Trends change in composition of feline uroliths

Epidemiology of feline uroliths: 1981-2002

In 1981, calcium oxalate was detected in only 2 percent of feline uroliths submitted to the Minnesota Urolith Center, whereas struvite was detected in 78 percent.

Table 1. Feline urolith distribution 1981-2004

However, beginning in the mid-1980s, a dramatic increase in the frequency of calcium oxalate uroliths occurred in association with a decrease in the frequency of struvite uroliths (Table 1). 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. During this period, the decline in appearance of naturally occurring struvite uroliths associated with a reciprocal increase in calcium oxalate uroliths may have been associated with:

• The widespread use of a calculolytic diet designed to dissolve struvite uroliths,

• Modification of maintenance and prevention diets 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 evaluation of efficacy of dietary management protocols by urinalysis.

Epidemiology of feline uroliths: 2003-2004

In 2003, the frequency of calcium oxalate uroliths declined to 47 percent, while the frequency of struvite uroliths increased to 42 percent. During 2004, the number of struvite uroliths (44.9 percent) submitted to the Minnesota Urolith Center nudged past those containing calcium oxalate (44.3 percent) in frequency of occurrence (Table 2). The decrease in occurrence of naturally occurring calcium oxalate uroliths during the past two years may be associated with:

• Reformulation of adult maintenance diets to minimize risk factors for calcium oxalate crystalluria,

• Improvements in formulation of therapeutic diets designed to reduce risk factors for calcium-oxalate uroliths, and

• Increased use of therapeutic diets designed to reduce risk factors for calcium oxalate uroliths.

The increase in appearance of naturally occurring struvite uroliths during the past two years may be associated with the reciprocal relationship between dietary risk factors for calcium oxalate and struvite uroliths. Diets that reduce urine acidity and provide adequate quantities of magnesium reduce the risk of calcium oxalate urolith formation, but increase the risk of struvite (magnesium ammonium phosphate) urolith formation. In addition, the increase in struvite urolith occurrence in 2003 and 2004 may be associated with decreased use of diets designed to dissolve sterile struvite uroliths as a consequence of the significant increase in occurrence of calcium oxalate uroliths in the 1980s and 1990s. However, it is likely that many of the 3,915 sterile struvite uroliths obtained from cats and submitted to the Minnesota Urolith Center in 2004 could have been readily dissolved by feeding a diet designed to promote formation of urine that is undersaturated with struvite. The following case report is a typical example.

Case report

Database: A 4.5-year-old spayed female domestic shorthair cat was referred to the Veterinary Teaching Hospital at the University of Minnesota because of gross hematuria and dysuria of four weeks' duration. Urocystoliths of unknown composition had been removed surgically when the cat was 1.5 years old. Since that time, the cat had been fed a commercially prepared dry adult maintenance diet.

Physical examination revealed that the cat was in good physical condition. She weighed 13 pounds. Rectal temperature, pulse rate and rhythm, and respiration rate and character were normal. Palpation of the abdomen revealed that the urinary bladder was thickened, contracted and painful. A grating sensation characteristic of uroliths was detected within the bladder lumen. Micturition induced during palpation revealed gross hematuria. No other abnormalities were detected.

Analysis of a urine sample collected by cystocentesis revealed findings typical of inflammation (i.e. hematuria, proteinuria and pyuria) and struvite crystalluria. The urine specific gravity was 1.050; the urine pH was 7.5 (reagent strip). Aerobic bacteria were not detected by the conventional quantitative culture method. A complete blood count and serum biochemistry profile were normal.

Problem list

Urocystoliths associated with inflammation of the lower urinary tract were identified on the basis of the history, physical examination and laboratory test results.

Initial diagnostic plans

To evaluate the entire urinary tract for evidence of uroliths and other possible causes of inflammation, survey abdominal radiography and double contrast cystography were planned.

Follow-up studies

Survey radiographs of the abdomen revealed two elliptical, radiodense uroliths in the bladder lumen (Figure 1). Double contrast cystography revealed slight thickening of the bladder wall and irregularity of the bladder mucosa (Figure 2).

Figure 1: Survey lateral abdominal radiograph of a 4.5 year-old spayed female domestic shorthair cat illustrating tow radiodense urocystoliths.

On the basis of available data, what is the probable mineral composition of the uroliths? The following observations were interpreted to indicate that the urocystoliths were composed of sterile struvite:

• Radiographic density of the uroliths,

• Struvite crystalluria,

• Alkaline urine pH,

• Negative in vitro culture of urine for aerobic bacteria.

Medical dissolution of uroliths

How would you manage this patient if it were you, a family member or your cat? Would you recommend surgery, medical dissolution or a combination of the two? In a prospective clinical trial of medical dissolution of feline struvite urocystoliths in 20 cats performed at the Minnesota Urolith Center, consumption of a magnesium restricted diet designed to promote formation of acid urine (Prescription Diet Feline s/d; Hill's Pet Nutrition Inc.) resulted in dissolution of struvite uroliths in four to five weeks (JAVMA 196: 697-733, 1990). After informing the client of the benefits and risks associated with surgery and medical management, the client requested medical management.

Figure 2: Lateral view of a double contrast cystogram illustrating the urocystoliths described in Figure 1.

Accordingly, the owners were instructed to feed the canned formulation of Feline s/d in sufficient quantity to maintain stable body weight. Plans were to continue this regimen of therapy for one additional "insurance month" following survey radiographic confirmation of urolith dissolution.

The owner indicated that the clinical signs of hematuria and dysuria gradually subsided during a two-week period following initiation of dietary therapy. Thirteen days after initiation of therapy, physical examination revealed no abnormalities. Evaluation of a urine sample collected by cystocentesis revealed that the urine specific gravity was 1.026 and the pH was 6.5. Crystalluria was not observed. Although the urine was yellow in color, hematuria was detected by microscopic examination of urine sediment. These findings (acid urine pH, no struvite crystalluria, and reduction in specific gravity) indicate owner and patient compliance with dietary recommendations. Survey radiography revealed that the urocystoliths were approximately one-third their original size (Figure 3). The owners were shown the pre- and post-treatment radiographs to reinforce the need for continued therapy, and to praise them for compliance with management recommendations. They were advised to continue therapy with the calculolytic diet.

Figure 3: Survey lateral abdominal radio-graph of the cat described in Figure 1 obtained 13 days after initiation of management with a struvitolytic diet. The urocystoliths are about one-third their premanagement size.

The cat was re-evaluated five weeks after initiation of therapy. During the three-week interval between evaluations, the cat remained asymptomatic. Its micturition pattern was normal. Evaluation of a urine sample collected by cystocentesis revealed no abnormalities (SG=1.049, pH=6.0). Uroliths were not detected by survey radiography (Figure 4, ). At that time, no evidence of hematuria, inflammation or crystalluria was detected by urinalysis; the urine pH was 6.0. After the insurance month of dietary management, therapy with Prescription Diet s/d was discontinued.

Figure 4: Survey lateral abdominal radiograph of the cat described in Figure 1 obtained five weeks after initiation of management with a struvitolytic diet. There are no radiodense uroliths.

Prevention of recurrence

Although there have been no reports of controlled studies designed to evaluate the frequency of recurrence of struvite uroliths, the general consensus of opinion based on clinical experience is that recurrence is a common but unpredictable event. Therefore, the owners were advised of the availability of diets designed to minimize several risk factors associated with formation of struvite uroliths.

Table 2. Mineral composition of 8,711 feline uroliths and 524 urethral plugs submitted to the Minnesota Urolith Center during 2004

In our clinical experience, diets formulated to reduce urinary concentration of magnesium and to acidify urine to a pH of approximately 6.0 to 6.3 are effective in minimizing recurrence of naturally occurring sterile struvite urocystoliths in male and female cats. No attempt was made to determine whether acidification and/or low magnesium diets were the major factors(s) responsible for beneficial results. Therefore, we recommended that the cat be fed a high-moisture (canned) struvite prevention diet. We emphasized to the owner that reduction of some risk factors for formation of struvite crystals, including promoting formation of less alkaline or more acidic urine, is one of several risk factors for calcium oxalate urolithiasis. Therefore, we recommended periodic re-evaluation of the patient. In the event that significant calcium oxalate crystalluria developed, appropriate adjustments in dietary management would be necessary.

Key point

Epidemiological data from the Minnesota Urolith Center revealed a dramatic increase in the occurrence of feline calcium oxalate uroliths associated with a decline in struvite uroliths for more than a decade. But during the past two years, the trend has been a decline in the number of calcium oxalate uroliths associated with an increase in the number of struvite uroliths. We hypothesize that dietary factors are influencing these changes.

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.

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