Table of Contents:
Bladder stones (or urolithiasis) are a common problem in dogs. They are cystic calculi that form and collect in the lower part of the urinary tract, mainly in the bladder or the urethra.
Bladder stones are more prevalent in dogs and cats than humans.The predominant theory concerning the high frequency of disease in dogs and cats is that their quadruped gait allows for the bladder to be more gravity dependent. This allows small stones and crystals to settle and not be urinated out as they are in humans who stand upright.
There is a genetic component to bladder stones in some breeds of dogs that predisposes them to stone formation, while some dogs will develop stones with no genetic component. Bladder stones form around a small nidus that starts the urolith formation and then needs a suitable environment in the bladder to continue the growth. The bladder stone also needs additional urine solutes to create layers of mineralized material, which eventually forms into a stone.
Bladder stones are characterized by the minerals that make up the highest percentage of the stone when analysed by a urolith center. Analyzing the type of stone and its composition can help to create a treatment plan to minimize recurrence. The five most common types of cystic calculi in dogs are:
- Calcium oxalate
The chemical name of struvite stones is magnesium ammonium phosphate hexahydrate (MgNH4PO4 6H2O). Struvite stones account for nearly 50% of the stones submitted for analysis from dogs. They form secondary to a urinary tract infection with urease-producing bacteria, of which Staphylococcus species is the most commonly responsible. Female dogs have a higher prevalence of urinary tract infections compared to male dogs, which makes struvite stones more common for their gender. A genetic component has been noted in English Cocker Spaniels.
Treatment for struvite stones includes:
- Medical dissolution
Medical dissolution can be attempted in some struvite stone cases, with the goal of treatment to address the underlying urinary tract infection with susceptible antibiotics. A urinary culture should be submitted and antibiotic choice is based on susceptibility, which is gathered from a panel of tests. If medical dissolution is elected, frequent follow-up urine cultures and radiographs to measure changes in stone size are recommended. In addition to antibiotics, dissolution diets will be prescribed by your veterinarian to decrease the magnesium/phosphate solutes in the urine, dilute the urine, and decrease urine pH to help dissolve the stones and prevent reoccurance.
Surgical removal is recommended in cases of urinary obstruction, lack of improvement with dissolution diet, or lack of ability to maintain a dissolution diet. The surgery to remove a bladder stone is called a cystotomy, which involves general anesthesia. An incision is made into the bladder to remove any stones and flush out any debris in the bladder. If stones are lodged in the urethra, they would be retropulsed back into the bladder or removed through a separate incision in the urethra (urethrotomy).
Lithotripsy involves breaking down stones into smaller pieces with a laser and cystoscopy (scope inserted into the urethra and bladder). This process allows for natural voiding and is performed under anesthesia. In cases with large stones or a large volume of stones, lithotripsy is not recommended due to prolonged anesthetic times.
Prevention for recurrence of struvite stones focuses on prevention of urinary tract infections. Dogs can have recurrent urinary tract infections due to congenital anatomical abnormalities that need to be surgically addressed to prevent recurrence of infection. Dogs with frequent urinary tract infections should have their urine evaluated frequently by a veterinarian to monitor for infections +/- stone formation. There are vet-prescribed bladder supplements that can be helpful for some dogs. Most dogs with struvite stones do not need to be on long-term prescription diets after the infection has been treated and the initial stones have been dissolved. In some cases, particularly those with recurrence, long-term stone diets may be recommended by a veterinarian.
Calcium Oxalate Stones
Calcium oxalate stones are the second most common type of stones in dogs and cats; they even accounted for 41% of stones submitted from dogs in 2007. Calcium oxalate stones are more prevalent in male dogs than female dogs with an overrepresentation in neutered males. Calcium oxalate stones form secondary to elevated calcium levels in the urine.
Predisposed breeds include:
- Miniature Schnauzer
- Lhasa Apso
- Yorkshire Terrier
- Bichon Frise
- Miniature Poodle
There is a hereditary link for calcium oxalate urolithiasis type I that has been identified in the following breeds:
- English Bulldog
- Boston Terrier
- Bull Mastiff
- American Staffordshire Terrier
Genetic testing can be submitted for these breeds to identify if there are carriers that need to be excluded from breeding populations.
Treatment of calcium oxalate stones is different than with struvite stones. Medical dissolution diets have not been successful for this type of stone. Surgical removal or lithotripsy is needed for calcium oxalate stones. If the ultimate cause for elevated calcium is identified and corrected, the likelihood of recurrence of stone formation is very low. If no obvious cause for stone formation is identified, long-term dietary management is needed.
Prevention once removed involves dietary changes and monitoring of urine pH to minimize the chances of recurrence. There are multiple prescription diets that have been formulated to minimize the risk of reforming calcium oxalate stones.
Generalized dietary recommendations for dogs prone to calcium oxalate stones include:
- Feeding a canned diet, since increased moisture helps to dilute urine
- Avoiding high-salt, high-protein, and phosphorus-restricting diets
- Avoiding diets that overly acidify urine
Further monitoring of urine pH every 3-6 months (to make sure that urine is not overly acidified) and radiographs or ultrasound every 6-12 months (to monitor for stone formation) may be done to ensure that stones aren’t reforming.
The normal metabolism of Uric acid in the body is through the liver cells, where an enzyme called uricase (uric acid oxidase) is converted to allantoin. Allantoin is highly soluble and is excreted normally in urine. Some dogs develop urate stones secondary to liver failure or abnormal blood vessels through the liver, as is seen with portosystemic shunts. Dalmations also form urate stones due their breed’s inability to properly transport uric acid, which causes a large amount of acid to accumulate in the urine and create stones. There is a genetic test available to look for hereditary hyperuricosuria, which has been identified in Dalmations, Labrador Retrievers, and English Bulldogs.
Treatment in Dalmations involves alkalinization of their urine, feeding a low-purine/low-protein diet, and addition of allopurinol to help dissolve stones and prevent further stone formation. Stones should be surgically removed if causing a urinary obstruction. Allopurinol should not be used in dogs with liver abnormalities and treatment plans for dogs with urate stones secondary to liver dysfunction should be tailored individually for each patient.
Prevention of urate stones involves dietary management as discussed above. In addition, urine pH should be checked every 3 – 6 months to monitor and ensure that it’s neutral (>7) and imaging (ultrasound) should be done every 6 – 12 months to monitor for stone formation.
Cystine stones (SCH2CHNH2COOH) are formed in dogs that have a renal tubular amino acid reabsorption defect, known as cystinuria. This causes a high proportion of cystine to be excreted into the urine, which subsequently causes urolith formation. Cystine stones are predominantly seen in male dogs and more commonly in dogs in Europe than in the U.S.
Cystinuria has a genetic component that has been linked with these breeds:
- Basset Hound
- English Bulldog
- Yorkshire Terrier
- Irish Terrier
- Mixed-breed dogs descended from the aforementioned breeds
Medical dissolution can be achieved by alkalinizing the urine and feeding a low-protein diet. Surgery may be needed if medical dissolution is not effective. Unfortunately, the recurrence rate of cystine stones is very high. There are some breeds of dogs that have an androgen component to cystine stones and neutering can be helpful in preventing recurrence. Breeds known to have this androgen component include:
- Staffordshire Bull Terrier
- French and English Bulldogs
- Scottish Deerhound
- Irish Terrier
Prevention for cystine stones involves feeding a low-protein diet and maintaining an alkaline urine pH (7 – 8). Monitoring urine pH every 3 – 6 months and stone formation every 6 – 12 months with ultrasound is ideal.
Silica stones are one of the least common types of bladder stones noted in dogs, and are predominantly seen in German Shepherds. There are no dissolution diet options for this type of stone and surgical removal is recommended. Diets that are high in plant protein should be avoided in dogs that have formed silica stones in the past. This includes diets that have silica, corn gluten feed, and soybean hulls.
At home prevention includes avoiding diets that contain inciting ingredients. In addition, monitoring urine pH every 3 – 6 months (with an ideal range between 7 – 8) and imaging every 6 – 12 months (to watch for stone formation) is recommended.
Work with your veterinarian to determine the type of bladder stones your pet has and create a treatment plan. Pets may need long-term to lifetime management to prevent recurrence of stones.