In the past three decades, the composition of uroliths in cats and dogs has been variable, while the composition of feline urethral plugs has remained consistent. In this article, the first of three parts, Jody P. Lulich, DVM, PhD, Dipl. ACVIM, and I evaluate trends in feline uroliths, feline urethral plugs, and canine uroliths to determine what may be causing this disparity and the implications for our patients.
Calcium oxalate vs. struvite
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. 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 1: Composition of feline uroliths
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 the frequency of naturally occurring struvite uroliths associated with a reciprocal increase in calcium oxalate uroliths may have been associated with:
Figure 2: Change in frequency of feline struvite and calcium oxalate
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), and
3) Inconsistent follow-up of efficacy of dietary management protocols by urinalysis and radiography.
During 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 continued into 2010 (Figure 3 and Table 1).
Figure 3: Mineral composition of feline uroliths, 2010