Improving management of urolithiasis: feline sterile struvite uroliths, urethral plugs

May 01, 2004

Table 1: Mineral composition of feline uroliths 2003
Recently, concern has been expressed that widespread use of diets to minimize struvite crystalluria has been associated with an increase in the frequency of calcium oxalate uroliths. As summarized in the March issue of DVM Newsmagazine (Volume 35, pages 12S to 17S), results of epidemiologic studies indicate that some dietary factors (including urine acidifying potential and magnesium restriction) associated with reduced risk for struvite urolith formation may increase the risk for calcium oxalate uroliths.

Because of the increased prevalence of calcium oxalate uroliths in cats, we have been asked by many veterinary practitioners whether diets designed to minimize struvite crystalluria should be used less frequently. The objective of this article is to provide insight into the risks and benefits associated with management strategies designed to minimize feline sterile MAP uroliths and urethral plugs.

Distinguishing uroliths from urethral plugs There are physical and probable etiopathogenic differences between uroliths and urethral plugs. Therefore these terms should not be used as synonyms.

Table 2: Mineral composition of feline plugs 2003
Uroliths are polycrystalline concretions composed primarily of minerals (inorganic and organic crystalloids) and smaller quantities (often less that 5 percent) of nonmineral matrix. Unlike urethral plugs, uroliths are not disorganized precipitates of crystalline material, but consist of crystal aggregates with a complex internal structure (Figure 1).

Feline urethral plugs commonly are composed of large quantities of nonmineral matrix (as much as 50 percent) mixed with minerals (Figure 2). Some plugs, such as those composed of sloughed tissue and/or blood clots, do not contain any minerals. The matrix components within the same urethral plug may vary (i.e. the distal end of the plug may be a combination of minerals and matrix, whereas more proximal portions may be composed of blood clots without crystals).

At this time, we recommend that the mineral composition of uroliths and urethral plugs be used to describe them since most therapeutic regimens designed to dissolve or prevent them are based on their mineral composition.

Table 3: Feline urolith distribution 1981-2003 and Table 4: Feline plug distribution 1981-2003
Epidemiology Of 8,822 uroliths submitted to the Minnesota Urolith Center by veterinarians in 2003, the mineral composition of approximately 42.5 percent were primarily struvite. In contrast, 47.4 percent of the uroliths were composed of calcium oxalate (Table 1).

Of 645 urethral plugs submitted to the Minnesota Urolith Center by veterinarians in 2003, the mineral composition of approximately 87 percent were primarily struvite (Table 2). Less than 1 percent were composed of calcium oxalate. Although the prevalence of calcium oxalate uroliths has increased during the past two decades (Table 3), the prevalence of calcium oxalate in urethral plugs always has been infrequent (Table 4). Struvite has consistently been the most common mineral in feline urethral plugs.

Clinical significance The explanation as to why the prevalence of feline calcium oxalate uroliths dramatically increased during the past 20 years, while the prevalence of calcium oxalate in feline urethral plugs remained extremely low is not obvious, especially in light of the observation that male gender appears to be a risk factor for calcium oxalate uroliths and struvite urethral plugs.

Table 5: Potential risk factors associated with sterile struvite urolith formation
However, the high prevalence of struvite in urethral plugs is of clinical significance in terms of dietary strategies to prevent their formation. The frequency of urethral obstruction of male cats with struvite plugs has been on the decline over the past two decades, and this trend coincides with widespread utilization of diets designed to minimize risk factors for struvite crystalluria.

For example, in North American university veterinary teaching hospitals, the yearly hospital proportional morbidity rate (HPMR) for urethral obstructions declined from 19 cases/1,000 feline evaluations in 1980 to seven cases/1,000 evaluations 20 years later. This trend coincides with a dramatic decline in the frequency with which perineal urethrostomies have been performed in male cats. The yearly HPMR for urethrostomies decreased from 13 cases/1000 feline evaluations in 1980 to four cases/1,000 evaluations 20 years later.

Need for change? Is their need for fundamental changes in dietary management of feline uroliths and urethral plugs?

In context of these epidemiological observations, let us return to the question about whether urine acidifying and magnesium restricted diets designed to minimize struvite crystalluria should be used less frequently because of their association with increased risk for calcium oxalate urolith formation.

To answer the question, let's look at canine distemper. Textbooks of the 1950s and 1960s mention canine distemper as the most serious disease of dogs, causing morbidity and mortality in all ages, breeds and genders.