Overview 

Cystinuria is an uncommon, inherited condition that causes an amino acid called cystine to build up in urine. Cystine can be excreted in urine and lead to the formation of bladder or kidney stones. Diagnosis and veterinary management of this condition in dogs can help avoid painful and potentially dangerous complications. 

Causes 

Cystine is a type of amino acid in the body that is normally reabsorbed by the kidneys. Cystinuria occurs when the kidneys are not able to properly reabsorb cystine, causing it to accumulate in the urine and form bladder or kidney stones.  

There are three types of cystinuria, two of which can occur in males and females, and one that is influenced by the presence of sex hormones common in intact males. 

While both males and females with cystinuria are equally affected from excess cystine in their urine, the obstruction of urine flow is more common in males due to differences in their anatomy.  

Clinical signs 

Dogs with cystinuria may not show any signs unless bladder or kidney stones form. The most common signs include: 

• Straining to urinate 

• Frequent, small amounts of urination 

• Accidents in the house  

• Blood in the urine 

The inability to urinate may also occur if stones develop and cause an obstruction in the urethra, which is a medical emergency. The signs for this condition will be much more severe — including a large and firm bladder, belly pain, vomiting and lethargy. If left untreated, a urinary obstruction can be fatal. 

Diagnosis 

A test called a urinalysis will be performed to look for the presence of cystine crystals, the pH of the urine and any coexisting issues, such as a urinary tract infection. Cystine crystals form in acidic urine (which has a lower pH).  

Another urine test called urine nitroprusside can screen for cystinuria. Specialized X-rays, ultrasounds or other urinary imaging tests may be used to diagnose cystine stones in the bladder or kidneys.  

Treatment 

The goal of treatment is to reduce the amount of cystine excreted in the dog’s urine, dissolve what cystine remains and avoid stone formation.  

Cystine can dissolve if the urine is made less acidic (by increasing its pH). This is achieved by feeding a prescription diet with reduced sodium and protein — particularly targeting an amino acid called methionine, which is one of the precursors involved in forming cystine stones.  

The urine should become diluted by feeding your dog a canned diet, adding water to their meals and encouraging them to drink more water. Medications may be needed if diet alone does not increase the urine pH enough to dissolve the cystine.  

After bladder stones develop, it is necessary to remove the stones and manage any secondary urinary tract infections or additional irritation. Removal often requires surgery. Neutering intact male dogs may be curative for certain types of androgen-dependent (sex hormone) cystinuria.   

Genetic testing is available for a few breeds known to be affected by cystinuria. And since cystinuria can be inherited, dogs suspected of having (or carrying) cystinuria should not be used for breeding without genetic testing and careful consideration of mate selection. 

Outcome 

Cystine stones commonly recur within 6-12 months if they are surgically removed and no changes are made in the diet. Even with dietary modifications, cystine stones can still recur at some point in a dog’s life, but the rate of stone formation tends to decrease with age.  

Your veterinarian will set a schedule for monitoring your dog for recurrence of clinical cystinuria by checking their urine or performing an X-ray or ultrasound to check for stones. This will allow your veterinarian to ensure that the dietary therapy is well-managed and help adjust your dog’s treatment if needed.  

Genetics 

Cystinuria is typically divided into three types. Type I occurs in Labrador Retrievers and Newfoundlands, each with a different variant of the same gene, called SLC3A1. These are both recessive variants, meaning that  two copies of the variant are needed for your dog to be considered at risk from the variant. 

Type II is caused when the inheritance is dominant, meaning only one copy of the variant is needed to cause the condition. Type II cystinuria can occur from a variant of two separate genes. In Australian Cattle Dogs, it is due to a variant in the SLC3A1 gene (Type II-A), and in Miniature Pinschers, it is due to a variant in the SLC7A9 gene (Type II-B). 

Type III is associated with the hormones of intact male dogs (making it androgen-dependent). A variety of breeds may be genetically at-risk, though the causative variants have not yet been discovered. Type III cystinuria in Mastiffs, English Bulldogs and French Bulldogs has a few linked markers that are recessively inherited genetic variants involving both the SLC3A1 and SLC7A9 genes.  

This health topic was developed as part of a collaboration between the Cornell Richard P. Riney Canine Health Center and Embark Veterinary, Inc. You can learn more about the hereditary risks of other canine health conditions by exploring our genetics articles.