Struvite Bladder Stones in Dogs

By Malcolm Weir, DVM, MSc, MPH; Tammy Hunter, DVM; Ernest Ward, DVM

What are struvite bladder stones?

Bladder stones (uroliths or cystic calculi) are rock-like formations of minerals that form in the urinary bladder and are more common than kidney stones in dogs. One of the more common uroliths in the dog is composed of magnesium ammonium phosphate hexahydrate. The more common name for this type of bladder stone is struvite bladder stone.

What causes struvite bladder stones?

Normal dog urine is slightly acidic and contains waste products from metabolism, including dissolved mineral salts and other compounds. Struvite is a normal component of dog's urine and will remain dissolved as long as the urine is acidic and is not too concentrated. If the urine becomes very concentrated, or if it becomes alkaline (opposite to acidic), struvite crystals will form.

In dogs, struvite bladder stones usually form as a complication of a bladder infection caused by bacteria that produce an enzyme known as urease. This enzyme breaks down the urea that is normally present in urine, causing an excess production of ammonia; this ammonia production then causes the urine to become alkaline. Ammonia in the urine also causes bladder inflammation. Under these environmental conditions, struvite crystals will form and collect around any cells or debris that may have formed in the bladder because of inflammation.

"In dogs, struvite bladder stones usually form as a complication of a bladder infection."

Female dogs get these types of bladder infections and stones much more frequently than males, probably because their shorter, wider urethra (the tube that transports the urine from the bladder to the outside of the body) makes it easier for bacteria to travel up into the bladder. In some studies, up to 85% of dogs with struvite bladder stones were female.

Small and toy breeds tend to be affected more frequently than large-breed dogs. Breeds reported to have a tendency for struvite stones include the cocker spaniel, miniature poodle, miniature schnauzer, shih tzu, Lhasa apso, Pekingese, dachshund, and bichon frise. Other studies have noted increased occurrence in the golden retriever, Labrador retriever, Bernese mountain dog, Saint Bernard, and rottweiler.

Other causes of alkaline urine can also predispose a dog to struvite bladder stones: certain kidney diseases, long-term use of diuretic drugs or antacids, and conditions that cause elevated urine pH, urinary phosphorus, or ammonia.

How common are struvite bladder stones?

Bladder stones are somewhat common in dogs, and struvite stones are the most common. In clinical studies, between 33% and 53% of all bladder stones were found to contain struvite. Based on the results of tens of thousands of stone analyses, it was found that the number of struvite bladder stones has been declining in dogs while the number of calcium oxalate stones has been increasing. Struvite uroliths were noted to be more common in female dogs, while calcium oxalate uroliths are more common in male dogs.

What are the signs of struvite bladder stones?

The signs of bladder stones are very similar to those of an uncomplicated bladder infection or cystitis. The most common signs that a dog has bladder stones are hematuria (blood in the urine) and dysuria (straining to urinate). Hematuria occurs because the stones rub against the bladder wall, irritating and damaging the tissue and causing bleeding. Dysuria may result from inflammation and swelling of the bladder walls or the urethra, from muscle spasms, or from a blockage urine flow. Dogs may demonstrate signs of discomfort by being less active, panting more, eating less or crying when urinating.

"A complete obstruction is potentially life threatening and requires immediate emergency treatment."

Large stones may act almost like a ball valve, causing an intermittent or partial obstruction at the neck of the bladder, the point where the bladder attaches to the urethra. Small stones may flow with the urine into the urethra where they can become lodged and cause an obstruction. If an obstruction occurs, the bladder cannot be emptied fully; if the obstruction is complete, the dog will be unable to urinate at all. If the obstruction is not relieved, the bladder may rupture. A complete obstruction is potentially life threatening and requires immediate emergency treatment.

How will my veterinarian diagnose struvite bladder stones?

If your dog is relaxed, and the bladder is not too painful, your veterinarian may be able to palpate (feel) struvite stones in the bladder. However, some stones are too small to be felt this way. Often, bladder stones are diagnosed by means of an X-ray (radiograph) of the bladder, or by means of an ultrasound. Struvite stones are almost always radiodense, meaning that they can be seen on a plain radiograph. However, bones or other overlying body parts will sometimes interfere with the ability to see bladder stones with regular X-rays, in which case, your veterinarian may recommend a bladder ultrasound or contrast study - a specialized technique that uses dye to outline the stones in the bladder.

These imaging procedures identify the presence of a bladder stone but do not definitively indicate its composition. The only way to be sure that a bladder stone is made of struvite is to have the stone analyzed at a veterinary laboratory.

In some cases, your veterinarian may make an educated guess about the type of stone that is present, based on the radiographic appearance and results of a urinalysis. For example, if X-rays show that there are one or more stones present in the bladder, and the results of the urinalysis show the presence of alkaline urine along with numerous struvite crystals, your veterinarian may make a presumptive diagnosis of struvite bladder stones and recommend treatment accordingly.

How are struvite bladder stones treated?

There are three primary treatment strategies for struvite bladder stones: 1) feeding a special diet to dissolve the stones, 2) non-surgical removal by urohydropropulsion, and 3) surgical removal.

Feeding a special diet

The use of special therapeutic diets to dissolve struvite bladder stones is often recommended in cases where the risk of a urinary tract obstruction is relatively low. These diets typically are restricted in protein, phosphorus, and magnesium, and are formulated to promote formation of acidic urine (with a pH less than 6.5).

Examples of these diets include Hills Prescription Diet® s/d® or c/d®, Royal Canin Urinary® SO™, Purina® Pro Plan® Veterinary UR Urinary™, and Rayne Clinical Nutrition Adult Health-RSS™. These formulations help dissolve struvite stones that are already present in the urine and prevent formation of further stones. Dissolution of the stones is further enhanced by increased water intake, which dilutes the urine.

"Dissolution of the stones is further enhanced by increased water intake, which dilutes the urine."

Since most dogs with struvite bladder stones developed them as a result of a bladder infection, your dog will also be placed on antibiotic therapy while the stones are being dissolved. This is important because, as the layers of stone are dissolved, bacteria that have become trapped in the layers of stone are released into the bladder. If left untreated, these bacteria can set up another infection. Some dogs may experience dissolution of struvite stones within two weeks while others may take up to 12 weeks. Your dog will need to have antibiotics during this entire period. If your dog is placed on dietary therapy to dissolve the bladder stones, your veterinarian will recommend that urinalysis and bladder X-rays be performed approximately every four to six weeks during treatment.

Some bladder stones can be mixed or composed of multiple layers of different types of minerals, which may complicate treatment. If follow-up X-rays show that the stones are no longer dissolving, this may indicate that the stones are mixed, and the treatment plan may need to be adjusted.

Non-surgical removal

If the bladder stones are very small, it may be possible to pass a special catheter into the bladder and flush the stones out in a technique called urohydropropulsion. In some cases, this procedure may be performed with the dog under heavy sedation, although general anesthesia is often necessary. Small stones in the bladder or urethra can sometimes be removed with a cystoscope, therefore avoiding the need for a surgical procedure to access the bladder. Either of these procedures may also be used to obtain a sample stone for analysis so that your veterinarian can determine if dietary dissolution is feasible.

Surgical removal

Surgery is indicated in dogs that have many stones in their bladder, if there is an increased risk that the patient will develop an obstruction in the urinary tract, or if the client wishes to have the problem resolved as quickly as possible. Male dogs are at a much higher risk of developing an obstruction in the urinary tract as a result of bladder stones, so when bladder stones are diagnosed in a male dog, your veterinarian will often strongly recommend surgical removal.

Surgery is also indicated if dietary treatment was not successful in eliminating the stones, or if it appears that the stones are composed of a mixture of mineral types. Your veterinarian will discuss the appropriate treatment strategy for your dog, based on your dog's individual situation.

Are there any other treatment options?

In some selected referral centers, another option may be available to treat bladder stones: ultrasonic dissolution, a technique in which high-frequency ultrasound waves are used to disrupt or break the stones into tiny particles that can then be flushed out of the bladder. It has the advantage of immediate removal of the offending stones without the need for surgery. Your veterinarian will discuss this treatment option with you if it is available in your area.

Will struvite bladder stones recur after treatment?

Dogs that have experienced struvite bladder stones will often be fed a therapeutic diet for life. Diets lower in protein, phosphorus, and magnesium, and that promote acidic urine, are recommended. The preventive diet is not the same as the diet that promotes dissolution of the stones. In certain cases, medications to acidify the urine may be required.

In addition, routine monitoring of the urine is also recommended to detect any signs of bacterial infection. Bladder X-rays and urinalysis will be performed one month after successful treatment whether dietary or surgical, and then every three to six months for life. Dogs displaying any clinical signs of urinary tract infections such as frequent urination, urinating in unusual places, painful urination, or the presence of blood in the urine should be evaluated immediately. Keep in mind that the greatest risk factor for developing struvite bladder stones in the dog is a urinary tract infection.

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