Analysis of 36,032 canine cases shows decline in struvite uroliths

Knowledge of urolith composition is important because contemporary methods of detection, treatment and prevention of the underlying causes of urolithiasis largely depend on knowledge of urolith composition.

The following discussion is based on quantitative analysis of 36,032 canine uroliths submitted to the Minnesota Urolith Center in 2006.

In 1981, calcium oxalate was detected in only 5 percent of canine uroliths submitted to the Minnesota Urolith Center, whereas struvite (magnesium ammonium phosphate) was detected in 78 percent.

However, evaluation of the prevalence of different types of minerals in canine uroliths during successive years reveals a gradual and consistent increase in calcium oxalate uroliths, and a gradual and consistent decline in struvite uroliths (Figure 1).

In fact, by 2003 the prevalence of calcium oxalate (41 percent) was approximately equal to struvite (40 percent). In 2004, calcium oxalate (41 percent) surpassed struvite (39 percent). In 2005, calcium oxalate was detected in 41 percent of urolith submissions while struvite was detected in (38 percent).

In other words, from 1981 to 2006, struvite uroliths declined by approximately 43 percent compared to a 78 percent increase for calcium oxalate (Figure 1).

Figure 1

Trends in 2006

What would be your estimate of the occurrence of calcium oxalate and struvite urolith submissions to the Minnesota Urolith Center in 2006? Even though the total submission (36,032) of canine uroliths was 3,147 more than for the year 2005, in 2006 the frequency of occurrence of calcium oxalate (41 percent) and struvite (39 percent) remained approximately the same (Table 1; Figs. 1 and 2).

Figure 2

Recall that the frequency of feline calcium oxalate and struvite occurrence during the same period was similar. (For additional details related to feline uroliths and feline urethral plugs, refer to the Diagnote titled "Changing trends in composition of feline uroliths and feline urethral plugs," February 2007 issue of DVM Newsmagazine.)

Table 1 Quantitative mineral composition* of 36,032 canine uroliths - 2006

Risk and protective factors

What is the underlying reason for such a dramatic change in the composition of canine uroliths? Although several hypotheses have been proposed, none has been proven to date. Evidence suggests an interaction between (1) demographic risk factors such as breed, age, gender, anatomy and genetic predisposition, and (2) environmental risk factors such as sources of food, water, exposure to certain drugs and living conditions. Some factors may be protective.

Not all risk and protective factors are of equal importance. It is apparent that each contributing risk or protective factor may play a limited or a significant role in the pathogenesis of urolithiasis. The chance of developing a specific type of urolith when exposed to one or more of the risk or protective factors often is expressed in terms of numerical probabilities (so-called "odds" or "odds ratios").

When used in a qualitative (rather than a quantitative) way, the significance of risk or protective factors should not be assigned an "all or none" or "always or never" interpretation.

In many situations, each risk factor contributes a limited role to the development of urolithiasis. In some situations, they may not be a factor in every exposed patient. Furthermore, identifying one event in a chain of etiologic events is not the same as identifying the entire etiologic chain.

Why identify risk, protective factors?

Our interest in recognizing the association of specific risk factors with urolithiasis is related to:

• Identifying healthy but susceptible populations of animals and trying to minimize their exposure to these risk factors.

• Identifying healthy, but susceptible, populations and trying to enhance their exposure to protective factors.

• Facilitating detection and treatment of subclinical urolithiasis that has already developed in susceptible patients.

With support from Hill's Pet Nutrition, studies are in progress at the Minnesota Urolith Center to identify risk and protective factors associated with calcium oxalate and struvite uroliths.

To learn more about our studies and how you can participate, visit our Web site, www.cvm.umn.edu. Click the link to department and centers to find Minnesota Urolith Center.

Carl A. Osborne DVM, PhD, Dipl. ACVIM

Dr. Osborne, a diplomate of the American College of Veterinary Internal Medicine, is professor of medicine in the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.

Carl A. Osborne

Jody P. Lulich DVM, PhD, Dipl. ACVIM

Dr. Lulich, a diplomate of the American College of Veterinary Internal Medicine, is a professor in the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.

Jody P. Lulich