Changing trends in composition of feline uroliths and urethral plugs

Perspectives from the Minnesota Urolith Center

Epidemiology of feline uroliths: 1981-2002


Table 1. Feline urolith distribution 1981-2005
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 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:
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.

Epidemiology of feline uroliths: 2003-2005


Table 2. Feline plug distribution 1981-2005
In 2003, the frequency of calcium oxalate uroliths declined to 47 percent, while the frequency of struvite uroliths rose 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 2005, the number of struvite uroliths (48.1 percent) surpassed those containing calcium oxalate (40.6 percent) in frequency of occurrence (Figure 1). The progressive decrease in occurrence of naturally occurring calcium oxalate uroliths during the past three years might 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.


Figure 1. Mineral composition of feline uroliths, 2005 and Figure 2. Mineral composition of feline plugs, 2005
The increase in appearance of naturally occurring struvite uroliths during the past two years may be associated with the reciprocal relationship between some 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 they increase the risk of struvite (magnesium ammonium phosphate) urolith formation.

In addition, the increase in struvite urolith occurrence in 2003, 2004 and 2005 might 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 4,435 sterile struvite uroliths obtained from cats and submitted to the Minnesota Urolith Center in 2005 could have been readily dissolved by feeding a diet designed to promote formation of urine that is undersaturated with struvite.