Bladder Stones in Dogs
Bladder stones are a collection of minerals and other materials. Most bladder stones in dogs are made from struvite, calcium oxalate, urate, or cystine crystals.
Oxalate Bladder Stones (Canine)
Oxalate Bladder Stones in Dogs
- 73% of calcium oxalate patients are male. This stone type is unusual in females.
- Breeds at especially high risk include: miniature schnauzers, Lhasa Apsos, Yorkshire terriers, miniature poodles, shih tzus, and Bichon frises.
- Most cases occur in dogs between ages 5 and 12 years of age.
How do we know these are Calcium Oxalate Stones?
Although a urinalysis can provide a clue, the only way to know for sure that a dog’s bladder stone is an oxalate stone is to retrieve a stone and have a laboratory analyze it. If the stones are very small, flushing the urinary bladder and forcefully expressing it may produce a stone sample for testing. The only other way to obtain a sample is to surgically open the bladder and remove the stones. The surgical method is invasive but provides the most rapid resolution of the bladder stone issue. Calcium oxalate stones cannot be made to dissolve over time by changing to a special diet, as can be done with struvite or uric acid bladder stones.
Why would my Dog Form Calcium Oxalate Stones?
It shouldn’t be too surprising that there is a strong hereditary component to the formation of oxalate bladder stones. This is also true in humans. There is a substance called nephrocalcin in urine that naturally inhibits the formation of calcium oxalate stones. This substance is defective in both humans and dogs who form calcium oxalate bladder stones. The production of defective nephrocalcin may be a genetic problem.
In humans, the genetic predisposition for stone formation is coupled with dietary issues (problem foods include: spinach, peanuts, chocolate, dairy products, calcium supplements, vitamin C supplements, and tea). Dogs are more likely to eat simply a commercial brand of dog food without nearly the dietary variety that humans experience. This means that diet can be used to manage the problem in dogs but unless the dog is receiving an unusual treat supply, diet is probably not a cause.
There are some metabolic diseases that might predispose a dog to creating a calcium oxalate stone. For example, a dog with Cushing's disease will over-produce cortisol (commonly known as cortisone). This hormone increases calcium excretion in urine. The extra calcium in the urine will promote formation of a stone. Some animals may have an elevated blood calcium level (hypercalcemia) for any number of reasons and this could lead to excess calcium in the urine as the body attempts to restore calcium balance. Again, extra calcium in the urine promotes a stone. Screening for such medical conditions is important and is generally part of the medical work up once stones are detected on a radiograph. If any such metabolic problems are uncovered, they will need to be treated separately and as these problems are resolved, no further stone prevention measures should be needed.
Why Can’t the Stones stay where they are?
The most immediate concern for a dog with bladder stones is that the urinary opening may get obstructed as the dog attempts to pass the stones; this is largely a problem with male dogs, but the results can be life-threatening uremic poisoning. In such cases, the veterinarian will try to dislodge the stone, flushing it back into the bladder to restore the patency of the urinary opening. If the stone cannot be dislodged, a new urinary opening may have to be surgically created. The urethra (the narrow tube connecting the urinary bladder to the outside world) is a difficult place to perform surgery so it is preferable to move the stone back into the bladder for removal rather than attempting removal from the urethra.
Bladder stones are irritating to the bladder simply by rubbing on the tender lining. Bleeding typically results and, of course, the chance of developing chronic bladder infections is markedly increased with bladder stones.
BECAUSE CALCIUM OXALATE STONES CANNOT BE DISSOLVED BY DIET CHANGE, SURGICAL REMOVAL OF THE STONES IS USUALLY NECESSARY.
STUDIES HAVE SHOWN THAT 50% OF DOGS WHO HAVE UNDERGONE SUCH SURGERY WILL DEVELOP NEW CALCIUM OXALATE STONES WITHIN 3 YEARS.
The Following Therapeutic Plan has been Devised to Minimize the Chance of Recurrence
Step One: Diet
While specific diets cannot dissolve existing stones, they do help prevent the development of new stones.
Prevention centers on creating a urinary environment with minimal calcium and minimal oxalate as well as creating a urine pH that is not conducive to calcium oxalate formation. Your veterinarian can recommend appropriate therapeutic diets.
For commercial therapeutic diets, the canned form is preferable to dry so as to increase water consumption and thus help dilute the urine. It is especially important to avoid table scraps when caring for an oxalate stone-forming dog. Still, no one wants their dog to live life without treats. The following treats are acceptable for oxalate stone forming-dogs:
- Plain, cooked chicken
- Plain, cooked turkey
- white potatoes
Many pet owners ask if any non-therapeutic diets are appropriate for this condition. None are, however, it may be possible to have a veterinary nutritionist provide a recipe for an appropriate home-cooked diet. Visit www.balanceit.com or www.petdiets.com for details.
Some medications can increase the risk of calcium oxalate stone development and these should be avoided. Prednisone and other cortisone-type medications (commonly used for itchy skin, arthritis, inflammatory bowel disease, and other inflammatory conditions) should be avoided if possible. Furosemide (brand name: Lasix or Salix) is a diuretic mostly used to treat heart failure. Both these medications lead to excess calcium in urine. Should a diuretic be needed for a patient at risk for calcium oxalate stones, one from the thiazide class (see below) would be a better choice. Supplementation with Vitamins D or C also increases risk of forming oxalate stones, so such supplementation should be discontinued.
Step Two: Urinalysis
Two to four weeks after surgery, a urine sample is checked. If the urine is not adequately dilute (meaning the specific gravity is less than 1.020), water consumption will have to be increased (either by adding water to the food or increasing the amount of canned food).
If urinary pH is less than 6.5 or if oxalate crystals are seen, it is time to go to Step Three. If all is well, a urinalysis should be performed every 2 months or so for the rest of the dog’s life (University of Minnesota’s current recommendation).
Radiographs are taken every 3 to 6 months so that any new stones will still be small enough to be retrieved from the bladder by flushing. If one skips this monitoring, one may find stones present have developed to a large size and surgery is again needed to remove them.
Step Three: Potassium Citrate
When the pet takes potassium citrate pills, citrate levels increase in the urine. Calcium binds to citrate instead of to oxalate. This is a desirable event since calcium citrate tends to stay dissolved whereas calcium oxalate tends to precipitate out as mineral deposit. Potassium citrate also helps create alkaline urine (in which calcium oxalate stones have difficulty forming). Potassium citrate supplements are typically given twice a day.
Step Four: Urinalysis Again
Two to four weeks after potassium citrate is added to the therapeutic diet, a urine sample is checked. If the urine is not adequately dilute, water consumption will have to be increased (either by adding water to the food or increasing the amount of canned food).
If urinary pH is less than 6.5 or if oxalate crystals are seen, it is time to go to Step Five. If all is well, a urinalysis should be performed every 2 months or so for the rest of the dog’s life (University of Minnesota’s current recommendation). Again, radiographs should be taken every 3 to 6 months.
Step Five: Thiazide Diuretics
A diuretic is a medication designed to increase the amount of urine produced. The thiazide class of diuretics does this in a way that reduces the calcium content of the urine. This medication is typically given twice a day and electrolytes are generally measured via blood test within the first two weeks of this treatment.
There will always be some dogs who continue to form stones no matter what is done. With radiography monitoring, it is hoped that new stones can be detected while they are still small. As research continues, it is possible that new developments will arise that will not require such frequent monitoring. Until then, these are the guidelines for oxalate prevention as proposed by the specialists at the University of Minnesota.
A Note on the Bichon Frise
The Bichon Frise appears to have a unique situation with oxalate bladder stones in that they seem to begin to recur virtually as soon as they have been removed. In a study presented by Jody Lulich's group at the University of Minnesota at the 2004 meeting of the American College of Veterinary Internal Medicine, 24 out of 33 Bichons had oxalate stone recurrence after cystotomy (37% had had their first recurrence in the first year after surgery, by the end of the second year 57% had recurred, and by the end of the third year 73% had recurred, some more than once.) Steps to prevent recurrence need to be particularly aggressive from the beginning in this breed.
Prevention of Male Dog Obstruction
In the male dog, a bone called the “os penis" is located in the penis, and it surrounds the urethra. Urine passes through this hollow bone like water through a pipe. Because it is bone, it cannot stretch or expand to accommodate a stone trying to pass. A stone blockage of this type leads to an emergency and death if it goes untreated.
In a perfect world, controlling the bladder stones controls the risk of obstructions, but controlling the bladder stones is not always easy and treating a dog over and over for obstruction becomes expensive. A surgical procedure called urethrostomy can be performed to prevent obstruction. This procedure involves creating a new urinary opening in the scrotum area. This allows urine to be expelled earlier in its course so that passage through the os penis does not occur. The flexibility of the non-bony part of the urethra plus the surgically enlarged urinary opening allows for smaller stones to pass rather than stick in the os penis.
In order to perform this surgery, the male dog must be neutered (which can be done at the time of the urethrostomy). The creation of the new opening usually requires removal of the empty scrotum.
While this is not as good an outcome as preventing stone recurrence, at least the emergency obstruction is prevented.
Calcium oxalate bladder stones can be very frustrating. Not only do they tend to recur but following monitoring recommendations involves a visit to the veterinarian's office every two months. How rigidly the monitoring schedule is followed depends on the owner, but keep in mind that the trouble and expense of a stone surgery weighs against the much less trouble and expense of monitoring.
A new technique of stone removal involves the use of a laser to blast the stone into pieces small enough to pass. This option is especially useful when the number of stones is small and/or when a stone is lodged in the urethra and cannot be flushed into the bladder.