Trends change in composition of feline uroliths



Table 1. Feline urolith distribution 1981-2004
Epidemiology of feline uroliths: 1981-2002 In 1981, calcium oxalate was detected in only 2 percent of feline uroliths submitted to the Minnesota Urolith Center, whereas struvite was detected in 78 percent.

However, beginning in the mid-1980s, a dramatic increase in the frequency of calcium oxalate uroliths occurred in association with a decrease in the frequency of struvite uroliths (Table 1). From 1994 to 2002, approximately 55 percent of the feline uroliths submitted to the Minnesota Urolith Center were composed of calcium oxalate, while only 33 percent were composed of struvite. During this period, the decline in appearance of naturally occurring struvite uroliths associated with a reciprocal increase in calcium oxalate uroliths may have been associated with:

  • The widespread use of a calculolytic diet designed to dissolve struvite uroliths,
  • Modification of maintenance and prevention diets to minimize struvite crystalluria (some dietary risk factors that decrease the risk of struvite uroliths increase the risk of calcium oxalate uroliths),
  • Inconsistent follow-up evaluation of efficacy of dietary management protocols by urinalysis.

Epidemiology of feline uroliths: 2003-2004 In 2003, the frequency of calcium oxalate uroliths declined to 47 percent, while the frequency of struvite uroliths increased to 42 percent. During 2004, the number of struvite uroliths (44.9 percent) submitted to the Minnesota Urolith Center nudged past those containing calcium oxalate (44.3 percent) in frequency of occurrence (Table 2). The decrease in occurrence of naturally occurring calcium oxalate uroliths during the past two years may be associated with:

  • Reformulation of adult maintenance diets to minimize risk factors for calcium oxalate crystalluria,
  • Improvements in formulation of therapeutic diets designed to reduce risk factors for calcium-oxalate uroliths, and
  • Increased use of therapeutic diets designed to reduce risk factors for calcium oxalate uroliths.

The increase in appearance of naturally occurring struvite uroliths during the past two years may be associated with the reciprocal relationship between dietary risk factors for calcium oxalate and struvite uroliths. Diets that reduce urine acidity and provide adequate quantities of magnesium reduce the risk of calcium oxalate urolith formation, but increase the risk of struvite (magnesium ammonium phosphate) urolith formation. In addition, the increase in struvite urolith occurrence in 2003 and 2004 may be associated with decreased use of diets designed to dissolve sterile struvite uroliths as a consequence of the significant increase in occurrence of calcium oxalate uroliths in the 1980s and 1990s. However, it is likely that many of the 3,915 sterile struvite uroliths obtained from cats and submitted to the Minnesota Urolith Center in 2004 could have been readily dissolved by feeding a diet designed to promote formation of urine that is undersaturated with struvite. The following case report is a typical example.

Case report

Database: A 4.5-year-old spayed female domestic shorthair cat was referred to the Veterinary Teaching Hospital at the University of Minnesota because of gross hematuria and dysuria of four weeks' duration. Urocystoliths of unknown composition had been removed surgically when the cat was 1.5 years old. Since that time, the cat had been fed a commercially prepared dry adult maintenance diet.

Physical examination revealed that the cat was in good physical condition. She weighed 13 pounds. Rectal temperature, pulse rate and rhythm, and respiration rate and character were normal. Palpation of the abdomen revealed that the urinary bladder was thickened, contracted and painful. A grating sensation characteristic of uroliths was detected within the bladder lumen. Micturition induced during palpation revealed gross hematuria. No other abnormalities were detected.

Analysis of a urine sample collected by cystocentesis revealed findings typical of inflammation (i.e. hematuria, proteinuria and pyuria) and struvite crystalluria. The urine specific gravity was 1.050; the urine pH was 7.5 (reagent strip). Aerobic bacteria were not detected by the conventional quantitative culture method. A complete blood count and serum biochemistry profile were normal.