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.