Kidney showing numerous fluid filled cysts
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 (chronic kidney disease - CKD).
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 (ie, 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 (ie, 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.
A note of caution: In humans there are at least six different genes that can cause different forms of PKD. It appears that AD-PKD in Persians and related breeds is all caused by a single autosomal dominant gene defect. However, other forms of PKD caused by a different, unrelated gene mutation may exist (in Persians, or in completely unrelated breeds). If so, these other forms of PKD would not be detected by the AD-PKD gene test and diagnosis would have to be undertaken by ultrasound.
Why has PKD become so common?
PKD doesn't usually cause CKD (kidney failure) until quite late in life, so an affected cat may have been used to produce a large number of litters of kittens before it becomes ill itself.
Can PKD be cured?
Unfortunately there is no treatment that will prevent the development of chronic kidney disease in a cat that is affected by PKD. The cysts are present from birth and cannot be removed, nor can they be prevented from growing.
Once kidney failure has developed, supportive treatment can be used to minimise the impact of the kidney disease, but it will inevitably be a progressive disease.
Do all cats with PKD die of kidney failure?
The number of cysts present in each kidney, and the rate at which the cysts grow, varies considerably from cat to cat. Severely affected cats or cats with rapidly growing cysts will develop chronic kidney disease at an early age, and will die from PKD. Most affected cats will appear to be quite healthy until later in life but will eventually succumb to renal failure and die from PKD. Some cats with few cysts or slowly growing cysts may remain healthy into old age, and may die from other conditions before kidney failure develops.
Unfortunately there is currently no way to predict how quickly the condition will progress in an individual cat, and at what age renal failure will occur.
What can be done about PKD?
All cats that carry the abnormal gene are affected with AD-PKD, and affected cats should be identified before they reach breeding age. The availability of the gene test makes it relatively easy to eliminate the disease provided all cats are tested before being used in a breeding programme.
Finding an AD-PKD negative cat
If you are considering purchasing a pedigree cat of a breed known to be affected by AD-PKD, it is sensible to request to see evidence that both the queen and tom cat used to produce the litter of kittens have been tested for AD-PKD and are negative. If both were negative then the litter of kittens should all be negative. If in any doubt about whether cats have been tested, or the interpretation of a test result, just contact your vet.
International Cat Care have established a AD-PKD Negative Registerto provide a list of cats that have been accurately tested for AD-PKD using the gene test (or by having their kidneys scanned by an approved specialist ultrasonographer in the UK before the gene test became available).
|Click here for AD-PKD Negative Register|