In the last three decades, the composition of uroliths in cats has been variable, while the composition of feline urethral
plugs has remained consistent. Here's a look at what may be causing this disparity and the implications for our feline patients.
Feline uroliths
Figure 1: MAP = Magnesium ammonium phosphate, or 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
struvite was detected in 78 percent. However, beginning in the mid-1980s, a rapid, substantial increase in the frequency of
calcium oxalate uroliths occurred in association with a reciprocal decrease in the frequency of struvite uroliths (Figure
1).
Figure 2: MAP = Magnesium ammonium phosphate, or struvite; CaOx = calcium oxalate; CaPO4 = calcium phosphate.
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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 (Figures 1 and 2). 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) the widespread use of a calculolytic diet designed 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 3) inconsistent follow-up evaluation of efficacy of
dietary management protocols by urinalysis and radiography.
In 2003, the frequency of calcium oxalate uroliths declined to 47 percent, while the frequency of struvite uroliths increased
to 42 percent (Figure 1). During 2004, the number of struvite uroliths (45 percent) submitted to the Minnesota Urolith Center
nudged past those containing calcium oxalate (44 percent). In 2005, the number of struvite uroliths (48 percent) surpassed
those containing calcium oxalate (41 percent) in frequency of occurrence (Figure 1).
Figure 3: MAP = Magnesium ammonium phosphate, or struvite; CaOx = calcium oxalate; Cap = calcium phosphate.
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Of 10,093 feline uroliths submitted to the Minnesota Urolith Center in 2006, 5,001 (50 percent) were struvite, and 3,914 (39
percent) were calcium oxalate. In 2007, of 11,174 uroliths submitted to the center, 5,432 (49 percent) were struvite, and
4,553 (41 percent) were calcium oxalate (Figure 2). In 2008, 49 percent were struvite, and 39 percent were calcium oxalate.
In 2009, 48 percent were struvite, and 40 percent were calcium oxalate (Table 1).
Figure 4: MAP = Magnesium ammonium phosphate, or struvite; CaOx = calcium oxalate; CaPO4 = calcium phosphate.
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The decrease in occurrence of naturally occurring calcium oxalate uroliths during the past seven 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. The increase in appearance of naturally occurring struvite
uroliths during the past seven 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 appear to reduce the risk of calcium oxalate urolith formation but increase the risk of struvite (magnesium ammonium
phosphate) urolith formation.
Figure 5: Various 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 sustained increase in occurrence of struvite uroliths from 2003 to 2009 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 most of the 5,578 struvite uroliths obtained from cats and submitted
to the Minnesota Urolith Center in 2009 could have been readily dissolved in two to four weeks by feeding diets designed to
promote formation of urine that is undersaturated with struvite (e.g., Prescription Diet c/d Multicare — Hill's Pet Nutrition).
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