Autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (AD-PKD) is an inherited condition (passed from parents to their kittens) that can cause progressive kidney failure in cats. The disease has become particularly common in Persian and Exotic Shorthaired cats.

In the future it may be possible to eliminate this potentially fatal disease by careful breeding from unaffected individuals.

Kidney: polycystic – pathology © Roger Wilkinson

Autosomal dominant polycystic kidney disease (known as AD-PKD) is an inherited condition that can cause severe kidney failure in cats. The disease causes small, fluid filled holes (cysts) to form in the kidney, and these gradually get larger as the cat gets older. As the cysts get bigger the kidneys are unable to work normally and kidney failure will eventually occur, but the time course of this is very variable.

The disease is caused by an abnormal gene. All cats with the abnormal gene will develop the disease but because the signs of AD-PKD usually do not develop until the cat is adult it is possible for a cat to breed extensively (and pass the disease on to its kittens) before the affected cat becomes ill. It is therefore essential to screen breeding cats of high risk breeds for the presence of the gene before they are used for breeding.

Cats with AD-PKD have progressive kidney disease that will ultimately lead to kidney failure. The disease cannot be treated but if your cat does develop renal failure, some treatments may help to improve its quality of life. Sadly the disease is ultimately fatal. The only way to prevent future cats suffering the same fate is to make sure that affected cats are not allowed to breed.

AD-PKD is an inherited disease passed from parents to offspring in the genes. The affected gene is an autosomal dominant gene, so it affects both males and females. Only one of the parents needs to have the disease for it to be passed onto some of the kittens and all cats that inherit even a single copy of the affected gene will have AD-PKD. Mildly affected queens can produce offspring with severe disease and vice versa.

AD-PKD is a very rare condition in breeds other than those that are, or are related to Persians and Exotic shorthairs. Persian cats throughout the world appear to have an especially high chance of having AD-PKD. Past figures identified that 1/3 of Persians cats are affected throughout the world. Other breeds, related to Persians, are also at high risk of the disease. The disease is common in Exotic shorthairs, with 3 in 10 testing positive for AD-PKD. Other breeds which may have imported the AD-PKD gene through previous outcrosses with Persian cats include British shorthairs, Himalayans, exotic shorthair breeds, Burmillas and possibly Maine Coons.

Many cats do not show any obvious signs of AD-PKD. Affected cats usually develop signs of kidney disease between 3-10 years of age, with most cats being around 7 years when they start to show signs. Signs may include increased thirst, increased frequency or volume of urination, reduced appetite, vomiting, constipation, weight loss and occasionally blood in the urine.

Cats can be screened for the presence of disease before they start to show signs of kidney failure. Breeding cats from the high-risk breeds should be screened for AD-PKD before they are used for breeding. If your cat belongs to one of the breeds at risk of AD-PKD then it may well have come with some sort of certification from the breeder and if both its parents are free of the disease then it will not have AD-PKD.

If your cat is in a high-risk breed group and its parents have not been tested then you can arrange for a gene test to be done. The test uses DNA extracted from a swab taken from inside the cat’s mouth or from a blood sample. Nursing kittens should have a blood sample performed. Cats over 10 months of age can also be screened for the disease by ultrasound scanning.

The gene test for AD-PKD involves collections of cells from the cat’s mouth, or from a blood sample. The sample is then sent to one of the accredited laboratories offering the gene test, including the Veterinary Genetics Laboratory, University College, Davis, California where the test was developed.

Ultrasound scanning can be used to identify the kidney cysts. If the cysts are large they can easily be identified by routine scanning (just like pregnancy testing in humans). This is useful in cats with advanced AD-PKD, eg those that have enlarged kidneys, or that have already developed kidney failure. A small patch of fur will need to be clipped so that the ultrasound can get good contact with the skin. Cysts may also be found in the liver.

Pre-breeding screening requires a more specialist approach as the cysts are likely to be very tiny and hard to identify. The scan must be done by a specialist ultrasonographer using a high definition machine. Cats must be 10 months old before they can be given a certificate to say that they do not have PKD, because the cysts may be too small to detect before this time.

To qualify for entry on the AD-PKD negative register the cat must be scanned by an approved ultrasonographer and must have an identification microchip which can be read at the time of scanning so that its identity can be checked.

For breeding cats the ultrasound test for AD-PKD must be done by a specialist ultrasonographer (so that you can be sure that the result is accurate).

If you are not planning to breed from your cat, your own veterinarian may be able to scan your cat and tell you whether or not your cat has large kidney cysts.

Will other tests be needed if my cat has AD-PKD?

Further testing may be needed to assess how well your cat’s kidneys are working. These may include blood and urine testing. Cats with AD-PKD should be monitored every 6-12 months ideally with ultrasound and urine testing to evaluate for disease progression.

Reputable breeders of Persian and Exotic shorthaired cats will have all their breeding cats tested for AD-PKD. If both parents are free of disease the offspring will all be unaffected. Occasionally a breeder may need to have a litter of kittens from an affected cat, and in this case it is predicted that a proportion of the kittens may be unaffected. The kittens can therefore be gene tested to identify which of them have the disease, and which have not.

Visit the Veterinary Genetics Laboratory (University College, Davis, California) website for further information at https://vgl.ucdavis.edu/test/pkd1-cat.