Understanding Bladder Stones in Dogs: Symptoms, Risks, and Prevention

Bladder stones, or uroliths, are increasingly common in pets. Often the first signs of uroliths is straining to urinate and blood in the urine, or recurrent bladder infections. If a stone is small enough to leave the bladder but large enough to block the urethra, the most serious consequence is uremic poisoning from urine that is trapped in the bladder. Stones are irritating to the delicate wall of the bladder and cause inflammation as well as predisposing to infections. The most common stone types are struvite and calcium oxalate. Their mineral composition and the conditions under which they form are completely different, as are the approaches to their prevention. Often an animal will have crystals in its urine, but these crystals are not always of the same type as the stone(s), nor do they necessarily lead to formation of stones. So the only way to determine the type of stone is to analyze a sample that has been passed naturally or removed via surgery.

Struvites have a mineral matrix consisting of magnesium, ammonium and phosphate. Struvite crystals are often found in normal urine but become a problem when there is a concurrent infection with certain bacteria which can digest the by-products of normal protein metabolism and generate ammonia. (The cocker spaniel is an exception to this rule in that it can form stones in sterile urine.) Ammonia promotes formation of struvites in three ways. First, it is directly toxic and irritating to the bladder and creates inflammation with influx of inflammatory proteins which serve as organic building material for a struvite stone. Secondly, ammonia creates a high pH which is the necessary condition for struvite formation. Thirdly, the ammonium ion serves as a mineral building block of the stones. As the stone forms, bacteria are trapped inside and become active as they are released. Struvite stones can be removed in three ways. Urohydropropulsion involves flushing the bladder with sterile saline and forcing the stones out together with urine, and works for smaller stones which do not risk blocking the urethra. Most (85%) cases of struvite affect female dogs since they are more prone to bladder infections, but their urethra is also wide enough to pass a stone up to 3-4mm. Larger stones can be surgically removed by cutting into the bladder. Finally, struvites can be dissolved through feeding a special diet together with antibiotic treatment which is necessary to kill bacteria as they are released from the dissolving stone. Dissolution diets are high in fat and salt and not suitable for dogs with pancreatitis, heart disease, kidney disease or high blood pressure; nor should they be fed for more than 6 months. Prevention and timely treatment of urinary tract infections with appropriate antibiotics is key to preventing formation of struvites. If your dog is diagnosed with a bladder infection, your veterinarian will offer to perform a culture and sensitivity test on the urine. This shows which bacteria are causing the infection and which antibiotics are best to eliminate them, and is a step well worth taking to prevent recurrence of the infection.

Calcium oxalate stones form in acidic urine, and diet plays a large role in their formation. Excess calcium in the diet and supplementation with Vitamin D (which facilitates uptake of calcium) increase the amount of calcium which ends up in the urine; peanuts and peanut butter provide oxalate, another mineral building block of the stones; adding urinary aciFisdifiers creates an acidic pH which is necessary for these stones to form. It is not uncommon for calcium oxalate stones to form as a result of overly aggressive therapy aimed and preventing struvites by adding acidifiers to the diet. (Dissolution diets for struvites are already acidifying and require no supplementation. Vitamin C, often given to dogs, is not required to begin with: unlike humans and some other species, dogs synthesize their own.) Some breeds are predisposed to forming calcium oxalate: the miniature schnauzer, Lhasa apso, Yorkshire terrier, bichon fries, Shih Tzu, and miniature poodle. A substance called nephrocalcin is needed to prevent formation of calcium oxalate stones, and dogs of the above breeds are often deficient in this substance; the defect is likely genetic. Some medications increase the risk of calcium oxalate formation because they increase the amount of calcium in the urine. Medications that should be avoided in dogs prone to forming calcium oxalate are: prednisone and other corticosteroids (used to treat inflammatory and immune-mediated diseases); and furosemide, a diuretic mostly used in treatment of heart failure.

Unlike the case with struvites, most dogs that form calcium oxalate stones are males. However, some dogs are prone to forming stones of both types. If this is the case, the priority is usually to prevent the occurrence of calcium oxalate since this type of stone cannot be dissolved. The most simple approach to prevention of either type of stone is dilution. If an animal drinks enough water to keep their urine dilute, crystals have no opportunity to form stones. Clean water must be easily available and canned diets are preferred over dry kibble. The good news is that canned diets do not put the animal at greater risk for dental disease. If a dog is used to eating only or mostly kibble, it can be soaked in warm water prior to feeding.