What are xanthine bladder stones?
Xanthine bladder stones are an uncommon type of urinary stone that can occur in both dogs and cats. Less than 1% of all urinary tract stones found in dogs are xanthine stones.
Why did my dog develop xanthine bladder stones?
Xanthine stones form in concentrated urine that has an abnormally high xanthine concentration. But how does a dog develop abnormal xanthine concentrations in the urine?
Xanthine is a substance that is produced in the breakdown of certain proteins. When xanthine is produced, there is a specific enzyme (xanthine oxidase) that breaks it down into other substances that can be readily eliminated from the body. If a dog does not have adequate levels of the xanthine oxidase enzyme, elevated levels of xanthine can build up in the urine. This leads to the formation of xanthine urinary tract stones.
Xanthine oxidase enzyme deficiency is a hereditary defect that can occur in dogs. Predisposed breeds include Toy Manchester Terriers, Cavalier King Charles Spaniels, English Cocker Spaniels, Dachshunds, and Chihuahuas.
"Dogs can also develop decreased xanthine oxidase activity when treated with allopurinol."
Dogs can also develop decreased xanthine oxidase activity when treated with allopurinol. This drug, which is used to treat urate urinary tract stones and an infection known as leishmaniasis, can lead to increased xanthine levels in the urine and the formation of xanthine urinary stones.
What are the signs of xanthine urinary tract stones?
Some dogs with xanthine urinary stones are completely asymptomatic.
When clinical signs do occur, however, they are similar to the signs associated with a urinary tract infection. Common signs include frequent urination, straining to urinate, visible blood in the urine, and urinary incontinence (dribbling urine). In some cases, xanthine stones may block the urethra and lead to signs of urinary obstruction.
"Some dogs may pass visible xanthine stones within the urine."
Some dogs may pass visible xanthine stones within the urine. These stones are mustard-yellow in color and often look like amorphous (shapeless) debris. The urine itself may also have a characteristic mustard-yellow color, due to large amounts of xanthine crystals dissolved within the urine.
How does my veterinarian diagnose xanthine urinary tract stones?
The first diagnostic test in a pet with urinary signs is a urinalysis. Microscopic examination of the urine sediment will likely show signs of inflammation, including red blood cells and white blood cells in the urine. If your veterinarian sees visible xanthine crystals under the microscope, this can confirm the diagnosis; however, crystals are not always visible on urinalysis. (Image via Wikimedia Commons / Joel Mills (CC BY-SA 3.0.)
"Ultrasound may be required in order to visualize xanthine stones within the bladder."
Your veterinarian may also perform radiographs (X-rays) to look for stones within the bladder. Unfortunately, while many types urinary stones show up clearly on radiographs, xanthine stones are composed of a material that is not visible on radiographs. Ultrasound may be required in order to visualize xanthine stones within the bladder.
If your veterinarian sees evidence of urinary tract stones, surgical removal of the stones may be recommended. In most cases, the identity of the stones cannot be confirmed until the stones are removed and sent to a laboratory for biochemical analysis.
What is the treatment for xanthine urinary tract stones?
Xanthine urinary tract stones cannot be dissolved with diet or medications. They must be removed physically. In most cases, this requires surgical removal of stones from within the bladder. In some cases, stones can be removed via cystoscopy (an endoscope inserted in the urethra) or broken down via laser lithotripsy, depending on the location of the stones.
Long-term management of pets with a history of xanthine urinary stones involves the use of a prescription diet. This diet is restricted in the proteins that act as xanthine precursors, in order to decrease xanthine formation. It is also formulated to help your pet produce dilute, alkaline (non-acidic) urine, which leads to a decreased likelihood of stone formation. Your veterinarian will recommend an appropriate diet for your pet. It is important to feed this diet exclusively, for the rest of your pet’s life. Without a prescription diet, most pets experience a recurrence of xanthine urinary stones within one year.
It is also beneficial to increase your pet’s water intake. Canned foods can help in the formation of dilute urine, but encouraging your pet to drink can play an even more significant role. Consider the following ideas to help make your pet’s water more appealing:
- add flavoring to your dog’s water, such as small amounts of chicken broth
- change your dog's water several times a day, keeping it cool and fresh
- clean your dog’s water bowl daily
Finally, any dog with a history of xanthine stones will require regular monitoring to prevent and detect recurrence. Your veterinarian will recommend an appropriate urinalysis monitoring schedule. Checking your dog’s urine regularly will ensure that its urine is remaining dilute and alkaline, while also allowing the early detection of a recurrence of xanthine crystals. Your veterinarian may also recommend regular bladder ultrasounds, as another way to allow the early detection of xanthine stones within the bladder.