Carl A. Osborne
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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 (Figure 1) .
From 1994 to 2002, approximately 55 percent of the feline uroliths submitted to the center were composed of calcium oxalate,
while only 33 percent were struvite (Figure 1). 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:
1) Widespread use of a calculolytic diet to dissolve struvite uroliths,
2) 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) and
Figure 1
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3) Inconsistent follow-up evaluation of efficacy of dietary management proto-cols by urinalysis and radiography
Figure 2
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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 urolith center nudged past those
containing calcium oxalate (44.3 percent). In 2005, the number of struvite uroliths (48.1 percent) surpassed those containing
calcium oxalate (40.6 percent) in frequency of occurrence (Figure 1).
Of 10,093 feline uroliths submitted to the center in 2006, 5,001 (50 percent) were struvite, and 3,914 (39 percent) were calcium
oxalate. In 2007, of 11,174 uroliths submitted, 5,432 (49 percent) were struvite and 4,553 (41 percent) were calcium oxalate
(Figure 2 and Table 1). The progressive decrease in occurrence of naturally occurring calcium oxalate uroliths during the
past five years may be associated with:
1) Reformulation of adult maintenance diets to minimize risk factors for calcium oxalate crystalluria,
2) Improvements in formulation of therapeutic diets designed to reduce risk factors for calcium oxalate uroliths and
3) Increased use of therapeutic diets designed to reduce risk factors for calcium oxalate uroliths.
Photo 1: Three different types of struvite that may occur in cats: A and B, sterile urocystoliths; C, infection-induced urocystolith;
D, matrix-struvite urethral plug.
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The increase in appearance of naturally occurring struvite uroliths during the past five years may be associated with the
reciprocal relationship between some dietary risk factors for calcium oxalate and struvite uroliths. For example, 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.
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