Feline urolith epidemiology: 1981 to 2010

In the past three decades, the composition of uroliths in cats and dogs has been variable, while the composition of feline urethral plugs has remained consistent. In this article, the first of three parts, Jody P. Lulich, DVM, PhD, Dipl. ACVIM, and I evaluate trends in feline uroliths, feline urethral plugs, and canine uroliths to determine what may be causing this disparity and the implications for our patients.

Calcium oxalate vs. struvite

In 1981, calcium oxalate was detected in only 2 percent of feline uroliths submitted to the Minnesota Urolith Center, whereas 78 percent of feline uroliths were composed of struvite. However, beginning in the mid-1980s, a rapid, substantial increase in the frequency of calcium oxalate uroliths occurred in association with a reciprocal decrease in the frequency of struvite uroliths (Figure 1).

Figure 1: Composition of feline uroliths

From 1994 to 2002, approximately 55 percent of the feline uroliths submitted to the Minnesota Urolith Center were composed of calcium oxalate, while only 33 percent were composed of struvite (Figures 1 and 2). During this period, the decline in the frequency of naturally occurring struvite uroliths associated with a reciprocal increase in calcium oxalate uroliths may have been associated with:

Figure 2: Change in frequency of feline struvite and calcium oxalate

1. Widespread use of a calculolytic food to dissolve struvite uroliths

2. 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), and

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

During 2004, the number of feline struvite uroliths (45 percent) submitted to the Minnesota Urolith Center nudged past those containing calcium oxalate (44 percent). These trends continued into 2010 (Figure 3 and Table 1).

Figure 3: Mineral composition of feline uroliths, 2010

Drop in dietary control

The increase in the frequency of feline struvite over calcium oxalate from 2003 to 2010 may be associated with decreased use of diets designed to dissolve sterile feline struvite uroliths as a consequence of the significant increase in calcium oxalate uroliths in the 1980s and 1990s.

Mineral composition of cat uroliths

The significance of struvite as the predominant mineral type emphasizes the importance of consideration of struvitolytic diet to manage cats diagnosed with urolithiasis. Currently, prescription diets are available that promote dissolution of struvite uroliths in a short period of time (Figures 4 and 5). It's likely that some, if not most, of the 5,631 feline struvite uroliths submitted to the Minnesota Urolith Center in 2010 could have been readily dissolved in one to four weeks by feeding diets designed to promote formation of urine that is undersaturated with struvite.

Figure 4: Survey lateral abdominal radiograph of a spayed female 11-year-old domestic shorthair cat.

However, not all uroliths can be dissolved with dietary manipulation. Uroliths that have not dissolved in a reasonable amount of time should be sent to a reputable laboratory for evaluation of mineral composition.

Figure 5: Survey abdominal radiograph taken three weeks after initiation of therapy with a struvitolytic diet.

Editor's note: This article was co-written by Jody P. Lulich, DVM, PhD, Dipl. ACVIM, the co-director of the Minnesota Urolith Center. The Center—with the support of an educational gift from Hill's Pet Nutrition as well as contributions from veterinarians and pet owners—provides quantitative urolith analysis at no charge. Online submission, e-mail notification and electronic retrieval of results are available. The 670,000-sample database offers veterinarians information on urolith trends, treatment and prevention suggestions. For details, visit urolithcenter.org.

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

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

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