[Translated by Jochgem van der Heijden, Ashamta cattery.]
By Dr. Ingrid Putcuyps, veterinarian. 2005.
What is Polycystic Kidney Disease Ė PKD?
PKD is a hereditary kidney disorder, which implicates that there are already cysts present at birth and usually in both kidneys. These cysts are cavities filled with fluids that originate from normal kidney tissue. In kittens these cavities are in the majority of cases very small (1 to 2 mm). As the animal matures these cavities will become larger (even larger than 2 cm) In one kidney there can be as many as 20 to 200 cysts present.
Breeds who carry PKD
The Persian is the most affected breed. Since this breed is and has been the most used breed for outcrossing, we are seeing PKD cases in other breeds as well. The breeds who have been outcrossed with Persians are: the Exotic Shorthair, the Selkirk Rex, the British Shorthair, The Scottish Fold, the Birman, the Ragdoll, the American Shorthair, the Devon Rex and the Maine Coon. In the past Persians were also used in the Norwegian Forest cat, the Sphynx, the Oriental Shorthair, the Cornish Rex, the Abyssinian, the Somali, the Manx and the Burmese, that is why we also see PKD in these breeds.
Symptoms of PKD
Whether a cat becomes ill of PKD or not depends on the size and number of cysts in both kidneys. A cat will display kidney failure (kidney insufficiency) when the cysts occupy too much room in the kidney, and normal kidney tissue is forced out. When there is too little normal kidney tissue left, the kidneys will not be able to function normally and the cat will fall ill. The first symptoms of disease usually occur between the age of 3 and 10 years, but sometimes it is seen at a much younger age.
Treatment of PKD
Until now there is no means available to prevent the development of PKD or to stop the growth of cysts. As a preventative measure the only option would be to remove PKD positive breeding animals from a breeding program. A treatment should only be considered when a cat displays symptoms of kidney failure. Dehydrated and/or vomiting animals should be put on IV for a couple of days. Once the cat is stable, a special kidney diet is the most important treatment. Such a prescription diet contains a lower percentage of protein and less phosphorus than normal cat food. In advanced patients the vet can decide to give additional medication like cardialgia inhibitors, calcium supplements and antibiotics when necessary. Motivated owners can administer hypodermic fluids by themselves at home.
Diagnosis of PKD
Recently the gene responsible for PKD in cats has been isolated by researchers in the US. A commercial test, which is said to be very reliable, is now available. Whether or not a cat is positive for PKD can be determined through a saliva or blood sample. At this moment the test is only developed for Persians and Exotics. A DNA test like this will not give any information about the size or number of cysts, and the DNA test is especially usuful for very young animals because they might have such small cysts that they can not be detected in the ultrasound, or for animals with a doubtful ultrasound testresult (for example an animal with only one cyst in one kidney).
To verify if there is already kidney insufficiency present in a cat, a blood test and urine test should be performed. Changes will only be seen when 2/3 of the normal kidney tissue is affected. A blood test will show us the levels of red blood cells (too low), ureum, creatine and phosphorus. The last three substances will be above normal levels if the kidney is not functioning normally. The urine test will show these concentrations (too low in badly functioning kidneys) and if there are no signs of urinary tract infection or protein loss through the kidneys.
Inheritance of PKD
A cat has 38 chromosomes, each paired to make 19 pairs. There are two of each chromosome present in a cat (one from each parent). These chromosomes contain genes. A gene is responsible for a certain characteristic like hair colour or a normal developed kidney and is also present in duplicate. At a certain moment something can go wrong with the gene, causing an abnormality within the body (e.g. the formation of cysts in the kidney). When this happens we are talking about a mutation. This mutation can be passed on to the kittens.
What can I do as a breeder?
First of all it is important to identify the PKD positive animals by means of ultrasound (or in the near future through DNA testing). Experienced vets may be able to detect cysts in kittens between 8 and 12 weeks. There is however no guarantee that the cat is PKD free when there are no cysts found at this age. It is still quite possible that the animal will develop PKD in the future. A final diagnosis can be done when the cat is 1 year or older. After the test the owner will be given a certificate which shows the catís name and pedigree registration number. In the future we should strive for identifying all our breeding cats by means of a microchip.