Feline uroliths (Proceedings)
Urolithiasis is common in the cat, causing morbidity and, occasionally, mortality. Diagnosis is based on clinical signs, imaging, gross physical characteristics of the uroliths, and quantitative analysis of the urolith. Therapy depends upon urolith composition and associated etiology as well as location of the urolith in the urinary tract. It should be remembered that increasing water intake is essential in the treatment and prevention of all uroliths regardless of composition. The following discussion covers many of the urolith types found in the cat.
The struvite (magnesium ammonium phosphate hexahydrate) urolith is one of the most common uroliths occurring in the cat. A study at the Minnesota Urolith Center analyzing feline uroliths from 1981 – 2002 revealed a change in the status of struvite urolith prevalence.(Osborne,2006) In 1981 uroliths composed of struvite represented the overwhelming majority of feline uroliths (78%), but a steady decline was seen over a period of several years with struvite decreasing to a prevalence of approximately 33%. Beginning in 2003 the frequency of feline struvite uroliths began to notably increase, reaching a level of 48.1% in 2005 and again establishing struvite as the most prevalent feline urolith composition identified at the Minnesota Urolith Center. A separate California study analyzing a large number of uroliths over a similar time period supports the trend in recent increases in feline struvite uroliths.(Cannon,2007) The shift in urolith prevalence in the cat 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. However, this theory does not seem to explain the fact that during the same 25-year time period, the mineral composition of feline urethral plugs has remained consistently and overwhelmingly struvite (87% in 2005).(Osborne, 2006)Occasionally calcium phosphate and calcium carbonate phosphate will also be associated with struvite uroliths. Struvite uroliths vary greatly in size, ranging from sand-like particles to specimens well over 1 cm in diameter. Struvite uroliths are generally sufficiently radiodense to be seen easily on survey films. These uroliths assume a variety of shapes, but are often spherical or pyramidal (flat-sided). Uroliths that have a pyramidal shape or that are > 1 cm in diameter are most often of struvite origin. In some cases a struvite urolith will take on the shape of the portion of the urinary tract surrounding it. Struvite crystals precipitate out more readily in alkaline urine and have an angular, prism-like appearance.
Struvite uroliths can either be infectious or sterile in origin. Infection-related uroliths occur more commonly in cats < 1year-of-age and > 10 years-of-age. The majority of feline struvite uroliths are sterile and are not associated with a known sex predisposition. Feline breeds predisposed to struvite formation include the foreign shorthair, ragdoll, Chartreux, Oriental shorthair, domestic shorthair, and Himalayan. The age predisposition for sterile struvite urolith formation in the cat is between 1 and 10 years-of-age with the incidence decreasing after approximately 7 years-of-age.
Sterile struvite uroliths may be associated with increased to normal levels of magnesium in the diet, but it is still unclear whether the presence of magnesium by itself triggers formation of struvite uroliths. Alkaluria and low urine volume also appear to put cats at risk for developing sterile struvite uroliths.
Feeding a calculolytic diet such as feline Hill's s/d® can dissolve both sterile and infection-related struvite urocystoliths and nephroliths, by creating an acidic, dilute urine undersaturated with magnesium, and ammonium. The time for urolith dissolution is variable, but averages about 2 - 3 months for infection-related uroliths and 3 – 6 weeks for sterile uroliths. 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. Diagnostic imaging is the primary means of determining the success of the dissolution therapy. 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. . 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 restrictions and increased sodium content of Hill's s/d® make it unsuitable as a diet for or kittens, pregnant pets, or lactating pets.
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 feline Control Formula®). Hill's feline w/d® can be used in struvite prevention programs when patient obesity is a concern. Preventative diets for sterile struvite uroliths are less restrictive than the calculolytic diets, but are still not satisfactory for kittens, 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.