Changing trends in composition of feline uroliths and urethral plugs

Epidemiology of feline uroliths: 1981-2002

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.

Table 1. Feline urolith distribution 1981-2005

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

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).

Table 2. Feline plug distribution 1981-2005

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.

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.

Figure 1. Mineral composition of feline uroliths, 2005 and Figure 2. Mineral composition of feline plugs, 2005

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.

Epidemiology of feline urethral plugs: 1981-2005

Of 593 urethral plugs submitted to the Minnesota Urolith Center by veterinarians in 2005, the mineral composition of almost 87 percent were primarily struvite (Figure 2). Less than 1 percent were composed of calcium oxalate. Since 1981, struvite has consistently been the most common mineral in feline urethral plugs; the prevalence of calcium oxalate in urethral plugs always has been infrequent (Table 2). The explanation as to why there have been significant shifts in the prevalence of calcium oxalate and struvite in feline uroliths during the past 25 years while the prevalence of struvite and calcium oxalate in feline urethral plugs has not significantly changed is not obvious. However, the high prevalence of struvite in urethral plugs is of clinical significance in terms of dietary strategies to prevent their formation.

Dr. Osborne, a diplomate of the American College of Veterinary Internal Medicine, is professor of medicine in the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.

Dr. Lulich, a diplomate of the American College of Veterinary Medicine, is a professor in the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.