Polycystic kidney disease

Polycystic kidney disease (PKD) is a disease in which a large number of fluid filled cysts form within the kidneys. These cysts are present from birth in affected cats but they start off very small. However, they gradually increase in size over time until eventually they compromise the surrounding normal kidney tissue and cause kidney failure.

In cats polycystic kidney disease is an inherited disease which is a particular problem in Persians, Exotic Shorthairs and related breeds. It is inherited as an autosomal dominant trait so its full name is autosomal dominant polycystic kidney disease (AD-PKD). The term ‘autosomal dominant’ means that there is a single pair of genes that determines whether PKD is present or not. The abnormal (mutated) gene that causes PKD is a ‘dominant’ gene, meaning that a cat only has to have one copy of the abnormal gene to be affected by the disease.

The presence of PKD can be demonstrated by an ultrasound scan of the kidneys in an older cat, but a genetic test to identify affected cats accurately at any age is now widely available and should be used in all breeds at risk. Breeders should avoid breeding from affected cats, and in this way it should be possible to eradicate the disease from the breed.

What is polycystic kidney disease?

Autosomal dominant polycystic kidney disease (AD-PKD) is an inherited condition that causes multiple cysts (pockets of fluid) to form in the kidneys. These cysts are present from birth but initially are tiny. Over time they grow progressively larger to the point where they severely disrupt the normal kidney tissue. When there is sufficient interference with normal kidney function, chronic kidney disease (CKD, or renal failure) will develop.

The cysts usually grow quite slowly, so most affected cats will not show any signs of kidney disease until they are middle-age or older, typically at around seven or eight years of age. However, in some cats kidney failure will occur at a much younger age and at the moment there is no way of predicting how rapidly the disease will progress in any particular cat.

How common is PKD in cats?

Unfortunately AD-PKD has become very common in some cat breeds. Persians and Exotic Shorthairs have the highest prevalence (frequency) of disease and studies around the world have shown that up to one in three cats from these breeds may be affected. However, the widespread adoption of pre-breeding testing by breeders does mean that the prevalence of the disease is dropping in many countries, although more still needs to be done.
Other cat breeds that have been developed using Persian bloodlines, and breeds that have allowed outcrossing to Persians (eg, British Shorthairs), may also have affected cats in their breeding stock. In other unrelated breeds though, it is an extremely rare condition.

How is PKD inherited?

AD-PKD is the result of a single, autosomal, dominant gene abnormality. This means that the disease is controlled by a single pair of genes and:
Every cat a copy of the abnormal (mutated) gene will have AD-PKD; there are no unaffected carriers of the gene.

Every cat with AD-PKD will have the abnormal gene, even if that cat only has a few small cysts in its kidneys.

A cat only needs one of its parents to be affected with AD-PKD in order to inherit the abnormal gene.

Every breeding cat with AD-PKD will pass the disease on to a proportion of its kittens, even if it is mated with an unaffected cat.

It appears that cats which inherit two copies of the abnormal gene (one from each parent, ie, being homozygous) will develop such severe disease that the affected kitten dies before birth and will be aborted or resorbed. All affected cats therefore carry one AD-PKD gene and one normal gene (i.e. they are heterozygous).

How can I find out if my cat is affected?

Testing for AD-PKD can be done in two ways:
A gene test is available which accurately identifies all cats with the abnormal gene. This test can be run on a blood sample, or a mouth swab.

This is a simple and very accurate test and can be performed on a cat of any age. International Cat Care believe that whenever genetic tests are run on cats for the selection of breeding stock, the gene test result should be linked to a method of permanently identifying the cat that has been tested (e.g., a standard, internationally recognised microchip number).

The disease can also be identified by ultrasound scanning of the kidneys. In advanced disease the cysts are large and the diagnosis is straightforward, but it can be difficult to identify the cysts in young cats (i.e. before breeding age) so for reliable pre-breeding diagnosis the scan must be undertaken by a specialist veterinary ultrasonographer, using a very high definition ultrasound probe, and the cat must be at least 10 months old. Unfortunately this limits the availability of this testing method.