A look at composition provides insight into ways to prevent plugs in the first place.
In the past three decades, the composition of uroliths in cats and dogs has been variable. In this article, the third of three installments, we evaluate current trends of feline urethral plugs to determine their implications for our patients.
The mineral composition of urethral plugs should be used to describe them since most therapeutic regimens have been based on their mineral composition. Of 362 urethral plugs submitted to the Minnesota Urolith Center by veterinarians in 2010, the mineral composition of about 90 percent primarily was sterile struvite (magnesium ammonium phosphate) (Table 1; Figures 1-3). Approximately 2 percent were composed of calcium oxalate.
Table 1 Mineral composition of 362 feline urethral plugs, 2010*
These data are consistent with the fact that since 1981 (the birth date of the Minnesota Urolith Center), struvite has consistently been the most common mineral in feline urethral plugs. In contrast, the prevalence of calcium oxalate in urethral plugs always has been infrequent (Table 1; Figures 1 and 2).
Figure 1: Note: MAP = struvite; CaOx = calcium oxalate; Cap = calcium phosphate.
Why have there been significant shifts in the prevalence of calcium oxalate and struvite in uroliths during the past 30 years, while the prevalence of struvite and calcium oxalate in feline urethral plugs has not significantly changed? The answer is not obvious to us. Whatever the reasons, in recent years there has been a dramatic decline in the frequency of urethral plugs submitted for quantitative mineral analysis. And there's been a parallel decline in the frequency of perineal urethrostomies associated with urethral obstruction.
Figure 2: Note: MAP = struvite; CaPO4 = calcium phosphate; CaOx = calcium oxalate.
The significance of struvite as the predominant mineral type in urethral plugs emphasizes the importance of considering struvitolytic and prevention foods to manage cats at risk for urethral obstruction caused by plugs. Currently, therapeutic diets are available that minimize formation of struvite plugs by reducing the supersaturation of urine with magnesium ammonium phosphate.
Extended study: Taking the time to analyze the mineral composition of urethral plugs in cats over time can help develop ways to treat and, hopefully, prevent them in the future.
It's likely that most of the 362 cats with obstructive uropathy associated with struvite plugs submitted to the Minnesota Urolith Center in 2010 could have been prevented by feeding 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 are prevented by dietary manipulation. The apparent ineffectiveness of dietary manipulation in some cats may be due to poor owner and/or patient compliance. We must remain vigilant. One important way to monitor future trends in the mineral composition of urethral plugs is to send them to a reputable laboratory that uses contemporary techniques for evaluating the mineral composition of uroliths and urethral plugs.
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 675,000 samples, the veterinary community is offered the latest information on urolith trends, treatment and prevention suggestions. For details, visit urolithcenter.org.
Carl A. Osborne, DVM, PhD, Dipl. ACVIM, is professor of medicine at the College of Veterinary Internal Medicine at the University of Minnesota. Jody P. Lulich, DVM, PhD, Dipl. ACVIM, is the co-director of The Minnesota Urolith Center and professor of Veterinary Internal Medicine at the University of Minnesota.