Stalking stones: An overview of canine and feline urolithiasis - Veterinary Medicine
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Stalking stones: An overview of canine and feline urolithiasis
Did you know that a new type of urolith has been found in cats? Should you institute preventive therapy if you identify only crystalluria? Is antibiotic therapy automatically warranted in animals with indwelling urinary catheters? This internist revamps your knowledge on diagnosing, treating, and preventing urolithiasis.


Infection-related struvite uroliths occur as a result of an underlying urinary tract infection. The bacteria involved are most commonly urease-producing Staphylococcus species, although urease-producing Proteus species and, rarely, ureaplasma organisms may also be involved.24 Occasionally, other bacteria are cultured from affected patients. The breakdown of urea by the bacteria releases ammonia into the urine, increasing the pH and availability of ammonium and phosphate ions, all of which predispose a patient to struvite urolith formation.

Treatment. Feeding a calculolytic diet (Table 1) can dissolve both sterile and infection-related struvite urocystoliths and nephroliths by creating an acidic, dilute urine undersaturated with magnesium, ammonium, and phosphorus.24-27 The dissolution time for uroliths varies, but it averages about two to three months for infection-related uroliths and three to six weeks for sterile uroliths.24-26 Dissolution of infection-related struvite uroliths occurs more rapidly if an appropriate antibiotic is administered concurrently. Antibiotic therapy is best determined by bacterial culture and antimicrobial sensitivity testing of urine obtained by cystocentesis. Ideally, a urine culture should be performed before antibiotics are initiated and periodically during the dissolution process.

By evaluating serial urinalyses and abdominal radiography or ultrasonography every four weeks, dissolution therapy can be altered as needed for individual patients. Urinalysis findings compatible with an ongoing infection (bacteriuria, pyuria, alkaluria) indicate a need to reassess antibiotic therapy through urine bacterial culture and antimicrobial sensitivity testing. Diagnostic imaging is the primary means of determining the success of the dissolution therapy. Continue the calculolytic diet and antibiotic administration for one month after complete dissolution of uroliths as established by imaging.19,24,25

A calculolytic diet is not a maintenance diet. For example, according to the manufacturer's product guide (2005 Hill's Key to Clinical Nutrition), Prescription Diet s/d feline and canine (Hill's Pet Nutrition) should not be fed for longer than six months. The nutrient restrictions, fat content, and increased sodium content of s/d make it unsuitable as a diet for puppies or kittens, pregnant or lactating pets, and pets with heart failure, hypertension, or renal failure.

In a recent study in which the results are unpublished, an alternative protocol for treating infection-related struvite urocystoliths in a small number of dogs has preliminarily shown dissolution of these uroliths by feeding a maintenance, noncalculolytic diet (e.g. Science Diet Adult Original [Hill's Pet Nutrition], Dog Chow [Purina]) combined with antibiotic therapy and the urinary acidifier d,l-methionine (Methio-Form—Vet-A-Mix; 100 mg/kg orally twice a day).26 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.24,28 Calcium phosphate and calcium carbonate phosphate are occasionally associated with struvite uroliths and may interfere with medical dissolution therapy.

Not all patients with suspected or confirmed struvite uroliths are candidates for medical dissolution because of noncompliant owners, complicating health circumstances (e.g. hypertension), or ureteral or urethral uroliths. Surgical removal, lithotripsy, or voiding urohydropropulsion should be considered in these patients depending on the circumstances.

Prevention. Prevent sterile struvite uroliths by having owners feed their pets one of the many available diets that increase urine volume, decrease urine pH to < 6.8, and decrease excretion of magnesium, ammonium, and phosphorus (Table 1). In struvite prevention programs in which patient obesity is a concern, canine and feline Prescription Diet w/d (Hill's Pet Nutrition) can be used. Preventive diets for sterile struvite uroliths are less restrictive than the calculolytic diets but should not be given to puppies or kittens, pregnant or lactating patients, or dogs at risk for pancreatitis.

Although these diets can be fed to patients with infection-related struvite uroliths, special dietary management is not specifically indicated. The key to preventing these uroliths is to eradicate the bacterial infection and periodically monitor for infection recurrence with urinalyses and bacterial cultures.26


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