneys in cats with chronic renal insufficiency are often shrunken and irregular.
Treatment: Treatment of renal failure has three main goals. The first is to keep the BUN and other "uremic" toxins low. Uremic toxins are chemicals and waste products normally excreted by the kidneys. BUN is one of these but it is important to realize it is not the only one. Uremic toxins are responsible for many of the signs of kidney disease. These are kept low with fluid therapy and dietary modification. When cats are ill, intravenous fluids are often required. For long term therapy, fluids can be given under the skin. I've had many owner give their cats fluids under the skin (Sub-Q) and help them manage their renal failure for years. The frequency and amount of fluids that a cat needs depends on the degree of damage to the kidneys. Some cats cannot be managed on sub-q fluids and cannot be managed off of intravenous fluids. I will discuss dietary manipulation separately in a moment. The second goal of renal failure treatment is to limit the phosphorous. Since these chemical can lead to mineralization and a worsening of renal failure it needs to be kept low. If the phosphorous is elevated I generally use drugs that bind the phosphorous in the intestine and do not allow it to be absorbed. Alternagel or other antacid products that bind phosphate are what I recommend. The third goal of therapy is to help prevent stomach ulcers that may occur secondary to the uremic toxins. Drugs that are useful include Tagamet (cimetadine), Zantac (ranitadine), carafate (sucralfate), and cytotec.
Diet is frequently discussed as a possible treatment and as a possible causes in renal failure in dogs and cats. It is true that protein in the diet is converted to BUN by the liver. In that sense minimizing protein may minimize BUN. Unfortunately the degree to which this is effective is probably nonsignificant. In normal dogs restricted protein does decrease the BUN from normal to low normal. In renal failure the BUN may decrease from 100 to 80, but it is still too high. In my opinion, if the BUN is high enough to require therapy, it needs more than dietary modification.
Do high protein diets causes renal failure? No. In dogs they have removed 7/8 of the renal mass and then placed them on diets of various protein level and quality. Dietary protein had no effect on the development of renal failure. In cats similar studies suggest that dietary protein level is not associated with renal failure.
Cats are not small dogs (despite what some of you may think) and have very different medical and physiological parameters. For instance, cats require a higher portion of calories to come from dietary protein. In dogs on low protein diets, they can use other sources (such as fat and carbohydrates) for energy, but because of the peculiarities of cats, cats cannot do this. So not only is protein restriction likely to be nonprotective, but will likely do more harm than good.
What can I do about the diet? Well, diets that are low in potassium can cause renal failure. There have been commercial diets in the past that had been poorly formulated and may have led to the development of renal failure. These diets have been removed from the market and/or reformulated to eliminate this problem. During developing renal insufficiency the phosphate content of the diet may be important. High phosphate levels may lead to worsening renal failure. Many diets today are acidified to help reduce the risk of FUS (or FLUTD as it is now known) this may cause increased loss of potassium in the urine and promote potassium depletion. Currently these problems (high phosphate and acidification) are being investigated in cats to see how important they are to the development of renal failure.
At this point I would not recommend any dietary modification prior to a diagnosis of renal failure or insufficiency. Dietary modification with a relatively low protein diet (20-25%) may help reduce clinical signs of uremic toxins at that point.
Another consequence of chronic renal failure is anemia. The kidneys are responsible for the production of the hormone erythropoetin. This hormone causes the bone marrow to produce new red blood cells. In cats with chronic renal failure this hormone may be deficient. The bone marrow production of red blood cells slows or stops. Since red blood cells are dying every day (normally to be replaced by new ones) an anemia results. The anemia is classified as nonregenerative, meaning that the body is not compensating by creating new cells. The anemia is slow to develop, and cats generally acclimate to the decrease. When the anemia is severe enough however, cats become clinical. Signs of anemia are pale gums, lethargy, depression, and decreased activity. Panting and increased heart rate may result as well. Fortunately, erythropoetin has been recently synthesized recombinately and it is available for use in cats. This medication called Epogen is given by injection multiple times a week and results in good regenerative responses in cats. Unfortunately, about 40% of cats given Epogen will eventually form antibodies against it and then fail to respond. Because of this possibility it is recommended that Epogen therapy be started only after the anemia is in a more severe stage.
The prognosis for cats with renal insufficiency depends on the degree of damage the kidneys have or more importantly, the degree of function they still have left. Fluid therapy and drug therapy can allow cats to leave relatively happy lives for months to a couple of years. Cats that can not be stabilized for home management carry a worse prognosis and euthanasia should be considered. Unfortunately, if a cat lives long enough it will likely have some degree of renal insufficiency. Regular veterinary exams and blood work in older cats may allow us to detect these patients early and keep them happy and comfortable for longer. Other disease that may make renal failure worse are hyperthyroidism, and hypertension.
Derek Duval, VMD Philadelphia, PA
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