Canine uroliths (Proceedings) - Veterinary Healthcare
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Canine uroliths (Proceedings)


CVC IN SAN DIEGO PROCEEDINGS


Diagnosis of urolithiasis may be aided by historical information such as the previous occurrence of uroliths or owner observation of symptoms compatible with the presence of uroliths. Predisposition to urolithiasis may be indicated by case signalment or pre-existing medical conditions favoring urolith formation. Laboratory data base information (CBC, biochemistry profile, urinalysis) does not indicate specifically the presence of a urolith but provides valuable information in regard to predisposing or complicating medical conditions. In addition, urinalysis findings such as pH, evidence of bacterial infection, and the presence of specific crystal types can also aid in determining the most likely composition of uroliths detected. Urinalysis is best performed within 30 minutes of sample collection. Refrigeration will help preserve the urine sample but can alter both the chemical and sediment findings. In particular, crystal formation can be enhanced by temperature and pH changes which may occur with time and refrigeration, causing misinterpretation of the urinalysis results. Physical examination of the patient may give an indication of the presence of uroliths if a firm object is palpated in the bladder or if very small uroliths are observed in voided urine or mucoid discharge from the penis or vulva. Rectal examination may reveal pelvic urethroliths. In most cases definitive diagnosis requires imaging procedures such as survey abdominal and/or urethral radiography, ultrasonography or contrast radiography. The entire length of the urinary tract should be surveyed to make certain that uroliths are not present in muliple locations and to identify any factors predisposing the patient to urolith formation. Ultrasonography is a good initial imaging tool to detect uroliths; however, ultrasonography does not provide information concerning the radiodensity of uroliths or the shape of uroliths. In addition, unless the size of the patient allows the use of a rectal transducer, the pelvic urethra cannot be imaged with ultrasonography. Survey radiographs can detect radiodensity of uroliths and give some information about the size and contour of uroliths. Survey radiographs are also generally the most cost effective and available noninvasive procedure for initial evaluation of uroliths. Although high-frequency ultrasonoraphy compared favorably to double contrast cystography in the detection of urocystoliths and the determination of urolith numbers in one study, double contrast cystography has the additional benefit of being a sensitive and specific indicator of urolith shape and size. Determining exact numbers of uroliths with any of the imaging modalities can be challenging in some cases.

Following the diagnosis of urolithiasis, therapy and prevention depend upon factors such as urolith composition, associated etiology, and location of the urolith in the urinary tract.

Struvite

The struvite (magnesium ammonium phosphate hexahydrate) urolith is one of the most common uroliths occurring in the dog. Although canine uroliths composed of struvite have been the most prevalent category of stone composition for a total time period spanning 25 years (1981 – 2005, 43.5%) according to one study20, a steady decline in occurrence of canine struvite uroliths during that same time period has resulted in struvite becoming the second most prevalent urolith composition in the dog beginning in 2004 (39%). The shift in urolith prevalence in the dog has been theorized to be the result of commercial maintenance diet reformulation as well as the increased use of therapeutic diets designed to dissolve or prevent uroliths.

Struvite uroliths are frequently spherical or pyramidal, radiodense, and vary greatly in size. Struvite crystals precipitate out more readily in alkaline urine and have an angular, prism-like appearance.

Unlike the cat, the majority of canine struvite uroliths have an infectious etiology although sterile sruvite uroliths can occur. Canine Infection-related uroliths occur more commonly in female patients between 2 and 9 years-of-age. Struvite urolith formation has a breed predisposition for the miniature schnauzer, shih tzu, bichon frise, miniature poodle, cocker spaniel, and lhasa apso.

Infection-related struvite uroliths occur as the result of underlying urinary tract infection. The bacteria involved are most commonly urease-producing Staphylococcus although urease-producing Proteus and, rarely, ureaplasma organisms may also be involved. Occasionally other bacteria are cultured from affected patients. The breakdown of urea by the bacteria causes release of ammonia into the urine, increasing the pH and the availability of ammonium and phosphate ions, all of which predispose the patient to struvite urolith formation.

Feeding a calculolytic diet such as Hill's canine s/d® can dissolve both sterile and infection-related struvite urocystoliths and nephroliths, by creating an acidic, dilute urine undersaturated with magnesium, ammonium, and phosphorus. The time for urolith dissolution is variable, but averages about 2 - 3 months for infection-related uroliths and 3 – 6 weeks for sterile uroliths.23,24,25 Dissolution will occur more rapidly with infection-related struvite uroliths when an appropriate antibiotic is administered concurrently. Antibiotic therapy is best determined by culture and sensitivity testing of urine obtained by cystocentesis. Ideally, urine culture should be performed prior to the initiation of antibiotic administration and periodically during the dissolution process. By evaluating serial urinalyses and abdominal radiographs or ultrasound at approximately 4-week intervals, dissolution therapy can be altered as needed for individual patients. Urinalysis findings compatible with ongoing infection (bacteriuria, pyuria, alkaluria) indicate the need to reassess antibiotic therapy through urine culture and sensitivity testing. Diagnostic imaging is the primary means of determining the success of the dissolution therapy. The calculolytic diet and antibiotic administration should be continued for one month following complete dissolution of uroliths as established by imaging. Hill's s/d® is not a maintenance diet and should not be fed for longer than 6 months. The nutrient restrictions, fat content, and increased sodium content of Hill's s/d® make it unsuitable as a diet for puppies, pregnant or lactating pets, and pets with heart failure, hypertension, or renal failure. In a recent study with as yet unpublished results, an alternative protocol for treating infection-related struvite uroliths in dogs has resulted in preliminary evidence that dissolution of these uroliths may be achieved by feeding a maintenance, non-calculolytic diet(e.g, Science Diet Adult Original {Hill's Pet Nutrition}, Dog Chow {Purina}) when combined with antibiotic therapy and the urinary acidifier d,l methionine (Methioform® - 100 mg/kg PO q12 hrs) This alternative protocol is still under evaluation but could be considered for treating struvite urocystoliths in dogs that refuse to eat the specially formulated calculolytic diets.

Failure to dissolve struvite uroliths with an appropriate therapeutic regimen and owner compliance may indicate that the targeted uroliths are not composed of struvite or have layers containing greater than 20% nonstruvite minerals. Calcium phosphate and calcium carbonate phosphate are minerals which are occasionally associated with struvite uroliths and may interfere with medical dissolution therapy.

Prevention of sterile struvite uroliths can be accomplished by feeding any of many available diets that increase urine volume, decrease urine pH to < 6.8, and decrease excretion of magnesium, ammonium and phosphorus (e.g., Royal Canin canine Control Formula®). Hill's canine w/d® can be used in struvite prevention programs when patient obesity is a concern. Preventative diets for struvite uroliths are less restrictive than the calculolytic diets, but are still not satisfactory for puppies, lactating patients, or pregnant patients. Although such a diet can be fed for prevention to patients with infection-related struvite uroliths, special dietary management is not specifically indicated because the key to prevention is eradication of bacterial infection and subsequent monitoring for recurrence of infection by periodically checking urinalyses and urine cultures.


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