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Cystinuria: An Overview

Cystinuria is a genetic defect in the tubules of the kidneys. In normal function the kidney tubules will reabsorb the amino acid cystine so it is not excreted in the urine. The genetic defect associated with cystinuria does not allow for this reabsorption to take place and cystine is excreted in the urine. Due to its structure cystine is insoluble in neutral or acidic substances (pH < 7.0) Urine is slightly acidic under normal conditions which makes it likely that the cystine will precipitate, or crystallize, in the urine. This crystallization can cause particles, which are sand like in nature, to form. These crystals can then stick together and form stones. The stones can be life threatening if they become lodged in the urinary tract and block it. Not all dogs with cystinuria will form stones, but there is always a chance of it occurring. Dogs that have cystinuria are born with it but may not show any signs for many years. Except for in Newfoundlands, the condition is primarily found in males.

Some breeds which are known to be affected by cystinuria are Newfoundlands, Dachshunds, Basset Hounds, English Bulldogs, Chihuahuas, Scottish Deerhounds, Labrador Retrievers, and Mastiffs. Genetic tests to determine carriers are available for Newfoundlands and Labrador Retrievers.

Causes of Cystinuria

Cystinuria is inherited, meaning it is passed down from generation to generation. There is currently a genetic test available for Newfoundlands and Labrador Retrievers which can be used to help breeders make informed decisions about their breeding practices. Unfortunately there are many other breeds for which this test is still not available.

Prevention of Cystinuria

For those breeds that do not have a genetic test available, prevention of cystinuria is difficult. All dogs with cystine in their urine are affected, though they may not be symptomatic (form stones). Unfortunately, there are no dietary recommendations to help prevent stone formation.

Symptoms of Cystinuria

You will not be able to tell if your dog has cystinuria just by looking at it. Many times the first sign pointing to cystinuria is a urinary tract blockage caused by a bladder stone. The analysis of a bladder stone in a cystinuria dog will be composed almost entirely of the amino acid cystine. Symptoms of a blocked urinary tract include:

  • frequent urination
  • small amount of urine when going
  • blood in urine
  • urine flow interrupted
If your dog exhibits any of these signs contact your local veterinarian immediately as a blocked urinary tract can be fatal with no treatment. In some cases reoccurring urinary tract disorders may be a sign of cystinuria. And in all cases dogs with cystinuria will have cystine in their urine which can be seen in a urinalysis.

Diagnosing Cystinuria

There are 3 tests which are available to diagnose cystinuria. All the tests are based on the urine composition. The first is a basic urinalysis which can be performed at any veterinary clinic and is the cheapest of the three tests. The second is a nitroprusside test, performed at the University of Pennsylvania. Urine must be sent to the university which adds some cost to the test. The third is a urine amino acid quantitation test which can only be performed in a human laboratory and is the most expensive of the tests. Will all the tests it is important to note that a test negative for cystine does not mean that your dog does not have cystinuria. Three to four tests may be done before a dog will present with cystine in their urine. This is a rare condition and therefore may not always be a quick diagnosis.

Treating Cystinuria

There is no treatment fore the genetic defect in the kidney tubules which causes cystinuria. There is however treatment to try and reduce the incidence of stones forming in the bladder along with treatment for bladder stones. A diet very low in protein can be fed to lower the amount of cystine being excreted in the urine. A buffer such as sodium bicarbonate can also be fed to raise the pH of the urine which helps keep stones from forming. The down side is that a diet low in protein can be unhealthy for large dogs and the diet can cause other stones to occur. There are 2 medications which can dissolve stones, Thiola and Penicillamine. Thiola has few side effects but is very expensive and hard to obtain. Penicillamine is easier to get and cheaper but has been known to cause sever side effects. If your dog has bladder stones treatment is necessary. If a stone has blocked the urinary tract a retrograde hydropulsion is performed to unblock the tract by pushing the stone back up into the bladder. If stones are found in the bladder, a cystotomy is performed to remove the stones.

Care for Dogs with Cystinuria

Dogs with cystinuria will not be affected in any way other than having cystine in their urine and can live normal happy lives. Although cystic stone formation is rare and few dogs with cystinuria form them, owners should always be aware that the stones can appear at any time. They should be on the look out for signs which point to stones being present (see symptoms) as these can be life threatening without treatment. Following the recommendations of your veterinarian on diet and medication will allow for your pet to have the best life they can while living with cystinuria.

The AKC Canine Health Foundation and Cystinuria

The AKC Canine Health Foundation has been funding research on cystinuria since 1996. The research funded has resulted in the genetic tests for cystinuria in Newfoundlands and Labrador Retrievers. With your help continued research will hopefully allow more tests to be developed for other breeds.

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