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

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.

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.

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).

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.

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.

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.

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.

But today, widespread use of highly effective vaccines has dramatically reduced clinical cases of canine distemper in most parts of the United States and Canada. In fact, some recent veterinary graduates have not encountered a clinical case of canine distemper. Does this change in disease prevalence mean that canine distemper vaccines should be used less frequently?

Table 6: Potential risk factors associated with calcium oxalate urolith formation

By analogy, lower urinary tract disease, especially obstruction of the male urethra with struvite plugs, was once a leading cause of morbidity and mortality in cats. Does the reduction in frequency of obstruction of the urethra of male cats with struvite urethral plugs mean that we should stop being concerned about events that were associated with this change?

Likewise, does the reduction in the frequency of occurrence of feline struvite uroliths mean that we can ignore events associated with this change? If our goal is to prevent increased occurrence of struvite related causes of feline lower urinary tract disease, we must not lose sight of events that helped to control them. We should learn from past events because history teaches us that those who are inclined to ignore the past must be prepared to repeat it. Although the recommendation to avoid diets associated with increased risk of calcium oxalate uroliths in susceptible cats (for example, neutered male Himalayan and Persian cats between 4 years and 15 years of age) is logical, we emphasize the observation that, unlike uroliths, struvite has consistently been the predominant mineral in urethral plugs during the past 20 years. Therefore, if a recommendation to reduce use of diets designed to minimize risk factors for struvite crystalluria was inappropriately applied to cats at increased risk for struvite plugs (for example, 1-year-old to 6-year-old neutered males), the frequencies of urethral obstructions and urethrostomies might increase to levels that occurred in the early 1980s. In our opinion, this would be unfortunate because, in addition to added medical expenses, urethrostomies have been associated with substantial short-term and long-term complications. These include bacterial urinary tract infections, infection-induced struvite urolithiasis and urethral strictures.

In our opinion, modification of diets to minimize risk factors (Table 5, p. 35) associated with struvite crystalluria has been associated with a reduction in the frequency of occurrence of struvite urolithiasis and reduction in frequency of occurrence of struvite urethral plugs.

However, we also recognize that modifying diets to reduce some risk factors associated with formation of struvite crystals may expose cats with a pre-existing nondietary predisposition to calcium oxalate urolithiasis to additional diet-related risk factors (Table 6) for calcium oxalate urolithiasis. How do we resolve this dilemma?

With the understanding that dietary changes cannot be expected to eliminate or control all risk factors for all types of uroliths, the logical answer is that practicing veterinarians and food manufacturers should strive to provide dietary modifications that will minimize formation of different types of uroliths. This requires knowledge of, and a search for, different risk factors associated with different types of uroliths (Tables 5 and 6).

With the exception of recommending moist rather than dry diets, it is unlikely that a single dietary formulation will minimize diet-related risks of all types of uroliths. Rather, the benefits and risks of different dietary prescriptions appropriate for specific types of uroliths and matrix-crystalline plugs should be considered.

Since struvite urethral plugs predominantly affect young to middle age male cats, this population would likely receive the greatest benefit from consumption of diet prescriptions designed to minimize struvite crystalluria. Since calcium oxalate uroliths predominately affect middle age and older cats, this population would likely receive the greatest benefit from consumption of diet prescriptions designed to minimize calcium oxalate crystalluria.

Following choice of a diet designed to minimize risk factors associated with a specific type of urolith, cats should be periodically re-evaluated to determine the efficacy of this strategy. Special emphasis should be placed on evaluation of urine specific gravity (the goal is to reduce formation of concentrated urine), urine pH, and in vivo crystalluria. If in-vivo calcium oxalate crystals develop in a patient consuming a diet designed to minimize struvite crystalluria, appropriate adjustments in management should be made.