Magnesium ammonium phosphate (struvite) is one of the most common minerals found in feline uroliths. Previous studies have shown the efficacy of acidifying calculolytic diets (inducing urine pH ,6·5), in dissolving struvite stones in cats. Recent work in our laboratory found that wet and dry test diets induce a struvite urinary relative supersaturation (RSS) , 1 and that the urine of healthy cats fed the dry test diet dissolved feline struvite stones in vitro. The objective of the present study was to demonstrate the efficacy of those test diets on naturally occurring struvite urocystoliths in cats. A total of twenty-one cats were used, of which seventeen completed the study. Of the seventeen cats, eight were fed the wet test diet and nine the dry test diet. Uroliths dissolved in a median of 18 (10-55) d. In the remaining four cats, uroliths failed to dissolve and were removed surgically. Quantitative analysis showed that these uroliths contained either calcium oxalate or calcium phosphate. The present study demonstrates that diets that induce a struvite RSS , 1 result in struvite stone dissolution in vivo.Key words: Feline: Dissolution: Struvite: Relative supersaturationIn cats, the majority of uroliths found in the bladder are composed of magnesium ammonium phosphate (struvite) or calcium oxalate (1 -3) . Surveys on the prevalence of struvite and calcium oxalate uroliths in stone laboratories worldwide indicate that the occurrence of those two stones represents approximately 95 % of submissions. Struvite and calcium oxalate are more or less equally represented, with a slight advantage to one of them depending on the country (1 -8) .The most important risk factors for struvite formation are urinary pH and urine dilution, which will affect the concentrations of magnesium, ammonium and phosphate (4) . Contributing factors to struvite uroliths in cats may include periods of inclement weather in which cats tend to remain indoors for prolonged periods of time, inactivity, and feeding of low moisture, non-acidifying dry cat foods (9) . In a retrospective case -control study, diets with high Mg, P, Ca, Cl and fibre, moderate protein and low fat content were associated with increased risk (10) . Previous studies have shown the efficacy of canned (11,12) and dry (12) urine-acidifying diets in dissolving feline struvite stones. A study in our laboratory has found that the urine of healthy cats fed a dry-extruded diet, formulated to generate a urinary struvite relative supersaturation (RSS) ,1, dissolved feline struvite uroliths in vitro (13) . Therefore, the present study was undertaken to confirm that diets inducing struvite RSS ,1 and effective in dissolving struvite stones in vitro were also effective in dissolving naturally occurring struvite stones in vivo.
Materials and methodsCats were recruited from veterinary hospitals across Canada. They entered the study on the basis of clinical signs of lower urinary tract disease including, but not limited to, haematuria, stranguria, pollakiuria, inappropriate ...