Struvite is the common name for magnesium ammonium phosphate uroliths (stones) found in dogs. These stones may occur anywhere in the urinary tract including the kidneys, ureters, urinary bladder and urethra. Struvite stones often form when the dog has a urinary tract infection with bacteria that produce urease. This enzyme leads to over production of ammonia which in turn, increases urinary pH. Alkaline urine (high pH) causes struvite crystals to form. Since females are more prone to infection, they are also more prone to developing struvite stones. According to the Minnesota Urolith Center, magnesium ammonium phosphate uroliths may also be caused by chronic use of antacids due to the excess magnesium, chronic diuretic use, hypoxemia and dogs who cannot produce acidic urine. I rarely see these causes in practice. Most of my patients develop struvite stones due to urinary tract infections.
Because of the magnesium, this type of urinary stone shows up well on x-rays. Stone analysis done by the Minnesota Urolith Center provides the final diagnosis. If the patient does not pass any stones in the urine, surgical removal may be necessary to obtain a sample. The urine is also cultured to find out what bacteria is present and its antibiotic sensitivity.
Treatment of struvite stones focuses on curing the urinary tract infection as well as removing the stones. After antibiotic therapy, I always recommend another culture of the urine to make sure the infection is gone. Existing stones are removed through surgery or dissolving them with diet. Hill’s SD is designed to dissolve the stone by having lower levels phosphorous and magnesium. It also lowers the pH to a slightly acidic level. I like to check a urine pH after a week of the diet to make sure the pH is around 6.0. If it goes too low (acidic), the patient is in danger of developing other types of stones including calcium oxalate. When the stones are shrinking, it is possible for them to flow out of the kidney and become trapped in the ureter or leave the urinary bladder and become trapped in the urethra. Owners must watch the dog closely for any signs of discomfort, straining or lethargy. If observed, the dog must be brought in for veterinary care immediately. Urinary obstruction is an emergency condition. After the initial check, I take x-rays and check urine every month until the stones are gone. The length of treatment depends upon the size of the stones. In my experience, dogs with average size stones require about 10 weeks of therapy.
To prevent the recurrence of struvite stones, dogs must be monitored closely for urinary tract infections. I recommend a urinalysis with culture every three months the first year then every six months thereafter. As long as re-infection is prevented, the dog may return to their normal diet. The dissolution diet should only be used for a short period of time. Please note, this recommendation does not apply to cats or dogs who have ‘sterile‘ struvite uroliths which means that an infection is not present. Animals with this condition are placed on a maintenance diet, formulated to produce a slightly acidic urine with a pH around 6.0. These diets also produce a less concentrated urine which helps to wash crystals out before they become stones.
If the struvite stones recur, further diagnostics are needed to find out what is causing re-infection. This includes a thorough vaginal exam looking for anything that might cause urine pooling. Obesity and/or a deeply set vulva are two common causes. If this exam is normal, the next step is often an ultrasound of the abdomen followed by a contrast study of the bladder.
-Minnesota Urolith Center, Canine Struvite Urolithiais, University of Minnesota, http://www.cvm.umn.edu/depts/minnesotaurolithcenter/prod/groups/cvm/@pub/@cvm/@urolith/documents/asset/cvm_asset_143354.pdf.
-Shell, Linda, Urolithiasis, Magnesium Ammonium Phosphate, VIN Canine Associate, March 15, 2010.