Typically, renal failure comes about as the kidneys slowly
undergo aging changes and begin to “wear out.”
It is a process that develops over months to years. Initially,
there may be no apparent signs, and the cat’s bloodwork
is normal. However, there are irreversible microscopic changes
underway in the aging kidney. Eventually, the kidneys will
begin to shrink because of scar tissue and will become small
and hard. By this time, there are usually signs of progressive
kidney disease, and the labwork will indicate associated changes.
The kidneys are nothing more than filters
which selectively keep certain compounds in the blood, while
allowing unnecessary waste products to escape into the urine.
When aging causes the filtration process to become progressively
less effective, blood flow to the kidneys increases in an
attempt to improve filtration. This is the reason that the
cat with kidney failure is producing a large volume of urine.
Because of the loss of excessive fluid through the urine,
the cat is obligated to drink more water to avoid becoming
dehydrated. This is called a compensatory change.
Thus, the typical clinical signs of kidney failure include
increased water consumption (polydipsia) and increased urine
production (polyuria).
Various clinical studies have evaluated the prevalence of
renal failure in cats. For most cats, onset of clinical signs
begins anywhere from 7-12 years of age. However, the prevalence
of overt kidney failure is highest in cats older than fifteen
years of age. One study found that approximately 30% of cats
over 15 years of age had some degree of chronic renal insufficiency
or failure.
The frequency of renal failure in male cats is essentially
the same as for female cats.
One study indicated no apparent breed predispositions; however,
a very large study conducted by 23 veterinary colleges found
that renal failure was recognized more than twice as often
in the following breeds: Maine coon, Abyssinian, Siamese,
Russian Blue, and Burmese.
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Chronic renal failure in cats is technically called chronic
tubulointerstitial nephritis. This term essentially describes
a microscopic finding and is not specific for a particular
cause. In most cats, a specific cause for renal failure cannot
be determined. In many cats, kidney failure is the end result
of several diseases or insults to the kidneys over a prolonged
period of time.
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At the present time, there are three contributing factors identified
that may hasten progression of renal disease.
1. Hyperthyroidism (overactive thyroid glands).
This hormonal disease is relatively common in older cats.
Hyperthyroid cats are usually hypertensive (have high blood
pressure), and this results in increased blood flow through
the kidneys. As mentioned above, extra blood flow helps the
aging kidney maintain function. With treatment of hyperthyroidism,
blood pressure normalizes and the extra blood flow to the
kidneys is no longer present. This can cause a sudden decompensation
of kidneys with already marginal function.
2. Long term intake of an acidified diet. Diets that
are used to make urine pH more acid are very common. These
diets have been developed to aid in management of lower urinary
tract (bladder) problems in cats; this is also called “cystitis.”
While these diets are useful for managing certain bladder
problems more common to young adult cats, they are potentially
very harmful to the kidneys of older cats. These diets cause
chronic loss of potassium from the body. Chronic low potassium
has been identified as a significant factor in progression
of kidney disease in older cats. Even when blood levels of
potassium are normal, older cats may have low levels of total
body potassium. Because total body potassium cannot be measured,
the contribution of potassium depletion can be overlooked.
3. Urinary tract infection. Infection in
the kidneys and/or bladder may be present without any apparent
signs. To prevent bacteria from further damaging the kidneys,
cats with kidney failure should have the urine checked for
the presence of bacteria.
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As described above, the classic signs of kidney failure are increased
urine output and a compensatory increase in water intake (thirst).
Also, with more advanced kidney failure, other signs may include
loss of appetite, weight loss and a poor haircoat, depression,
vomiting, diarrhea, and very bad breath. Occasionally, ulcers
will be found in the mouth. A heart murmur may be present
when the anemia of renal failure develops.
High blood pressure is relatively common in cats with failing
kidneys. The sustained high blood pressure causes some cats
to detach the retinas in the back of the eye or to have strokes.
These situations result in sudden blindness and loss of equilibrium.
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The diagnosis of kidney failure is made by determining the
level of two waste products in the blood, blood urea nitrogen
(BUN) and blood creatinine, and urine specific gravity. Urine
specific gravity is obtained from a urine sample. It helps
assess the ability of the kidneys to “do work.”
The normal cat has very concentrated urine (high specific
gravity), whereas the cat with renal failure has dilute urine
(low specific gravity).
When the BUN and creatinine reach certain levels, they are
very specific indicators of renal failure. However, the problem
with these tests is that they do not become abnormal until
late in the disease. Over 75% of kidney function must be lost
before the test results are substantially elevated.
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Treatment is in two phases.
Phase 1 - Diuresis
In the first phase of treatment, large volumes of intravenous
fluids are given in an attempt to flush toxins from the body.
This flushing process, called diuresis, is designed to maximize
the function of all remaining kidney tissue. If enough functional
kidney cells remain, they may be able to adequately meet the
body’s needs for waste removal with the help of this
additional fluid. Also, the fluid therapy helps to replace
various electrolytes, especially potassium.
Other important aspects of initial treatment include proper
nutrition and drugs to control vomiting and diarrhea.
Our goal is that intensive fluid therapy will substantially
decrease the blood levels of BUN and creatinine (the serum
markers for kidney function). If there is improvement in the
blood tests after 3-4 days of fluid therapy, the prognosis
is good as long as continued treatment at home occurs. If
there is no improvement after 3-4 days of fluid therapy, the
prognosis is not good. Unfortunately, there is not a test
that will predict which cats will respond and which will not.
Phase 2 - Ongoing Medical Therapy at Home
The second phase of treatment is designed to continue supporting
the kidneys, realizing that if treatment is not continued
the kidneys will fail again very soon. This is accomplished
with one or more of the following, depending on the situation:
1. A kidney failure diet. This helps in three
ways. First, it helps to minimize excess protein waste products
that require the kidneys to work excessively. The result is
that the BUN will improve, and the cat will usually feel better.
These diets also have restricted amounts of phosphorus, and
they do not have products that create an acid pH of the urine.
There are several commercially available prescription diets
available. Please note that the available brands of cat food
which are labeled “For Urinary Health” are designed
to manage bladder problems and are harmful
to cats with kidney failure. These diets are usually found
in grocery stores and pet shops. You will need to purchase
the special kidney failure diet from the veterinarian.
2. Potassium supplementation. Potassium is
lost in the urine when urine production becomes excessive.
A potassium supplement will replace that loss. As mentioned
above, depletion of body potassium can worsen kidney function.
3. Fluids given at home. After your cat has completed
a course of intravenous fluid therapy in the hospital, fluid
can be given at home. The fluid is dripped under the skin,
or subcutaneously. This serves to continually “restart”
the kidneys as their function continues to decline. This is
done once daily to once weekly, depending on the severity
of kidney failure. Most owners easily master this technique
so don’t be afraid to consider this very helpful option.
4. A phosphate binder. As the filtering ability of
the kidneys declines, phosphorous begins to accumulate in
the blood. High serum phosphorous contributes to depression
and anorexia. Phosphate binders will attach to excess dietary
phosphorous in the intestine so that it is not absorbed with
the food. Blood levels of phosphorous can be monitored to
help tailor the drug dosage. These drugs are used when the
kidney failure diets are not able to control phosphorus levels.
5. A drug to regulate the parathyroid gland and calcium levels.
Calcium and phosphorus must remain at about a 2:1 ratio in
the blood. The increase in the blood phosphorus level, as
mentioned above, stimulates the parathyroid gland to increase
the blood calcium level by removing it from bones. This can
be helpful for the sake of the normalizing calcium:phosphorus
ratio, but it can make the bones brittle and easily broken.
Calcitriol can be used to reduce the function of the parathyroid
gland and to increase calcium absorption from the intestinal
tract.
6. A drug to stimulate the bone marrow to produce
new red blood cells. The kidneys produce erythropoietin,
a hormone that stimulates the bone marrow to make red blood
cells. Consequently, many cats in kidney failure have a low
red blood cell count (anemia). Epogen, a synthetic form of
erythropoietin, will correct the anemia in most cats. Unfortunately
for some cats, the drug cannot be used long term because the
immune system recognizes the drug as "foreign" and
will make antibodies (immune proteins) against it.
7. Drugs to stimulate appetite. Famotidine
is a drug that helps to neutralize excess stomach acid. This
is often helpful in improving appetite because excess stomach
acid is a cause of nausea that adversely affects the appetite.
Anabolic steroids (such as Winstrol) may also be helpful in
improving appetite in cats with kidney failure. Both drugs
may be given, but most cats need only one.
8. Drugs to control high blood pressure. About 65% of cats
that have kidney failure also have hypertension. Its control
is important in preventing blindness and strokes.
Kidney Transplants
This procedure is being done at a few locations in the country.
Generally, the cat must still be in good condition and not
ill from the kidney failure in order to be accepted for a
transplant. The cat cannot can have other diseases that might
complicate the transplant or be life-threatening.
Many transplant centers require that the owner adopt the cat
that has donated a kidney for the procedure. Some find this
undesirable; others find it very rewarding. Also, multiple
medications must be given daily for the duration of the cat’s
life; the antirejection drugs can be extremely expensive.
Repeated blood tests are required to monitor function of the
transplanted kidney and to monitor blood levels of the antirejection
drug. The cost for the transplant procedure, medicines, blood
monitoring, and follow up care can ultimately be thousands
of dollars.
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Prognosis can depend on response to the initial stage of treatment
and your ability to perform the follow-up care. However, we
encourage treatment in most situations because many cats will
respond and have the potential for good quality life for months
to years.
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For the most part, kidney failure is not a preventable disease.
It occurs as a consequence of aging. However, known causes
that can be prevented or managed include the following:
1. Urinary tract infections should be identified and treated.
2. Middle aged and older cats should not be fed acidified
diets or those labeled “For Urinary Health.” It
is usually safe to give these cats the diets formulated for
renal disease as a prophylactic measure, although it is not
clear if they may slow the onset or prevent development of
kidney disease.
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