Since we've been analyzing uroliths in dogs and cats over the past three decades, the composition has varied. In this article, the third of three installments, we evaluate current trends of urethral plugs in cats and examine the implications for our feline patients.
A look at the numbers
It is important to determine the mineral composition of urethral plugs since most therapeutic regimens are based on this composition. Of 283 urethral plugs submitted to the Minnesota Urolith Center by veterinarians in 2011, the mineral composition of just over 90 percent was primarily sterile struvite (magnesium ammonium phosphate) (Table 1; Figures 1-3). Just under 2 percent were composed of calcium oxalate.
These data are not surprising. Since the founding of the Minnesota Urolith Center in 1981, struvite has consistently been the most common mineral in feline urethral plugs, while the prevalence of calcium oxalate in urethral plugs has always been infrequent (Table 1; Figures 1 and 2).
Figure 1: Note: MAP = struvite; CaOx = calcium oxalate; Cap = calcium phosphate.
There have been significant shifts in the prevalence of calcium oxalate and struvite in uroliths during the past 30 years, but the prevalence of struvite and calcium oxalate in feline urethral plugs has not significantly changed. Why this is occurring is not obvious to us. In recent years, there has also been a dramatic decline in the frequency of urethral plugs submitted for quantitative mineral analysis and a parallel decline in the frequency of perineal urethrostomies associated with urethral obstruction.
Figure 2: Note: MAP = struvite; Cap = calcium phosphate; CaOx = calcium oxalate.
What the findings mean for our patients
Since struvite continues to be the most common mineral type in urethral plugs, practitioners should consider struvitolytic and prevention foods to manage cats at risk for urethral obstruction caused by such plugs. Therapeutic diets are available that can minimize formation of struvite plugs by reducing the supersaturation of urine with magnesium ammonium phosphate. It is likely that most of the 283 struvite plugs submitted to the Minnesota Urolith Center in 2011 could have been prevented by feeding the cats these diets designed to promote formation of urine that is undersaturated with struvite (e.g., Prescription Diet c/d Multicare Feline—Hill's Pet Nutrition).
Figure 3: Note: MAP = struvite; CaOx = calcium oxalate.
Not all struvite urethral plugs can be prevented by dietary manipulation alone. In some cases dietary manipulation may be ineffective because of poor owner or patient compliance.
These data also tell us that we must remain vigilant. To help monitor future trends, be sure to send urethral plugs to a reputable laboratory that uses contemporary techniques for evaluating the mineral composition of uroliths and urethral plugs. Keep in mind that air-dried plugs are preferred for evaluating the crystalline component in urethral plugs.
Editor's note: The Minnesota Urolith 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, email notification and electronic retrieval of results are available. With access to our database of 730,000 samples, the veterinary community is offered the latest information on urolith trends, treatment and prevention suggestions. For details, visit
Dr. Osborne is the director of The Minnesota Urolith Center and a professor at the College of Veterinary 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. Dr. Nwaokorie recently completed his MS and is pursuing a PhD at the University of Minnesota.