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
Table 1: Mineral composition of feline uroliths 2003
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.
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).
Table 2: Mineral composition of feline plugs 2003
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.
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).
Table 3: Feline urolith distribution 1981-2003 and Table 4: Feline plug distribution 1981-2003
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.
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.
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
Table 5: Potential risk factors associated with sterile struvite urolith formation
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.