In the last three decades, the composition of uroliths in dogs has been variable. In this article, the second in a series of three, we evaluate the canine uroliths submissions to the Minnesota Urolith Center to determine if trends during 2010 were different from previous years as well as the implications of this information when considered in context of our patients.
Calcium oxalate on the rise; struvite declining
In 1981, calcium oxalate was detected in only 5 percent of canine uroliths submitted to the Minnesota Urolith Center, whereas struvite was detected in 78 percent of canine uroliths. At that time, canine calcium oxalate uroliths were considered rare. During subsequent years, canine calcium oxalate uroliths were thought to be associated with trends similar to those that we have documented in cats. As with cats, during the past 30 years, there has been a substantial increase in calcium oxalate uroliths (42 percent) and a substantial decrease in the number of struvite (magnesium ammonium phosphate) uroliths (39 percent; Table 1 and Figures 1-4).
Table 1 Mineral composition of 52,562 canine uroliths, 2010*
Some clinicians have speculated that the increase in calcium oxalate occurrence in dogs and cats is an artifact that has occurred as a result of the widespread use of struvolytic diets and antimicrobial drugs beginning in the mid-1980s. However, the observation that the number of purine uroliths (uric acid and salts of uric acid) has remained relatively constant during the same time interval does not support this conclusion (Figures 2 and 3). Likewise, the observation that a similar trend in purine urolith occurrence has also been observed in domestic cats during the same time span does not support this generality.1
Figure 1: Composition of canine uroliths, 1981-2010. Note: MAP = struvite; CaOx = calcium oxalate; Capoh = calcium phosphate.
During this period, the decline in the frequency of naturally occurring struvite uroliths associated with a reciprocal increase in calcium oxalate uroliths may also have been associated with:
1. The widespread use of a calculolytic food designed to dissolve struvite uroliths
2. The widespread use of 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 evaluation of efficacy of dietary management protocols by urinalysis and radiography.
Likewise, the probability that > 95 percent of the canine struvite uroliths submitted to our center were induced by microbes that produced urease-producing enzymes, while < 95 percent of the feline uroliths were not associated with urease-positive microbes, is problematic. One hypothesis that might explain (at least in part) the trend toward increased calcium oxalate occurrence in both groups is the treatment of canine infection-induced uroliths with an appropriate diet and antimicrobial drugs and the treatment of sterile feline uroliths with an appropriate diet.
Figure 2: Mineral composition of 493,286 canine uroliths, 1981-2010. Note: CaOx = calcium oxalate; MAP = struvite; CaPO4 = calcium phosphate; Cmpd = compound.