Feline urolith epidemiology update: 1981 to 2012

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In the past three decades, urolith composition in cats and dogs has varied, while feline urethral plug composition has remained consistent. In this article, we evaluate trends in feline urolith composition to determine what may be causing this disparity and examine the implications for our patients. Epidemiology of feline urethral plugs will be discussed in a future article.

Calcium oxalate versus struvite uroliths

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 (magnesium ammonium phosphate). But in the mid-1980s, a rapid, substantial increase in the frequency of calcium oxalate uroliths occurred, while the frequency of struvite uroliths decreased (Figure 1).

From 1994 to 2002, about 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).

Figure 1: Mineral composition of feline uroliths 1981-2012. CaOx=calcium oxalate; MAP=struvite; Cap=calcium phosphate. (All figures and photos courtesy of Dr. Carl Osborne.)

Figure 2: Change in frequency of feline struvite and calcium oxalate uroliths. CaOx=calcium oxalate; MAP=struvite.

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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 due to:

• Widespread use of a calculolytic food to dissolve struvite uroliths

• 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)

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

In 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 have continued into 2012 (Figure 3).

Figure 3: Mineral composition of feline uroliths. CaOx=calcium oxalate; MAP=struvite; CaPO4=calcium phosphate.

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Observations on global differences

When the epidemiological data for 2012 feline urolith submissions to the Minnesota Urolith Center was evaluated based on geographic location (continent), some significant differences are noted.Struvite uroliths were more common in feline submissions from Australia and Oceania and North America. Calcium oxalate uroliths were more common in Africa and Europe. The percentages of calcium oxalate and struvite uroliths submitted from colleagues in Asia and South America were about equal (Figure 4).

Figure 4: Feline urolith composition by continent 2012.

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The role of diet control

The increase in the frequency of feline struvite over calcium oxalate from 2003 to 2012 may be associated with a 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.

Photo 1: A calcium oxalate urolith in a 10-year-old spayed female cat. The markings represent millimeters.

Photo 2: A struvite urolith in a 3-year-old spayed female cat. The markings represent millimeters.

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The significance of struvite as the predominant mineral type emphasizes the importance of considering a struvitolytic diet to manage cats at risk for struvite urolithiasis. Currently, therapeutic diets are available that promote struvite urolith dissolution in a short period. In a double-blind study, a therapeutic diet (Prescription Diet c/d Multicare Feline—Hill’s Pet Nutrition) performed as expected in the dissolution of sterile struvite uroliths.1

It’s likely that some, if not most, of the 6,317 feline struvite uroliths submitted to the Minnesota Urolith Center in 2012 could have been readily dissolved in one to four weeks by feeding diets designed to promote formation of urine that is undersaturated with struvite.

However, not all uroliths can be dissolved with dietary manipulation. Those uroliths that have not shown evidence of dissolution in three to four weeks (provided the owner is compliant with feeding instructions) should be removed and sent to a reputable laboratory for evaluation of mineral composition.

We are still searching for a veterinary diet that is safe and consistently effective in minimizing calcium oxalate uroliths.

Author’s note: With the support of an educational gift from Hill’s Pet Nutrition, as well as contributions from veterinarians and pet owners, the Minnesota Urolith Center provides quantitative urolith analysis at no charge. Online submission, e-mail notification and electronic retrieval of results are available.

With access to our database of 830,000 samples, the veterinary community is offered the latest information on urolith trends, treatment and prevention suggestions. For details, visit www.cvm.umn.edu/depts/MinnesotaUrolithCenter/.

Reference

1. Lulich JP, et al. Struvite urolith dissolution in cats: a double blind randomized clinical trial of 2 foods, in Proceedings. Am Coll Vet Intern Med Forum, June 2011.

Dr. Osborne is director 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 is a post-doctoral associate in the Veterinary Clinical Sciences Department at the College of Veterinary Medicine at the University of Minnesota.

Dr. Hunprasit is a Ph.D. candidate in the Veterinary Clinical Sciences Department at the College of Veterinary Medicine at the University of Minnesota.