Cystinuria is a urinary-tract disorder involving an entirely different type of stone.
Tue, 11/02/2021 - 4:54pm

Learning About Cystinuria

Everything you need to know about this urinary-tract disorder

What is cystinuria?

 

Cystinuria is an inherited genetic disorder that prevents the dog’s ability to effectively filter cystine (an amino acid) out of urine. Normally, the tubules in the kidney are responsible for reabsorption of cystine from the urine. Dogs with cystinuria have tubules that are unable to transport the cystine, which allows it to accumulate in the urine. When too much cystine builds up, it crystallizes and forms calculi, also known as stones.

These tiny irregular stones can get stuck anywhere in the urinary tract, including the kidneys, bladder and ureters. The resulting irritation and potential obstruction can be painful and even life-threatening if the dog is unable to urinate. Not every dog that has the mutation that causes cystinuria will exhibit symptoms. Obstruction from cystine stones is more common in males due to their long, narrow urethra. Females rarely exhibit signs of urinary discomfort and may be completely asymptomatic.

 

Which breeds are affected by cystinuria?

 

Canine cystinuria affects more than 60 breeds of dogs. The most commonly affected are Newfoundlands, Labrador Retrievers, Dachshunds, Basset Hounds, Irish Terriers, English Bulldogs and Miniature Pinschers. Crystal formation may not occur until four to five years of age; however, Newfoundlands can start showing symptoms as early as six months old.

This is a recessive disorder, so the dog must carry two copies of the defective gene to suffer from the disease. Because of this recessive inheritance, a dog can also be a carrier of this disease, meaning it carries one copy of the mutation but does not display any symptoms. If two carriers are bred together, each puppy has a 25% chance of developing cystinuria.

 

What are the signs of cystinuria?

 

Elevated levels of cystine in the urine and cystine crystals in the urine do not cause clinical signs. When cystine stones form, they irritate the lining of the urinary tract, resulting in frequent, painful urination, urinary accidents, straining while urinating, bloody urine and recurring urinary tract infections. Over time, the stones accumulate and block the urinary tract. Dogs that are unable to urinate need immediate medical attention. The pressure from the obstruction can result in rupture of the bladder and cause permanent, potentially fatal damage to the kidneys and bladder.

 

How is cystinuria diagnosed?

 

Cystine stones are small, round, greenish-colored stones that are often found in the dog’s urine when the urine is analyzed. Unlike other bladder stones, or “uroliths,” these stones can be “radiolucent,” meaning they are not always visible on X-rays. Other imaging studies may be needed, such as an ultrasound of the bladder or a radiographic contrast study that uses dye to outline stones in the bladder. Stones should be analyzed at a veterinary diagnostic laboratory to determine the specific composition.

Genetic tests are available to identify carriers and affected dogs. Cystinuria in some dogs may be androgen (testosterone) dependent. A genetic marker test for androgen-dependent cystinuria has been developed for Mastiffs, Bulldogs and French Bulldogs. Surgical or medical castration will resolve cystinuria in male dogs with the androgen-dependent form of cystinuria. It also excludes these dogs from breeding programs so they do not pass on this genetic disease.

There are other breeds with androgen-dependent cystinuria, as well as other breeds in which castration will not reduce cystinuria. These are classified as TypeIa, reported in Newfoundlands and Labrador Retrievers; Type IIa, reported in Australian Cattle Dogs and Border Collies, and Type IIb, reported in Miniature Pinschers.

The urine nitroprusside test is an effective screening test for cystinuria. Some dogs with cystinuria will be negative on this test even though they have the disease. If the nitroprusside test is only evaluated after castration, it is difficult to determine if castration reduced the cystine excretion. The results of this test are also affected by urine concentration and kidney function. Certain medications, such as ampicillin and sulfur-containing drugs, have been reported to cause false-positive results. This test may also be affected by ascorbic acid (vitamin C), which dogs are able to synthesize within their body.

 

What is the treatment for cystinuria?

 

If large numbers of stones are present, surgical removal is recommended. At some referral centers, ultrasonic dissolution may be available as a treatment option. This is a technique in which high-frequency ultrasound waves are used to break the stones into tiny particles that can then be flushed out of the bladder. Cystine uroliths have a high incidence of recurrence.

Dogs that have developed cystine bladder stones should be fed a special diet for life. Therapeutic diets, such as Hills Prescription Diet u/d, which cause the urine to have a neutral to alkaline pH (7.0-8.0), have been shown to reduce urine cystine by 25%. Canned formulations are often more effective because increased water consumption results in decreased crystals in the urine.

Diets rich in methionine, which is a precursor of cysteine and a common amino acid in animal protein sources, may contribute to cystine build-up. Lower protein foods with reduced quantities of animal protein would be preferable. Nutritional studies suggest high-sodium diets encourage cystinuria and should be avoided.

If diet alone is not effective in controlling cystinuria, your veterinarian may recommend adding urine alkalinizers to maintain a urine pH of 7.5 or above. Thiola is the brand name for the drug tiopronin. This medication binds with cysteine molecules to form a complex that is more easily dissolved in urine than cystine. Thiola is available in tablet form from compounding pharmacies.

Cystinuria can be difficult to resolve. Dogs that were able to maintain a low concentration of cystine in the urine did not reform stones. Also, on a positive note, dogs appear to have a decrease in cystine urolith formation as they age.

Additional information regarding canine urolithiasis can be found at my alma mater: vetmed.umn.edu/centers-programs/minnesota-urolith-center/recommendations

Resources for cystinuria testing (urine nitroprusside/genetic testing/metabolic screening): PennGen Laboratories, http://research.vet.upenn.edu/penngen

 

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