Polycystic Kidney Disease
Polycystic kidney disease (PKD) is a slowly progressive inherited kidney disorder that affects Persian and Persian-related breeds. It is the most prevalent inherited genetic disease in cats. The prevalence of PKD varies in several countries, ranging from 36% (Slovenia) to 49.2% (United Kingdom). In the United States, the prevalence of PKD in Persian and Persian-related cats is approximately 38%
Although most normal cats eventually develop signs of kidney disease as they age, cats with PKD experience the signs of chronic kidney disease (CKD) at an earlier age. Typically, cats begin to show the first signs of CKD around the age of 13 or 14. Cats with PKD, on average, will develop signs of CKD around 7 years of age.
The job of the kidney is to filter toxins from the bloodstream and put them into the urine. As cats age, normal kidney tissue is slowly replaced by scar tissue, and the kidneys gradually become less effective at filtering. As the scar tissue contracts, the kidneys become progressively smaller in size. The kidneys of cats that are affected by PKD also gradually lose their ability to filter, however, the kidneys of these cats become enlarged rather than become smaller. This is due to the existence of multiple cysts (hence the “polycystic” in the name) that are present in both kidneys at birth. The cysts grow slowly over time, causing the kidneys to enlarge. These cysts compress the kidney tissue surrounding them, reducing their function. The cysts can vary in size from less than 1 mm to greater than 1 cm. Because the cysts enlarge over time, older cats will have larger cysts. Cats with PKD occasionally will develop cysts in the liver as well, although this has no clinical consequence.
The signs of CKD in cats include increased thirst and urination, poor appetite, weight loss, vomiting, and lethargy. Blood and urine tests will confirm that a cat has chronic kidney disease, but they do not establish that PKD is the underlying cause. Late in the course of the disease, the detection of enlarged, irregular kidneys in a Persian or Persian-related cat is certainly suggestive that PKD is present, however, early in the course of disease the cysts are not detectable on physical examination. To make a diagnosis of PKD early in the course of a cat’s life, some sort of diagnostic imaging procedure will need to be employed, with ultrasound being the most sensitive and noninvasive technique for this.
Ultrasound has the potential to detect PKD very early in the course of disease, i.e. as early as 6 to 8 weeks of age. In some instances, cysts have been detected in kittens as young as 4 weeks old. In the hands of an experienced ultrasonographer, PKD can be diagnosed with approximately 75% sensitivity in cats less than 16 weeks of age. This increases to 91% sensitivity in affected cats at 36 weeks of age (because the cysts may be larger and easier to detect).
In Persian cats, PKD is inherited as an “autosomal dominant” trait. This means that if a cat with PKD is bred to a normal cat, 50% of the offspring will develop PKD. Because kidney disease resulting from PKD develops later in life, an affected cat may have been used to produce a large number of kittens before it becomes ill from PKD itself. Fortunately, PKD can now be diagnosed via genetic testing. The test requires submitting a cheek swab or a blood sample to an appropriate testing facility. When performing the genetic test, cats of any age can be tested. However, if testing kittens that have not yet been weaned, a blood sample is necessary, since nursing kittens will have traces of the mother cat’s DNA in their mouths, which may lead to inaccurate results if using a cheek swab. The genetic test accurately identifies all cats carrying the abnormal gene.
Early identification of PKD, whether through ultrasound or genetic testing, is helpful because it allows the detection of the disorder before cats are bred. If all affected cats are neutered and spayed once the disorder is detected, PKD could be eliminated completely from breeding populations. Anyone planning to purchase a Persian kitten from a breeder should ask the breeder to give proof that the cats used to produce the kitten were screened as negative.
There is no treatment for PKD per se. Treatment of the chronic kidney disease that results from PKD is similar to that of cats that develop CKD as a result of the natural aging process, or of any other cause of CKD. This may include subcutaneous fluid administration, potassium supplementation, phosphorus binders, anti-nausea medications, appetite stimulants, drugs to control elevated blood pressure, drugs to reduce excessive protein loss in the urine, and the feeding of prescription diets that have reduced levels of protein and phosphorus. Although there is no specific treatment for PKD, the presence of PKD can be reliably demonstrated by ultrasound in an older cat, or via genetic testing in any age cat. If breeders remove all affected cats from their breeding stock, it should be possible to eradicate the disease from the breed.