Calcium oxalate stones in dogs form when excess calcium and oxalate combine in urine that is too concentrated, too acidic, or both. Unlike struvite stones, which can often be dissolved with diet changes, calcium oxalate stones cannot be dissolved once formed and must be physically removed. Understanding what drives their formation is the key to preventing recurrence, which can reach rates as high as 90% within three years.
How These Stones Form
Calcium and oxalate are normal components of urine. Problems start when their concentrations rise high enough to exceed what the urine can hold in solution. At that point, tiny crystals begin to form, a process called nucleation. If conditions remain favorable, those crystals grow and clump together into stones that can lodge in the bladder or kidneys.
Several factors tip the balance toward stone formation: high levels of calcium being filtered through the kidneys, concentrated urine from inadequate water intake, and a lack of natural inhibitors that normally keep crystals from aggregating. When multiple factors overlap, the risk climbs sharply.
Breed and Genetic Risk
Genetics play a major role. Miniature schnauzers and bichon frises have more than 20 times the risk of developing calcium oxalate stones compared to mixed breed dogs. Other commonly affected breeds include shih tzus, lhasa apsos, pomeranians, miniature and toy poodles, dachshunds, Yorkshire terriers, Cairn terriers, and Jack Russell terriers.
Researchers at the University of Minnesota have identified two specific inherited disorders, called CaOx1 and CaOx2, that greatly increase stone risk. CaOx1 involves a mutation in a protein that is excreted in urine and normally helps prevent crystal formation. CaOx2 involves a mutation in a protein responsible for transporting electrolytes between urine and kidney cells. Both are autosomal recessive, meaning a dog needs to inherit the defective gene from both parents.
These mutations are rare across the general dog population but are common causes of calcium oxalate stones in English bulldogs. CaOx1 has also been identified in American Staffordshire terriers, border collies, Boston terriers, bullmastiffs, Havanese, rottweilers, and Staffordshire bull terriers. CaOx2 appears in American bulldogs, basset hounds, beagles, bullmastiffs, and English mastiffs. Genetic testing is available for both mutations.
Metabolic and Hormonal Causes
Several metabolic conditions raise urinary calcium levels enough to trigger stone formation. Primary hyperparathyroidism, where the parathyroid glands overproduce parathyroid hormone, causes persistently elevated blood calcium. That excess calcium gets filtered through the kidneys and into the urine, creating ideal conditions for stones.
Cushing’s disease (hyperadrenocorticism) is another significant contributor. The excess cortisol produced in this condition decreases calcium absorption from the intestines while simultaneously increasing how much calcium the kidneys dump into the urine. Research in Frontiers in Veterinary Science found that Cushing’s is associated with enhanced urinary excretion of calcium, which helps explain why dogs with this condition are predisposed to calcium-containing stones.
Vitamin D Processing Defects
A subset of stone-forming dogs appears to have a problem with how their bodies process vitamin D. Normally, after vitamin D is activated in the liver and kidneys, the body breaks it down through an enzyme called 24-hydroxylase when levels exceed what’s needed. In some dogs with calcium oxalate stones, this breakdown pathway is sluggish. A study in the Journal of Veterinary Internal Medicine found that about 32% of stone-forming dogs had abnormally low conversion of vitamin D to its less active form, and that this correlated with higher urinary calcium levels. Stone status was the strongest predictor of this impaired vitamin D processing. The result is more active vitamin D circulating in the body, which drives greater calcium absorption from food and ultimately more calcium in the urine.
Diet Factors That Increase Risk
This is where the science can feel counterintuitive. You might assume that feeding less calcium would reduce calcium oxalate stones, but the opposite appears to be true. A study published in the Journal of the American Veterinary Medical Association found that dry diets with the lowest concentrations of calcium, protein, phosphorus, magnesium, sodium, potassium, and chloride were all associated with increased risk of stone formation.
The likely explanation: dietary calcium binds oxalate in the gut before it can be absorbed. When dietary calcium is too low, more free oxalate gets absorbed into the bloodstream and ends up in the urine. Similarly, adequate sodium in the diet encourages water intake and urine dilution, which helps keep calcium and oxalate from reaching concentrations high enough to crystallize. Diets formulated with higher levels of these minerals may actually minimize stone formation rather than promote it.
Medications That Raise Risk
Certain commonly prescribed medications increase how much calcium the kidneys excrete. Loop diuretics like furosemide are known to boost urinary calcium. Systemic glucocorticoids (steroids like prednisone) have a similar effect, reducing intestinal calcium absorption while increasing renal calcium loss. Even topical steroids and thyroid hormone supplements have been flagged as potential contributors. If your dog is on any of these medications long-term and belongs to a high-risk breed, that combination deserves a conversation with your veterinarian about monitoring.
Urine Concentration and pH
Highly concentrated urine is one of the most modifiable risk factors. The less water in the urine, the closer calcium and oxalate get to each other, and the more likely they are to crystallize. The American College of Veterinary Internal Medicine recommends aiming for a urine specific gravity of 1.020 or lower in dogs at risk for stones. Achieving this usually means increasing water intake through wet food, water added to kibble, or encouraging drinking with flavored broths.
Urine pH also matters. Calcium oxalate crystals form more readily in acidic urine. The ideal urine pH for dogs is 7.0 to 7.5, which is slightly alkaline. Diets that acidify urine, including some therapeutic diets designed for other types of stones, can inadvertently increase calcium oxalate risk.
Why Recurrence Rates Are So High
Calcium oxalate stones have one of the highest recurrence rates of any stone type in dogs. Studies report recurrence as high as 90% within three years and approaching 100% within four years without preventive measures. This is because removing stones surgically or through other procedures does nothing to change the underlying metabolic conditions that caused them. The same supersaturated, acidic urine that built the first stone will build another unless the chemistry changes.
Prevention requires addressing as many contributing factors as possible simultaneously: diluting the urine through increased water intake, feeding a diet that provides adequate calcium, protein, and minerals rather than restricting them, maintaining a mildly alkaline urine pH, and managing any underlying endocrine conditions like Cushing’s disease or hyperparathyroidism. For breeds with known genetic mutations, testing can help identify at-risk dogs before their first stone ever forms.