Efficacy of two commercially available, low-magnesium, urine-acidifying dry foods for the dissolution of struvite uroliths in cats

Jody P. Lulich Minnesota Urolith Center, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, University of Minnesota, Saint Paul, MN 55105.

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 DVM, PhD, DACVIM
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John M. Kruger Department of Small Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824.

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Jennifer M. MacLeay Hill's Pet Nutrition Center, 1035 NE 43rd St, Topeka, KS 66617.

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Jane M. Merrills Department of Small Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824.

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Inke Paetau-Robinson Hill's Pet Nutrition Center, 1035 NE 43rd St, Topeka, KS 66617.

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Hasan Albasan Minnesota Urolith Center, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, University of Minnesota, Saint Paul, MN 55105.

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Carl A. Osborne Minnesota Urolith Center, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, University of Minnesota, Saint Paul, MN 55105.

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Abstract

Objective—To compare the efficacy and safety of using 2 commercially available, low-magnesium, urine-acidifying dry foods to dissolve sterile struvite uroliths in cats.

Design—Prospective, multicenter, randomized clinical trial

Sample—37 cats with presumed struvite uroliths.

Procedures—Cats were randomly assigned to be fed 1 of 2 low-magnesium, urine-acidifying dry foods (food A or B). For each cat, physical examination, urinalysis, and abdominal radiography were performed weekly to assess treatment response.

Results—32 cats had complete urolith dissolution. Mean ± SD times for a 50% reduction in urolith size (0.69 ± 0.1 weeks) and complete urolith dissolution (13.0 ± 2.6 days) were significantly shorter for cats fed food A, compared with those (1.75 ± 0.27 weeks and 27.0 ± 2.6 days, respectively) for cats fed food B. At study termination, mean ± SD urine pH (6.083 ± 0.105) for cats fed food A was lower than that (6.431 ± 0.109) for cats fed food B. In 5 cats, uroliths did not dissolve and were subsequently determined to be composed of 100% ammonium urate (n = 4) or 100% calcium oxalate (1). Adverse events associated with diet were not observed in any of the cats.

Conclusions and Clinical Relevance—Results indicated that dietary dissolution is safe and effective for eradication of sterile struvite uroliths in cats. Cats fed food A had faster urolith dissolution than did cats fed food B. Lack of a reduction in urolith size at 2 weeks after diet initiation was indicative of misdiagnosis or noncompliance.

Abstract

Objective—To compare the efficacy and safety of using 2 commercially available, low-magnesium, urine-acidifying dry foods to dissolve sterile struvite uroliths in cats.

Design—Prospective, multicenter, randomized clinical trial

Sample—37 cats with presumed struvite uroliths.

Procedures—Cats were randomly assigned to be fed 1 of 2 low-magnesium, urine-acidifying dry foods (food A or B). For each cat, physical examination, urinalysis, and abdominal radiography were performed weekly to assess treatment response.

Results—32 cats had complete urolith dissolution. Mean ± SD times for a 50% reduction in urolith size (0.69 ± 0.1 weeks) and complete urolith dissolution (13.0 ± 2.6 days) were significantly shorter for cats fed food A, compared with those (1.75 ± 0.27 weeks and 27.0 ± 2.6 days, respectively) for cats fed food B. At study termination, mean ± SD urine pH (6.083 ± 0.105) for cats fed food A was lower than that (6.431 ± 0.109) for cats fed food B. In 5 cats, uroliths did not dissolve and were subsequently determined to be composed of 100% ammonium urate (n = 4) or 100% calcium oxalate (1). Adverse events associated with diet were not observed in any of the cats.

Conclusions and Clinical Relevance—Results indicated that dietary dissolution is safe and effective for eradication of sterile struvite uroliths in cats. Cats fed food A had faster urolith dissolution than did cats fed food B. Lack of a reduction in urolith size at 2 weeks after diet initiation was indicative of misdiagnosis or noncompliance.

Successful dietary dissolution of a naturally occurring struvite urolith in a cat was first described in 1983.1 Since then, 3 case-series reports2–4 have been published that describe the efficacy of calculolytic diets for the dissolution of struvite uroliths in cats. Nevertheless, uroliths that are submitted for quantitative analysis are still frequently identified as struvite,5–7 which suggests that many cats with urolithiasis undergo invasive urolith extraction despite the fact that noninvasive dietary dissolution would have likely resolved the disease.

To our knowledge, studies to elucidate the reasons for selection of urolith extraction rather than dietary dissolution for the treatment of cats with urolithiasis have not been conducted; however, the choice of treatment for a cat with urolithiasis likely involves several factors. Selection of cats with uroliths amenable to dietary dissolution requires an accurate prediction of urolith composition by the use of abdominal radiography and other diagnostic information, such as urine pH and the presence of crystalluria. Although diagnostic algorithms for cats with urolithiasis are available8 and dietary dissolution can generally be attempted prior to urolith extraction in affected cats, some clinicians may perceive that surgery is less technically demanding and provides a more timely resolution of urolithiasis than does dietary dissolution. Also, cats with urolithiasis often have a history of signs of pain during urination and periuria, clinical signs that might prompt clinicians to opt for cystotomy to extract uroliths as a means to rapidly resolve the problem; however, clinical signs such as hematuria and dysuria may not be immediately resolved until several days after surgery.9 Some clinicians perceive urolith removal prior to its potential migration into the urethra as a management strategy to minimize the risk of urethral obstruction, especially in male cats. Other clinicians might anticipate that poor compliance with the dietary dissolution treatment, either because of patient refusal to eat the recommended food or noncompliance on the part of the owner, will result in prolonged disease for the patient and additional costs for the owner. Finally, some veterinary clinics may lack sufficient resources to maintain inventories of various calculytic diets, although this concern has been alleviated because individual foods that both dissolve and prevent struvite uroliths in cats are now commercially available.

The purpose of the study reported here was to evaluate the efficacy, safety, and speed with which 2 commercially available, low-magnesium, urine-acidifying dry foods dissolved naturally occurring, sterile struvite uroliths in cats. We hypothesized that both foods would dissolve struvite uroliths, dietary dissolution of uroliths in affected cats would coincide with resolution of clinical signs associated with urinary calculi, and no cat would develop urethral obstruction during the study.

Materials and Methods

Animals—Between April 2009 and January 2011, client-owned cats were recruited for the study through advertisements on cat-related email lists and in newspapers and trade journals that were distributed in the metropolitan areas of Minneapolis and Saint Paul, Minn, and East Lansing, Mich. Veterinarians in those areas were also encouraged to refer cats for the study. Neutered cats with clinical signs of lower urinary tract disease (eg, dysuria, hematuria, or periuria) were considered for inclusion in the study. An investigator conducted a telephone interview with each interested cat owner to determine the eligibility of each cat for study enrollment and discuss the study protocol and requirements. Following that interview, eligible cats were brought to either the Veterinary Medical Center at the University of Minnesota or the Veterinary Teaching Hospital at Michigan State University. Consent for evaluation and study enrollment was obtained from the owner of each cat. Cats were evaluated for the presence of urolithiasis as described8 by means of physical examination, CBC, serum biochemical analysis, survey abdominal radiography, urinalysis, and culture of a urine sample. A cat was enrolled in the study if it had radiographic evidence of struvite uroliths (ie, moderately radiopaque, round, or discoid stones with a smooth to slightly irregular contour) within the urinary bladder and was otherwise deemed healthy on the basis of results of the other diagnostic tests performed. A cat was excluded from the study if it had evidence of nephroliths, urethroliths, urethral obstruction, urinary tract infection, or systemic disease (eg, azotemia, liver disease, or anemia) or had been administered the following types of medications within 14 days before the initial evaluation: antimicrobials, antihistamines, steroidal or nonsteroidal anti-inflammatories, anticholinergics, antidepressants, urinary acidifiers, diuretics, diethyl sulf-oxide, or cyclophosphamide. Cats were also excluded from the study if they had been fed foods formulated to dissolve struvite uroliths or administered any vitamins, nutritional supplement-type products, or injectable long-acting corticosteroids or polysulfated glycosamino-glycans within 30 days before the initial examination.

Study protocol—All study protocols were reviewed and approved by the Institutional Animal Care and Use Committees of the University of Minnesota and Michigan State University. All study expenses, including those associated with urolith removal, were paid by an extramural grant. Additionally, owners of study cats received a stipend to help compensate them for their time and travel expenses associated with study participation.

Cats enrolled in the study were randomly assigned to 1 of 2 treatments: food A,a a commercially available dry cat food formulated to achieve a urine pH in the range of 5.9 to 6.1 and that causes the dissolution of struvite uroliths,2 or food B,b another commercially available dry cat food formulated to achieve a urine pH in the range of 6.2 to 6.4. On the basis of standards established by the American Association of Feed Control Officials, food A met the requirements for intermittent feeding and food B met the requirements for maintenance feeding. The nutrient profiles of the 2 foods are provided (Appendix). The calcium, fat, and fatty acid concentrations and the omega 3-to-omega 6 fatty acid ratio differed substantially between the 2 foods.

Each cat's owner and clinical care team (primary care veterinarian and veterinary-associated personnel) remained unaware of (ie, were blinded to) the assigned treatment throughout the study observation period. Both foods were identical in appearance and form (dry kibble). Following nutrient content analysis, the manufacturer packaged the foods specifically for the study; the foods were distinguished from each other on the basis of a colored (gray or peach) square on the front of the package, but were otherwise packaged identically and shipped to the 2 study centers.

For each cat, the assigned food was dispensed to its owner at the end of the initial examination. The daily amount of food intake required to maintain the cat's body weight was calculated as described,10 and owners were instructed to feed the cat only the assigned food. Feeding of the assigned food was to begin immediately after the initial examination without a gradual transition from the food normally fed to the cat. To improve study compliance, the amount of assigned food dispensed was sufficient to feed the study cat and all other healthy cats in the household.

Physical examination, urinalysis, and survey abdominal radiography were performed weekly for each cat to assess its response to treatment. Owners were not required to record food intake for each cat. Instead, during the weekly examination, owners were asked the following question to assess treatment compliance: “How confident are you that your cat is only eating the diet prescribed?” The owner's response was recorded on a numerical scale from 0 to 100, where 0 indicated that the cat refused to eat the prescribed food and 100 indicated that the cat ate only the prescribed food. For each cat, a CBC and serum biochemical analysis were performed at the end of the observation period and at any time during the observation period when deemed necessary by the attending investigator.

During the observation period, study cats did not receive any medication except buprenorphinec (0.015 mg/kg [0.033 mg/lb]) via buccal transmucosal administration every 8 to 12 hours as needed to alleviate clinical signs of pain. Buprenorphine was offered as an analgesic for all study cats and dispensed to the owners to be administered at their discretion. Additionally, each owner was instructed to bring their cat to the respective study center for immediate veterinary care if it became ill at any time during the observation period.

Owners could withdraw their cat from the study at any time for any reason. Cats were also withdrawn from the study if they would not consume the assigned treatment or were fed food other than the assigned treatment. Likewise, all study cats that required additional medical intervention were withdrawn from the study; however, those cats continued to receive appropriate care for management of urolithiasis.

Data collection—During each weekly examination, the following data were recorded: body weight, body condition score (on a scale of 1 to 5, with 1 indicative of emaciation and 5 indicative of obesity), results of urinalysis, urine pH as determined by a pH meter,d and results of evaluation of survey abdominal radiographs. All survey abdominal radiographs were digitally acquired and evaluated by board-certified veterinary radiologists who were unaware of the assigned treatment for each cat. Data were also recorded regarding secondary or adverse events associated with treatment such as urethral obstruction, incomplete urolith dissolution, and changes in serum biochemical values from those obtained during the initial examination.

The rate of urolith dissolution was monitored by the measurement of urolith diameter on the survey abdominal radiographs that were obtained weekly for each cat. Urolith diameter was selected for monitoring the rate of urolith dissolution because most sterile struvite uroliths are disk shaped8; therefore, diameter would be unaffected by urolith orientation in the bladder. For each urolith, the longest radiographic dimension was measured as the diameter, and for cats with more than 1 urolith, the diameters for all uroliths were summed to yield a single value. Each week, the percentage urolith dissolution was calculated for each cat by use of the following formula: 100 – (100 × [urolith diameter/urolith diameter at initial examination]). The time to 50% dissolution of uroliths was calculated as the mean week number of the last examination during which the urolith was > 50% of its original size and the first examination during which the urolith was < 50% of its original size. For example, if a urolith was > 50% of its original size at the beginning of week 2 and < 50% of its original size at the beginning of week 3, the time to 50% dissolution was 2.5 (ie, [2 + 3]/2). This method was chosen for calculation of the time to 50% urolith dissolution instead of the calculation of the mean time from treatment initiation to complete urolith dissolution because urolith dissolution is not a linear process (ie, small uroliths generally dissolved faster than did larger uroliths).

The time to complete urolith dissolution was defined as the number of days from treatment initiation until the uroliths were no longer visible on survey abdominal radiographs as determined by the board-certified radiologists. Study cats in which uroliths were unaffected by treatment were withdrawn from the study, and the owners were offered the option of having the uroliths surgically removed from their cats at no charge. Removed uroliths were quantitatively analyzed to determine their mineral composition. Cats with undissolved uroliths composed of struvite were categorized as treatment failures, whereas cats with undissolved uroliths not composed of struvite were categorized as diagnostic failures.

Statistical analysis—Descriptive statistics were calculated. Categorical variables (ie, body condition score, multicat household, hematuria, periuria, increased frequency of urination, vocalization during urination, and number of uroliths) were expressed as frequency counts or the mode and range. Continuous variables were expressed as mean ± SD, except when the data were not normally distributed, in which case the mode and range were provided. Data were plotted and visually evaluated to identify potential outliers and assess distribution normality, and the F-max test was used to assess equality of variances. For pretreatment descriptive variables, comparisons between cats in the 2 treatment groups were made with a 2-sample t test for continuous variables and a 2-tailed Fisher exact test for categorical variables. The efficacy (ie, treatment was successful) and rapidity of urolith dissolution (ie, time to complete urolith dissolution) were compared between the 2 treatment groups by means of ANCOVA. For each outcome, independent variables assessed in the model included age, body weight, and body condition score of cats; initial urolith size; single or multiple uroliths; urine pH and specific gravity at urolith dissolution; and study site. A Pearson correlation coefficient was calculated to assess the strength of the relationship between time to complete urolith dissolution and initial urolith size. All analyses were performed with commercially available statistical software,e and values of P < 0.05 were considered significant.

Results

Animals—On the basis of results of initial physical examination and diagnostic testing, 37 cats were considered healthy except for the presence of uroliths and enrolled into the study. Sixteen cats were assigned to be fed food A, and 21 cats were assigned to be fed food B. In 5 cats that were fed food B, the uroliths did not dissolve and were surgically removed and quantitatively analyzed. None of the removed uroliths were composed of struvite; the removed uroliths from 4 cats were composed of 100% ammonium urate, and the uroliths removed from the remaining cat were composed of 100% calcium oxalate. Those 5 cats were categorized as diagnostic failures and were excluded from further analyses. Complete urolith dissolution was achieved in the other 32 cats (Figure 1); thus, no cats were categorized as treatment failures. Sixteen cats were treated with food A, and 16 cats were treated with food B. Pretreatment descriptive characteristics did not differ significantly between the cats assigned to be fed food A and those assigned to be fed food B (Table 1).

Figure 1—
Figure 1—

Representative right lateral radiographic images of the caudal aspect of the abdomen of a 5-year-old male domestic shorthair cat with dysuria that were obtained during initial examination (A) and at 6 (B) and 13 (C) days after initiation of dietary dissolution treatment with food A, a commercially available, low-magnesium, urine-acidifying dry cat food formulated for the dissolution of struvite uroliths. Notice the 3 moderately radiopaque, disk-shaped objects (arrow) characteristic of struvite uroliths present in the urinary bladder in panel A, the reduced size and number of those objects (arrow) in panel B, and the complete absence of those objects in panel C.

Citation: Journal of the American Veterinary Medical Association 243, 8; 10.2460/javma.243.8.1147

Table 1—

Pretreatment descriptive characteristics for cats with presumed struvite uroliths that were treated with 1 of 2 commercially available, low-magnesium, urine-acidifying dry cat foods formulated for the dissolution of struvite uroliths (food A, n = 16; food B, 16) until the uroliths were completely dissolved.

 Treatment 
VariableFood AFood BP value
Age (mean ± SD; y)5.3 ± 0.584.3 ± 0.520.21
Sex (No.)  
 Female1411
 Male25
Weight (mean ± SD; kg [lb])5.4 ± 1.3 (11.9 ± 2.9)5.5 ± 1.1 (12.1 ± 2.4)0.71
Body condition score (mode [range])*4 (2–5)3 (3–5)0.79
Multicat household (No.)9140.13
Urine pH (mean ± SD)6.12 ± 0.16.34 ± 0.10.13
Urine specific gravity (mean ± SD)1.051 ± 0.0031.050 ± 0.0030.68
Hematuria (No.)15100.08
Periuria (No.)12100.70
Increased frequency of urination (No.)10130.71
Vocalization during urination (No.)331.0
Crystalluria  
 Struvite331.0
 Nonstruvite111.0
Urolith diameter (mean ± SD; mm)9.0 ± 4.89.9 ± 5.00.61
No. of uroliths  0.72
 1109
 223
 ≥ 344

For each cat, body condition score was subjectively assigned on a scale of 1 to 5, with 1 indicative of emaciation and 5 indicative of obesity.

Only amorphous crystals were observed.

— = Not determined.

None of the study cats developed urethral obstruction during the observation period. For all cats, all values assessed on CBC and serum biochemical analyses before treatment and at complete urolith dissolution were within reference ranges, and CBC and serum biochemical results obtained at complete urolith dissolution did not vary significantly from those obtained prior to treatment. Eight cats (3 fed food A and 5 fed food B) were administered buprenorphine to alleviate signs of pain during the study. A urinary tract infection was diagnosed in 1 cat that was fed food B at the examination, during which complete dissolution of the urolith was determined. Culture of 2 serial urine samples obtained from that cat prior to antimicrobial administration yielded growth of Pasteurella sp (> 105 colony-forming units/mL) and Pseudomonas sp (> 105 colony-forming units/mL). Administration of enrofloxacinf (5 mg/kg [11 mg/lb], PO, q 24 h for 7 days) successfully resolved the infection.

Treatment efficacy and rapidity—The mean ± SD time required for a 50% reduction in urolith size from initial size was 0.69 ± 0.1 weeks for cats fed food A and 1.75 ± 0.27 weeks for cats fed food B (Figure 2). The mean ± SD time to complete urolith dissolution was significantly (P = 0.002) shorter for cats fed food A (13.0 ± 2.6 days [range, 6 to 28 days]), compared with that for cats fed food B (27.0 ± 2.6 days [range, 7 to 52 days]). The time to complete urolith dissolution was not confounded by age, body weight, body condition score, the presence of multiple uroliths, urine specific gravity at dissolution, or study site; however, initial urolith size was positively correlated (r = 0.42; P = 0.02) with time to complete urolith dissolution. At the time complete urolith dissolution was determined, cats fed food A had a significantly (P = 0.029) lower urine pH (6.083 ± 0.105) than did cats fed food B (6.431 ± 0.109), whereas body weight, body condition score, and urine specific gravity did not differ significantly between cats in the 2 treatment groups.

Figure 2—
Figure 2—

Mean ± SD percentage urolith dissolution by week for cats with presumed struvite uroliths that were treated with 1 of 2 commercially available, low-magnesium, urine-acidifying dry cat foods (food A, n = 16 [squares]; or food B, 16 [circles]) and 5 cats fed food B (diamonds) that had uroliths composed of 100% ammonium urate (4) or 100% calcium oxalate (1).

Citation: Journal of the American Veterinary Medical Association 243, 8; 10.2460/javma.243.8.1147

Treatment compliance—For all study cats, 92 weekly examinations were performed during which owners indicated that their cats consumed only the assigned treatment with 99 ± 6% confidence (mode, 100%; range, 55% to 100%). During 1 examination for each of 2 cats that were fed food A, the owner indicated that the cat might have consumed < 95% of the assigned treatment; however, the time to complete urolith dissolution was < 14 days for both cats.

Discussion

Results of the present study supported our hypothesis that dietary dissolution is an effective, safe, and rapid method for the eradication of sterile struvite uroliths from the urinary bladder of cats and were similar to those of other studies.1–4 Both of the commercially available, low-magnesium, urine-acidifying dry cat foods evaluated in the present study resulted in complete dissolution of presumed struvite uroliths in 32 of 37 cats. The 5 cats in which complete urolith dissolution was not achieved had uroliths that were composed of ammonium urate or calcium oxalate instead of struvite and were therefore classified as diagnostic failures rather than treatment failures. Uroliths dissolved faster in cats that were fed food A, the food specifically formulated for struvite dissolution, than did uroliths in cats that were fed food B, the food formulated for prevention as well as dissolution of struvite uroliths. Also, the mean time for complete urolith dissolution for cats fed food A in the present study was shorter than that determined in other clinical trials that involved the same2 or other calculytic foods.3,4

Results of a study11 that was conducted in vitro indicate that increasing urine pH and magnesium concentration have the greatest influence on struvite precipitation. As urine pH increases, the proportion of the total phosphorus in its trivalent form also increases, which, along with increasing concentrations of magnesium and ammonium, eventually exceeds the urine saturation point for struvite, resulting in the precipitation of struvite and formation of uroliths. The magnesium content of foods A and B was identical; however, the mean urine pH at complete urolith dissolution for cats fed food A was significantly lower than that for cats fed food B. The induction of a lower urine pH induced by food A may be 1 possible explanation for the shorter mean time to complete urolith dissolution for cats fed food A, compared with that for cats fed food B.

Struvite uroliths in cats can be eliminated by invasive (eg, cystotomy), minimally invasive (eg, transurethral laser lithotripsy), or noninvasive (eg, nutritional) methods. Cystotomy remains a common method for removal of uroliths, and although major complications are rare, minor ones are common. Investigators of a retrospective study9 reported the development of complications following cystotomy in 46% (66/144) of dogs and cats; the complications for 97% (64/66) of those animals were considered minor and included hematuria, dysuria, and urinary incontinence. In that study,9 only 1 cat developed a major complication (ie, urethral obstruction because of failure to remove all uroliths during the cystotomy). In another study,g failure to remove all uroliths during cystotomy was detected in 5 of 20 cats. Other complications associated with surgical removal of uroliths may not become clinically apparent for several months to years. In a retrospective study12 that evaluated risk factors associated with recurrent uroliths in dogs and cats, 5% (99/2,036) of uroliths evaluated had visible suture material that served as a nidus. The investigators of that study12 hypothesized that exposure of suture in the lumen of the bladder was an important risk factor for urolith recurrence in veterinary patients following cystotomy. In the present study, the outcome for cats with urolithiasis that were treated with dietary dissolution versus cystotomy was not compared; however, no adverse events were detected in the 32 cats with presumed struvite uroliths in which dietary dissolution resulted in complete elimination of uroliths. Furthermore, anesthesia-associated risks are avoided for cats with urolithiasis that are managed with dietary dissolution.

Unlike surgical removal of uroliths, the success of dietary dissolution of uroliths is dependent on the accurate prediction of the composition of the uroliths. Although ultrasonography is more sensitive than radiography for detecting uroliths, we chose to use the radiographic appearance of uroliths to predict their mineral composition in this study because ultrasonography does not provide accurate data about the radiopacity or shape of the uroliths, information that is necessary for the prediction of urolith composition.13 Also, results of a study14 that was conducted in vitro suggest that urolith size is more accurate when measured by means of survey radiography, compared with that measured by means of ultrasonography. During urinalysis, the presence of struvite crystals or a neutral or basic urine pH are suggestive of struvite uroliths; however, results of the urinalysis performed during the initial examinations of the cats in the present study indicated that only 19% (6/32) and 40.6% (13/32) of cats had struvite crystalluria and a urine pH > 6.5, respectively.

In the present study, a diagnosis of struvite uroliths by means of abdominal radiography was incorrect for 5 of 37 (14%) cats, and this rate of diagnostic failure was similar to that of other studies in which the same (2/30 [7%])2 or similar (5/39 [13%] and 4/21 [19%])3,4 urolith dissolution foods were evaluated. In the present study, uroliths that were composed of urate were most frequently misdiagnosed as struvite uroliths, likely because the radiographic appearance (ie, few radiolucent to moderately radiopaque, round to ovoid stones with a smooth contour) of urate uroliths is similar to that of struvite uroliths.8 However, because the prevalence of struvite uroliths is substantially higher than that of urate uroliths in affected cats,5–7 the use of abdominal radiography to predict urolith composition will infrequently yield an incorrect diagnosis, as evidenced by the relatively low diagnostic failure rates of the present and other studies.2–4

To our knowledge, the present study was the first multicenter, double-blinded (ie, the owner and veterinary clinical care team were unaware of the treatment assigned to each cat), randomized, clinical trial to evaluate dietary dissolution of sterile struvite uroliths in cats. Additionally, this study was the first to measure urine pH with a pH meter, which allowed us to obtain more precise urine pH measurements than those obtained by other studies2–4 that used colorimetric dipsticks, which measure pH in 0.5 increments. Traditionally, feeding cats with urolithiasis a diet containing > 1.1% sodium on a dry-matter basis was recommended to encourage water consumption and thereby reduce the struvite saturation of urine. Both of the foods evaluated in the present study contained less than half (0.41% and 0.34%) of that recommended amount of sodium and resulted in complete dissolution of uroliths, which suggested that feeding a diet with a high sodium content to cats with uroliths is not necessary for successful resolution of urolithiasis. This finding was consistent with those of other studies,h,i in which the feeding of diets with 0.4% to 1.2% sodium on a dry-matter basis to healthy cats had no effect on urine supersaturation of and activity product ratios for struvite, despite a significant increase in urine volume from that before treatment.

Limitations of the present study included the unknown composition of the dissolved uroliths, the over-representation of female cats in the study population, the inability to definitively confirm that the study cats consumed only the assigned treatment, and potential error in the measurement of urolith diameter. We cannot absolutely affirm that the uroliths that dissolved in the 32 cats that responded to dietary dissolution treatment were composed of struvite because the noninvasive methods used to diagnose urolithiasis precluded quantitative analysis of the uroliths. Uroliths composed of biogenic minerals other than struvite were not expected to dissolve. We concede that uroliths composed primarily of struvite with small quantities of other minerals would dissolve; however, we cannot predict whether the proportion of other minerals present within a struvite urolith would affect the time required for its dissolution on the basis of the results of this study. In the present study, 78% (25/32) of the study cats with presumed struvite uroliths were female, whereas data from laboratories that conduct quantitative analysis of uroliths indicate that the proportion of cats with struvite uroliths that are female ranges from 57% (1,293/2,279)5 to 58% (4,105/7,077).15 Although disproportionately more female cats were represented in the population of the present study, the sex distribution is unlikely to affect our conclusions regarding treatment efficacy. However, the low number of male cats represented might have biased our conclusions regarding adverse events associated with dietary urolith dissolution because male cats are more likely to develop urethral obstruction secondary to urolithiasis owing to the smaller diameter of their urethra, compared with that of the urethra of female cats. During the present study, most owners reported that their cats consumed only the assigned food; nevertheless, a couple of owners did report that their cats might have consumed other types of food. Regardless, urolith dissolution was successfully achieved in the 32 cats presumed to have struvite uroliths, which suggested that minor deviations in food consumption would have minimal impact on the efficiency of dietary dissolution. Finally, in some cats with thin uroliths, measurement of urolith diameter might have been underestimated because of incomplete attenuation of x-rays at the margins of those uroliths.

Both of the commercially available, low-magnesium, urine-acidifying dry cat foods evaluated in the present study were successful in dissolving sterile struvite uroliths. The decision of which food to feed to cats with urolithiasis should be made on the basis of the individual needs of the patient, management circumstances of the household, and the likelihood that the owner will remain compliant with dietary and follow-up recommendations. Use of food B, which is formulated for long-term maintenance feeding, eliminates the need to transition cats back to a maintenance food following urolith dissolution and allows for the convenience of feeding all cats in a household a single food. Use of food A, which is formulated specifically for urolith dissolution and not for long-term maintenance feeding, may be advantageous when rapid urolith dissolution is necessary for the optimal well-being of the patient or a different food is indicated for long-term dietary management of other health problems (eg, obesity). Irrespective of the food selected for dietary dissolution of uroliths in cats, we recommend that survey abdominal radiographs should be obtained 2 weeks after initiation of treatment, at which time the size of the uroliths should have decreased by approximately 50% from the pretreatment size. If, after 2 weeks, urolith size has not changed or changed only minimally, it is likely that the owner or patient is noncompliant with dietary recommendations or the uroliths are primarily composed of minerals other than struvite.

a.

Prescription Diet s/d Feline Dissolution dry food, Hill's Pet Nutrition Inc, Topeka, Kan.

b.

Prescription Diet c/d Multicare Feline Bladder Health dry food, Hill's Pet Nutrition Inc, Topeka, Kan.

c.

Buprenorphine, Reckitt Benckiser Healthcare Ltd, Hull, Yorkshire, England.

d.

Corning pH meter 430, Corning Inc, Scientific Products Division, Corning, NY.

e.

SAS, version 9.2 for Windows, SAS Institute Inc, Cary, NC.

f.

Enrofloxacin, Bayer HealthCare LLC, Animal Health Division, Shawnee Mission, Kan.

g.

Lulich J, Osborne C, Polzin D, et al. Incomplete removal of canine and feline urocystoliths by cystotomy (abstr). J Vet Intern Med 1993;7:124.

h.

Xu H, LaFlamme DP, Bartges JW, et al. Effect of dietary sodium on urine characteristics in healthy cats (abstr). J Vet Intern Med 2006;20:738–739.

i.

Gluhek T, Bartges JW, Callens A, et al. Evaluation of 3 struvite-oxalate prevention diets in healthy cats (abstr). J Vet Intern Med 2012;26:801.

References

  • 1. Osborne CA, Abdullahi S, Polzin DJ, et al. Current status of medical dissolution of canine and feline uroliths, in Proceedings. 1983 Kal Kan Symp Treatment Small Anim Dis 1984; 5381.

    • Search Google Scholar
    • Export Citation
  • 2. Osborne CA, Lulich JP, Kruger JM, et al. Medical dissolution of feline struvite urocystoliths. J Am Vet Med Assoc 1990; 196: 10511063.

    • Search Google Scholar
    • Export Citation
  • 3. Houston DM, Rinkardt NE, Hilton J. Evaluation of the efficacy of a commercial diet in the dissolution of feline struvite bladder uroliths. Vet Ther 2004; 5: 187201.

    • Search Google Scholar
    • Export Citation
  • 4. Houston DM, Weese HE, Evason MD, et al. A diet with a struvite relative supersaturation less than 1 is effective in dissolving struvite stones in vivo. Br J Nutr 2011; 106:S90S92.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 5. Cannon AB, Westropp JL, Ruby AL. Evaluation of trends in urolith composition in cats: 5,230 cases (1985–2004). J Am Vet Med Assoc 2007; 231: 570576.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 6. Houston DM, Moore AEP. Canine and feline urolithiasis: evaluation of over 50,000 urolith submissions to the Canadian veterinary urolith centre from 1998 to 2008. Can Vet J 2009; 50: 12631268.

    • Search Google Scholar
    • Export Citation
  • 7. Osborne CA, Lulich JP, Nwaokorie E. Feline urolith epidemiology update: 1981 to 2011. DVM Newsmagazine. Available at: veterinarynews.dvm360.com/dvm/Veterinary+news/Feline-urolith-epidemiology-update/ArticleStandard/Article/detail/775263. Accessed Jul 25, 2012.

    • Search Google Scholar
    • Export Citation
  • 8. Lulich JP, Osborne CA. Changing paradigms in the diagnosis of urolithiasis. Vet Clin North Am Small Anim Pract 2009; 39: 7991.

  • 9. Thieman-Mankin KM, Ellison GW, Jeyapaul CJ, et al. Comparison of short-term complications between dogs and cats undergoing appositional single-layer or inverting double-layer cystotomy closure: 144 cases (1993–2010). J Am Vet Med Assoc 2012; 240: 6568.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10. Gross KL, Becvarova I, Armstrong PJ, et al. Feeding young adult cats: before middle age. In: Hand M, Thatcher K, Remillard R, et al, eds. Small animal clinical nutrition. 5th ed. Topeka, Kan: Morris Animal Institute, 2012; 373387.

    • Search Google Scholar
    • Export Citation
  • 11. Scott WD, Wrigley TJ, Webb KM. A computer model of struvite solution chemistry. Talanta 1991; 38: 889895.

  • 12. Appel SL, Lefebvre SL, Houston DM, et al. Evaluation of risk factors associated with suture nidus cystoliths in dogs and cats: 176 cases (1999–2006). J Am Vet Med Assoc 2008; 233: 18891895.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13. Weichselbaum RC, Feeney DA, Jessen CR, et al. Urocystolith detection: comparison of survey, contrast radiography and ultrasonographic techniques in an in vitro bladder phantom. Vet Radiol Ultrasound 1999; 40: 386400.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14. Bly KM, Kruger JM, Kinns J, et al. In vitro comparison of plain radiography, double contrast cystography, ultrasonography, and computer tomography for estimation of cystolith size. Am J Vet Res 2010; 71: 374379.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 15. Lekcharoensuk C, Lulich JP, Osborne CA, et al. Association between patient-related factors and risk of calcium oxalate and magnesium ammonium phosphate urolithiasis in cats. J Am Vet Med Assoc 2000; 217: 520525.

    • Crossref
    • Search Google Scholar
    • Export Citation

Appendix

Comparison of key nutritional variables on a dry-matter basis between 2 commercially available, low-magnesium, urine-acidifying dry cat foods formulated for the dissolution of struvite uroliths.

VariableFood AFood B
Protein (%)35.1436.96
Fat (%)26.8816.07
Crude fiber (%)0.760.64
Calcium (%)1.120.68
Phosphorus (%)0.840.73
Magnesium (%)0.070.07
Sodium (%)0.410.34
Potassium (%)0.870.83
Docosahexaenoic acid (%)00.15
Eicosapentaenoic acid (%)0.010.23
n–6:n–3 ratio245
Vitamin E (U/kg)1,1021,251
Tryptophan (%)0.300.29
  • Figure 1—

    Representative right lateral radiographic images of the caudal aspect of the abdomen of a 5-year-old male domestic shorthair cat with dysuria that were obtained during initial examination (A) and at 6 (B) and 13 (C) days after initiation of dietary dissolution treatment with food A, a commercially available, low-magnesium, urine-acidifying dry cat food formulated for the dissolution of struvite uroliths. Notice the 3 moderately radiopaque, disk-shaped objects (arrow) characteristic of struvite uroliths present in the urinary bladder in panel A, the reduced size and number of those objects (arrow) in panel B, and the complete absence of those objects in panel C.

  • Figure 2—

    Mean ± SD percentage urolith dissolution by week for cats with presumed struvite uroliths that were treated with 1 of 2 commercially available, low-magnesium, urine-acidifying dry cat foods (food A, n = 16 [squares]; or food B, 16 [circles]) and 5 cats fed food B (diamonds) that had uroliths composed of 100% ammonium urate (4) or 100% calcium oxalate (1).

  • 1. Osborne CA, Abdullahi S, Polzin DJ, et al. Current status of medical dissolution of canine and feline uroliths, in Proceedings. 1983 Kal Kan Symp Treatment Small Anim Dis 1984; 5381.

    • Search Google Scholar
    • Export Citation
  • 2. Osborne CA, Lulich JP, Kruger JM, et al. Medical dissolution of feline struvite urocystoliths. J Am Vet Med Assoc 1990; 196: 10511063.

    • Search Google Scholar
    • Export Citation
  • 3. Houston DM, Rinkardt NE, Hilton J. Evaluation of the efficacy of a commercial diet in the dissolution of feline struvite bladder uroliths. Vet Ther 2004; 5: 187201.

    • Search Google Scholar
    • Export Citation
  • 4. Houston DM, Weese HE, Evason MD, et al. A diet with a struvite relative supersaturation less than 1 is effective in dissolving struvite stones in vivo. Br J Nutr 2011; 106:S90S92.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 5. Cannon AB, Westropp JL, Ruby AL. Evaluation of trends in urolith composition in cats: 5,230 cases (1985–2004). J Am Vet Med Assoc 2007; 231: 570576.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 6. Houston DM, Moore AEP. Canine and feline urolithiasis: evaluation of over 50,000 urolith submissions to the Canadian veterinary urolith centre from 1998 to 2008. Can Vet J 2009; 50: 12631268.

    • Search Google Scholar
    • Export Citation
  • 7. Osborne CA, Lulich JP, Nwaokorie E. Feline urolith epidemiology update: 1981 to 2011. DVM Newsmagazine. Available at: veterinarynews.dvm360.com/dvm/Veterinary+news/Feline-urolith-epidemiology-update/ArticleStandard/Article/detail/775263. Accessed Jul 25, 2012.

    • Search Google Scholar
    • Export Citation
  • 8. Lulich JP, Osborne CA. Changing paradigms in the diagnosis of urolithiasis. Vet Clin North Am Small Anim Pract 2009; 39: 7991.

  • 9. Thieman-Mankin KM, Ellison GW, Jeyapaul CJ, et al. Comparison of short-term complications between dogs and cats undergoing appositional single-layer or inverting double-layer cystotomy closure: 144 cases (1993–2010). J Am Vet Med Assoc 2012; 240: 6568.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10. Gross KL, Becvarova I, Armstrong PJ, et al. Feeding young adult cats: before middle age. In: Hand M, Thatcher K, Remillard R, et al, eds. Small animal clinical nutrition. 5th ed. Topeka, Kan: Morris Animal Institute, 2012; 373387.

    • Search Google Scholar
    • Export Citation
  • 11. Scott WD, Wrigley TJ, Webb KM. A computer model of struvite solution chemistry. Talanta 1991; 38: 889895.

  • 12. Appel SL, Lefebvre SL, Houston DM, et al. Evaluation of risk factors associated with suture nidus cystoliths in dogs and cats: 176 cases (1999–2006). J Am Vet Med Assoc 2008; 233: 18891895.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13. Weichselbaum RC, Feeney DA, Jessen CR, et al. Urocystolith detection: comparison of survey, contrast radiography and ultrasonographic techniques in an in vitro bladder phantom. Vet Radiol Ultrasound 1999; 40: 386400.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14. Bly KM, Kruger JM, Kinns J, et al. In vitro comparison of plain radiography, double contrast cystography, ultrasonography, and computer tomography for estimation of cystolith size. Am J Vet Res 2010; 71: 374379.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 15. Lekcharoensuk C, Lulich JP, Osborne CA, et al. Association between patient-related factors and risk of calcium oxalate and magnesium ammonium phosphate urolithiasis in cats. J Am Vet Med Assoc 2000; 217: 520525.

    • Crossref
    • Search Google Scholar
    • Export Citation

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