Carl A. Osborne, DVM, Phd, Dipl. ACVIM
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In the past three decades, urolith composition in cats and dogs has varied, while feline urethral plug composition has remained
consistent. In this article, the second of three parts, we evaluate trends in feline urolith composition to determine what
may be causing this disparity and examine the implications for our patients.
Calcium oxalate vs. struvite
Figure 1: Composition of feline uroliths, 1981-2011. Key—MAP = struvite, CaOx = calcium oxalate; Cap = calcium phosphate.
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In 1981, calcium oxalate was detected in only 2 percent of feline uroliths submitted to the Minnesota Urolith Center, whereas
78 percent of feline uroliths were composed of struvite (magnesium ammonium phosphate). But in the mid-1980s, a rapid, substantial
increase in the frequency of calcium oxalate uroliths occurred, while the frequency of struvite uroliths decreased (Figure
1).
Figure 2: Change in frequency of feline struvite and calcium oxalate. Key—CaOx = calcium oxalate; MAP = struvite.
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From 1994 to 2002, about 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 (Figures 1 and 2). During this period, the decline in the frequency
of naturally occurring struvite uroliths associated with a reciprocal increase in calcium oxalate uroliths may have been due
to:
1. Widespread use of a calculolytic food to dissolve struvite uroliths
2. Modification of maintenance and prevention foods to minimize struvite crystalluria (some dietary risk factors that decrease
the risk of struvite uroliths increase the risk of calcium oxalate uroliths)
3. Inconsistent follow-up of efficacy of dietary management protocols by urinalysis and radiography.
Figure 3: Mineral composition of feline uroliths, 2011. Key—CaOx = calcium oxalate; CaPO4 = calcium phosphate; MAP = struvite.
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In 2004, the number of feline struvite uroliths (45 percent) submitted to the Minnesota Urolith Center nudged past those containing
calcium oxalate (44 percent). These trends have continued into 2011 (Figure 3).
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