Canine urolith epidemiology: 1981 to 2010

In the last three decades, the composition of uroliths in dogs has been variable. In this article, the second in a series of three, we evaluate the canine uroliths submissions to the Minnesota Urolith Center to determine if trends during 2010 were different from previous years as well as the implications of this information when considered in context of our patients.

Calcium oxalate on the rise; struvite declining

In 1981, calcium oxalate was detected in only 5 percent of canine uroliths submitted to the Minnesota Urolith Center, whereas struvite was detected in 78 percent of canine uroliths. At that time, canine calcium oxalate uroliths were considered rare. During subsequent years, canine calcium oxalate uroliths were thought to be associated with trends similar to those that we have documented in cats. As with cats, during the past 30 years, there has been a substantial increase in calcium oxalate uroliths (42 percent) and a substantial decrease in the number of struvite (magnesium ammonium phosphate) uroliths (39 percent; Table 1 and Figures 1-4).

Table 1 Mineral composition of 52,562 canine uroliths, 2010*

Some clinicians have speculated that the increase in calcium oxalate occurrence in dogs and cats is an artifact that has occurred as a result of the widespread use of struvolytic diets and antimicrobial drugs beginning in the mid-1980s. However, the observation that the number of purine uroliths (uric acid and salts of uric acid) has remained relatively constant during the same time interval does not support this conclusion (Figures 2 and 3). Likewise, the observation that a similar trend in purine urolith occurrence has also been observed in domestic cats during the same time span does not support this generality.1

Figure 1: Composition of canine uroliths, 1981-2010. Note: MAP = struvite; CaOx = calcium oxalate; Capoh = calcium phosphate.

During this period, the decline in the frequency of naturally occurring struvite uroliths associated with a reciprocal increase in calcium oxalate uroliths may also have been associated with:

1. The widespread use of a calculolytic food designed to dissolve struvite uroliths

2. The widespread use of modification of maintenance and prevention foods to minimize struvite crystalluria (some dietary risk factors that decrease the risk of struvite uroliths increase the risk of calcium oxalate uroliths)

3. Inconsistent follow-up evaluation of efficacy of dietary management protocols by urinalysis and radiography.

Likewise, the probability that > 95 percent of the canine struvite uroliths submitted to our center were induced by microbes that produced urease-producing enzymes, while < 95 percent of the feline uroliths were not associated with urease-positive microbes, is problematic. One hypothesis that might explain (at least in part) the trend toward increased calcium oxalate occurrence in both groups is the treatment of canine infection-induced uroliths with an appropriate diet and antimicrobial drugs and the treatment of sterile feline uroliths with an appropriate diet.

Figure 2: Mineral composition of 493,286 canine uroliths, 1981-2010. Note: CaOx = calcium oxalate; MAP = struvite; CaPO4 = calcium phosphate; Cmpd = compound.

Implications for practice

The significance of infection-induced struvite as the predominant mineral type in dogs and sterile struvite as the predominate mineral type in cats—and the fact that the etiology of sterile struvite and infection-induced struvite is different—is of great importance when formulating therapeutic plans. An appropriate antimicrobial agent in addition to an appropriate diet (e.g., Prescription Diet s/d Canine canned—Hill's Pet Nutrition) is required to consistently induce dissolution of infection-induced uroliths, while the prevention of urinary tract infections is essential to minimize recurrence of infection-induced uroliths. A special diet is not necessary. Consumption of an appropriate diet (e.g., Prescription Diet c/d Multicare—Hill's Pet Nutrition) is usually effective in dissolving and minimizing recurrence of sterile struvite uroliths.

Figure 3: Mineral composition of 52,562 canine uroliths, 2010. Note: CaOx = calcium oxalate; MAP = struvite; CaPO4 = calcium phosphate; Cmpd = compound.

A surprising trend

During the past 30 years, the number of canine urolith submissions has exceeded the number of feline urolith submissions by a ratio of about 4 to 5:1. This trend is surprising given that the number of cats living with families in the United States is greater than the number of dogs living with families. For example, in 2010, we received 52,562 canine uroliths and 12,312 feline uroliths. This results in a ratio of 4.3 canine uroliths for every feline urolith that we received. We welcome the ideas of readers to explain this seemingly paradoxical trend.

Figure 4: CaOx = calcium oxalate; MAP = struvite.

Editor's note: With the support of an educational gift from Hill's Pet Nutrition, as well as contributions from veterinarians and pet owners worldwide, the Minnesota Urolith Center is providing quantitative urolith analysis at no charge. Online submission, e-mail notification and electronic retrieval of results are available. With a database of more than 670,000 samples, the veterinary community is offered the latest information on urolith trends, treatment and prevention suggestions. For details, visit urolithcenter.org.

Dr. Osborne is professor of medicine at the College of Veterinary Internal Medicine at the University of Minnesota.

Dr. Lulich is the co-director of The Minnesota Urolith Center and professor of Veterinary Internal Medicine at the University of Minnesota.

REFERENCE

1. Osborne CA, Lulich JP. Feline urolith epidemiology: 1981 to 2010. DVM Newsmagazine June 2011; 50-53.

For a complete list of articles by Dr. Osborne, visit dvm360.com/osborne.