• 85% of patients with struvite bladder stones are female
   
• Breeds felt to have an increased risk for the formation of struvite stones are the beagle, miniature schnauzer, and English cocker spaniel.
   
• The average age of patients with struvite bladder stones is 2.9 years.

WHEN TO SUSPECT BLADDER STONES

Some patients with bladder stones show no symptoms of any kind and the stones are discovered incidentally but there are some symptoms that might promote a search for stones. Bloody urine, recurrent bladder infection (especially by the same organism), or straining to urinate all would raise suspicion.  Fortunately, struvite stones are “radio-opaque” which means they show up readily on radiographs.

Occasionally stones are simply passed.If this occurs, it is important to
have radiographs taken to check for the presence of more stones. If possible a
sample stone should be turned in for analysis to determine for certain the stone
type.

WHEN TO SUSPECT STRUVITE STONES

Bladder stones come in several mineral compositions.  The most common stone types are Oxalate and Struvite and since the approach is completely different for each type, it is crucial to determine the stone type.  The stone type can be absolutely confirmed if a sample stone is available (either passed naturally or obtained via surgery); a laboratory analysis can easily determine the content of the stone and even determine if the stone consists of layers of different mineral types.  Without a sample stone, there are some hints that can be obtained through other tests that might help.

Struvite stones in the dog are almost always formed because of the urinary changes that occur with specific types of bladder infection: almost always Staphyloccocal infection but occasionally a Proteus infection.  If a urine culture from a patient with a bladder stone should grow either Staph or Proteus, this would make struvite more likely than oxalate.  Also, struvite requires an alkaline pH to form while oxalate requires an acid pH to form; urine pH is a part of any urinalysis and thus provides another clue as to the stone identity.

It should be noted that an educated guess is better than nothing but does not replace the actual analysis of a stone.  (Remember, occasionally a stone of one type forms the center of a stone of another type. A complete analysis is needed to effectively prevent recurrence.)

HOW DO STRUVITE STONES FORM?

Struvite is the name given to the crystal composed of Magnesium, Ammonium, and Phosphate. (Struvite is also occasionally referred to as “Triple Phosphate” due to an old erroneous belief that the phosphate ion was bonded to 3 positive ions instead of just magnesium and ammonium.) Struvite crystals are not unusual in normal urine and are usually of no consequence but when the are present in very large amounts, they can form stones.

Urea is an important biochemical excreted in urine.  When urine is infected with bacteria that are able to digest urea, urea is broken down into ammonia (NH3).  Ammonia in water ionizes into ammonium (NH4+).  Ammonia is toxic to the cells of the bladder wall and its presence generates inflammation (though the infection present also generates inflammation as well).  The proteins released in the inflammatory reaction form a matrix which the struvite crystals use to form an actual stone.  The reaction takes place only in an alkaline pH but the presence of ammonia creates just the alkaline pH needed for stone formation.

Bacteria capable of digesting urea are called “urease positive” bacteria and in most cases we are talking about Staphylococci.  In the dog, the general rule is:  No infection, no bladder stone.

(The hereditary situation of the English cocker spaniel represents an exception as in at least one genetic line of this breed the tendency to form a purely metabolic struvite stone has been documented).

WHAT SHOULD BE DONE ABOUT STRUVITE BLADDER STONES?

Struvite stone can be removed surgically, removed with a special technique called “voiding urohydropropulsion,” or dissolved via diet.

Surgical removal is the most direct method of removal. The advantage is that the stones are removed and healing may commence all in one day.  The chief disadvantages are those inherent to surgery: anesthetic risks, post-operative pain, risk of contaminating the abdomen with infected urine, possibility that not all stones will be removed, possibility that the bladder stitches will not properly hold.  These risks are generally considered minor and complications associated with “cystotomy” (opening of the urinary bladder) are very unusual.

If the stones present are small enough to pass, the bladder can be manipulated in a way to promote expulsion of the stone through the urethra.   This is called “voiding urohydropropulsion” and involves filling the bladder, agitating the bladder so the stones float freely in the urine, and then generating a high pressure urine stream to force the stones out.  This technique only works if the stones are small and if there are numerous stones present, often several attempts are needed if this is to be the only means of removal.  Often this technique is used to obtain a sample stone for analysis to determine if dietary dissolution is feasible.

Dietary dissolution of the stone is possible with struvite bladder stones. A special food called S/D diet® is made by Hills for the specific purpose of dissolving struvite stones. The food is of a gel-like consistency and may not be palatable to the animal but if dietary dissolution is attempted, S/D must be the only food fed to the dog during the period of dissolution. Antibiotics are needed as long as stones are present in the bladder (bacteria are encrusted within the stone and as the stone dissolves, they are released).  On the average, 3 and a half months are needed to dissolve the stone but the diet should be continued for a full month after the stones are no longer visible on radiographs because small stones may be present but not large enough to see. Radiographs are taken monthly to monitor progress. S/D diet is not meant to be continued as a regular diet after the stone has been dissolved; Hills recommends not feeding S/D diet any longer than 6 months.  Aside from the long treatment time, an important disadvantage of this approach is the possibility of urinary tract obstruction as the stone gets smaller and an unsuccessful attempt to pass the stone occurs. This is potentially a life-threatening hazard for male dogs as they possess the narrow urethra.

S/D diet is very high in fat and high in salt.  It should not be fed to patients at risk for pancreatitis, patients with heart disease, kidney insufficiency, or high blood pressure.

RECURRENCE OF STONES?

After stones are removed one way or another, the focus shifts to prevention. Often patients are somehow predisposed to bladder infection which means they are also predisposed to form more struvite bladder stones. A stone can form as quickly as 2 weeks after infection with a urease positive bacterium sets in.

After surgery, antibiotics must be continued until the infection is confirmed to have cleared (i.e., a negative urine culture is obtained). After this, a follow up schedule of radiographs and/or urine testing is recommended. For a single stone episode, only a few follow-up visits may be necessary. One must realize though that some individual animals are predisposed to recurring bladder infections and these individuals may form new struvite stones repeatedly. Obviously, if stones were to recur, a more regular monitoring schedule would have to be revised.

Dietary therapy in the prevention of struvite stones is of secondary importance in dogs; the focus is on prevention of infection (the exception being the rare individuals who form struvite stones metabolically). If your dog has had a history of struvite bladder stones, be sure to discuss long term monitoring and understand what schedule of testing is best for your pet

Page last updated: 3/18/04