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. These are most commonly found in the bladder and urethra but may be found in the kidneys and ureters also.
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 purines, which are found in animal proteins. There is a specific enzyme (xanthine oxidase) that then breaks it down further into uric acid, which is 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 blood and spill over into 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 a medication called Allopurinol. This medication, which is used to treat another type of urinary tract stone called urate, and an infection called 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 do not show any symptoms. When clinical signs do occur they are similar to the signs associated with urinary tract infections. 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, which is a life-threatening condition. Straining without passing any urine, abdominal pain, distended abdomen, lethargy, and vomiting are among the signs of a urinary tract obstruction.
Some dogs may pass visible xanthine stones in 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 for 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.
Your veterinarian may also perform X-rays to look for stones within the bladder. Unfortunately, while many types of urinary stones show up clearly on an X-ray, xanthine stones are composed of a material that is not visible. Ultrasound may be required 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 analysis.
What is the treatment for xanthine urinary tract stones?
Most xanthine urinary tract stones cannot be dissolved with diet or medication. They must be removed physically. However, xanthine stones that occur secondary to Allopurinol treatment may dissolve with discontinuation of the medication and a switch to a low purine food.
In most cases, stones must be surgically removed 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. If the stones are very small, they may be removed by voiding urohydropropulsion.
Long-term management of pets with a history of xanthine urinary stones involves the use of a prescription diet. These diets are restricted in the purines that act as xanthine precursors, in order to decrease xanthine formation. They are also formulated to help your pet produce dilute, alkaline (non-acidic) urine, which leads to a decreased likelihood of stone formation.
"Without a prescription diet, most pets experience
a recurrence of xanthine urinary stones within one year."
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. Examples of these prescription diets include Hills u/d™, Royal Canin UC™, Royal Canin Vegetarian™, and some lower protein diets formulated for renal failure. Without a prescription diet, most pets experience a recurrence of xanthine urinary stones within one year. Medications to increase the alkalinity of the urine may also be needed if diet alone is not sufficient.
It is also very important to increase your pet’s water intake, so they form a diluted urine. This will help flush the urinary tract. Feeding canned food and adding water directly to the kibble can be helpful, 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 low sodium 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 remains 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.