Between January 1, 1981, and December 31, 2007, uroliths retrieved from 94,776 cats were submitted to the MUC for quantitative mineral analysis.1 Of these, 4,713 (5%) were primarily composed of purines (defined2 as the parent substance of the uric acid group of compounds, which includes ammonium urate), 43,707 (46%) were primarily composed of calcium oxalate (monohydrate and dihydrate), and 40,554 (43%) were primarily composed of struvite (magnesium ammonium phosphate hexahydrate). Only 2,445 (3%) of the uroliths were from the upper urinary tract (ie, nephroureteroliths).1
It is known that recurrence of uroliths is a frequent manifestation of urolithiasis. For example, in humans with urolithiasis, recurrence rates of 25% to 75% within 10 years after an initial episode of urolith formation have been reported.3 Although recurrence of uroliths after detection of the initial episode is thought to be a common event in cats, we are not aware of any reports of the frequency of recurrence on the basis of quantitative analysis of uroliths in this species.
The primary purposes of the study reported here were to determine the frequency of and interval until recurrence for ammonium urate, calcium oxalate, and struvite uroliths in cats for which uroliths were submitted to the MUC. Breed, age, and sex were also evaluated to determine whether these variables were associated with an increase or decrease in the risk for recurrence.
Materials and Methods
Sample population—Cats for which uroliths were submitted to the MUC for analysis comprised the study population. Case cats consisted of cats for which initial uroliths were submitted to the MUC between January 1 and December 31, 1998, and subsequent uroliths were submitted to the MUC between January 1, 1998, and December 31, 2003. Control cats consisted of cats for which initial uroliths were submitted to the MUC between January 1 and December 31, 1998, but without any subsequent uroliths submitted to the MUC between January 1, 1998 and December 31, 2003.
Urolith analysis—Uroliths were grouped on the basis of mineral composition. Mineral composition of uroliths was determined by use of optical crystallography and infrared spectroscopy.4 A urolith without a nidus or shell that contained ≥ 70% of a single mineral was identified by that mineral. A urolith without a nidus or shell that contained < 70% of any single mineral was referred to as a mixed urolith. A compound urolith was defined as having 1 or more layers of a mineral that differed from the mineral or minerals identified in the nidus or shell. In this study, records of cats with uroliths composed only of ammonium urate, calcium oxalate, or struvite were included.
Urolith location—Because ureteral uroliths originate in the kidneys, relevant information about nephroliths and ureteroliths was combined and categorized as upper urinary tract uroliths. Because urethral calculi originate primarily from the urinary bladder, relevant information about cystoliths and urethral calculi was combined and categorized as lower urinary tract uroliths.
Urolith recurrence—Urolith recurrence was defined as a subsequent submission or submissions of uroliths obtained from the same cat. To detect recurrences of uroliths in cats within a 5-year period, all submissions of uroliths to the MUC between January 1, 1998, and December 31, 2003, were searched for matching patient name and owner name of all case cats. Case cats were verified further by matching breed and sex at each submission. In addition, the age of case cats at the time of submission of a subsequent urolith had to be consistent with the interval between submissions. Additional uroliths for case cats resubmitted within 6 months of a previous submission were designated as incomplete urolith removal and were not counted as a recurrence.a Results of quantitative mineral analysis at each submission were recorded to evaluate continuity of urolith type between submissions.
Validation of recurrence—To validate the method for determining recurrence, a questionnaire was mailed to the veterinary clinics that submitted uroliths from all case cats whose initial uroliths were composed of ammonium urate. Other mineral types of uroliths were not included in the mailing list because of expense. This mail survey was designed to determine when ammonium urate case cats were subsequently evaluated, whether immediate postsurgical radiographs were evaluated, whether signs of urinary tract disease had redeveloped after removal of the initial urolith, and whether additional uroliths were removed but not submitted to the MUC for analysis.
Statistical analysis—Crude ORs and 95% CIs were calculated by use of the Woolf method5 with computer softwareb to assess whether breed, age (< 4, 4 to < 7, 7 to < 10, 10 to < 15, and ≥ 15 years old), and sex (male vs female) were risk factors for urolith recurrence. If any expected cell frequency in a contingency table was < 5, the Fisher exact test was used.5 Domestic longhair, domestic medium-hair, domestic shorthair, and mixed-breed cats were collectively grouped as mixed-breed cats. In this study, mixed breed, age < 4 years old, and males were set as the reference groups during statistical analysis. Age of each cat at the time of the initial urolith was considered for statistical analyses in age groups for recurrent and nonrecurrent urolith case cats. Intervals between the initial episode and recurrent episodes of uroliths were calculated.
The estimation of ORs was considered significant when the 95% CI for the OR did not include a value of 1.0.6 On the basis of the recommendations in another study,7 we classified significant ORs between 1.1 and 1.9 and between 0.5 and 0.9 as weak associations. Similarly, we interpreted significant ORs > 2 (ie, risk) or < 0.5 (ie, protective) as clinically (biologically) important that would require further prospective studies to support causality. Results were considered significant at values of P < 0.05.
Results
Sample population—In 1998, uroliths from 4,760 cats were submitted to the MUC for quantitative mineral analysis. We excluded 325 cats because the uroliths consisted of mineral other than ammonium urate, calcium oxalate, and struvite and their numbers were insufficient for meaningful statistical analyses. Of the remaining 4,435 cats, 221 had ammonium urate uroliths, 2,393 had calcium oxalate uroliths, and 1,821 had struvite uroliths (Table 1). Of the 4,435 cats, uroliths were located in the upper urinary tract (ie, nephroureteroliths) in 92 (2.1%), the lower urinary tract (ie, urocystoliths and urethroliths) in 4,236 (95.5%), and both the upper and lower urinary tract in 22 (0.5%); location of the remaining 85 (1.9%) uroliths was not specified. Of these 4,435 cats, recurrent uroliths were submitted from 242 (5.5%) cats between January 1, 1998, and December 31, 2003.
Numbers of cats with an initial episode of ammonium urate, calcium oxalate, and struvite uroliths and and recurrent episodes of a urolith.
| Urolith episode | Ammonium urate | Calcium oxalate | Struvite | Total |
|---|---|---|---|---|
| Initial | 221 | 2,393 | 1,821 | 4,435 |
| First recurrence | 29 | 169 | 49 | 247 |
| Same urolith type* | 28 | 163 | 41 | 232 |
| Other urolith type* | 1 | 6 | 8 | 15 |
| Second recurrence | 9 | 15 | 3 | 27 |
| Same urolith type* | 7 | 14 | 2 | 23 |
| Other urolith type* | 2 | 1 | 1 | 4 |
| Third recurrence | 0 | 2 | 0 | 2 |
| Same urolith type* | 0 | 2 | 0 | 2 |
Compared with the urolith type for the initial episode.
Recurrence for ammonium urate uroliths—Of 221 case cats with ammonium urate uroliths during the initial episode, 24 (10.9%) had a first recurrence of a urolith. Only 6 of the 221 (2.7%) cats had a second recurrence of a urolith.
To detect recurrent episodes of uroliths not submitted to the MUC, questionnaires were mailed in 2006 to clinics that submitted the uroliths associated with the initial episode. Of 221 questionnaires mailed, 121 (54.8%) were returned. First recurrence of uroliths was identified in an additional 5 cats. Of these 5 case cats, 3 had a second recurrence. With this additional information, 29 of 221 (13.1%) cats had a first recurrence of a urolith (Table 1).
Of 29 uroliths at first recurrence, 28 were composed of ammonium urate, and 1 was composed of struvite. Mean ± SD age of cats at the time of the initial urolith was 4.6 ± 1.6 years. Mean interval between submission of the initial ammonium urate urolith and first recurrence of a urolith was 1.8 ± 1.0 years. Compared with the reference group (ie, mixed-breed cats), only Persians had a significantly (P = 0.047) higher risk of a first recurrence of a urolith (Table 2). Compared with the reference group (ie, < 4 years old), no age group was at increased risk for recurrence of uroliths (Table 3). Female cats were 2.2 times as likely as male cats to have a first recurrence of a urolith; however, this difference was not significant (P = 0.069; Table 4). Two hundred thirteen (96.4%) of the initial ammonium urate uroliths were retrieved from the lower urinary tract, and 2 (0.9%) of the initial ammonium urate uroliths were retrieved from the upper urinary tract. Location of the initial ammonium urate uroliths was not specified in 6 (2.7%) cats. All of the first recurrent uroliths were retrieved from the lower urinary tract.
Crude ORs and 95% CIs for breed of 247 case cats* with urolith recurrence and 4,193 control cats without urolith recurrence.
| Initial urolith type | Breed | Recurrence (No. of cats) | No recurrence (No. of cats) | OR | 95% CI |
|---|---|---|---|---|---|
| Ammonium urate | |||||
| First recurrence | Mixed-breed | 23 | 171 | 1.00 | Reference category |
| Himalayan | 1 | 0 | NA | NA | |
| Persian | 3 | 4 | 5.58‡ | 1.17–26.51† | |
| Siamese | 1 | 5 | 1.49 | 0.17–13.30† | |
| Turkish Angora | 1 | 1 | 7.44 | 0.45–122.98† | |
| Calcium oxalate | |||||
| First recurrence | Mixed-breed | 132 | 1,690 | 1.00 | Reference category |
| Bengal | 1 | 0 | NA | NA | |
| Burmese | 3 | 13 | 2.95 | 0.83–10.50† | |
| European Shorthair | 2 | 20 | 1.28 | 0.30–5.54† | |
| Himalayan | 14 | 150 | 1.19 | 0.67–2.12 | |
| Maine Coon | 2 | 10 | 2.56 | 0.56–11.81† | |
| Manx | 2 | 7 | 3.66 | 0.75–17.79† | |
| Norwegian Forest Cat | 1 | 2 | 6.40 | 0.58–71.06† | |
| Persian | 9 | 201 | 0.57 | 0.29–1.14 | |
| Scottish Fold | 1 | 11 | 1.16 | 0.15–9.08† | |
| Siamese | 1 | 54 | 0.24 | 0.03–1.73† | |
| Unknown | 1 | 30 | 0.43 | 0.06–3.15† | |
| Second recurrence | Mixed-breed | 9 | 1,690 | 1.00 | Reference category |
| Burmese | 1 | 13 | 14.44 | 1.70–122.39† | |
| Himalayan | 2 | 150 | 2.50 | 0.54–11.69† | |
| Manx | 1 | 7 | 26.83‡ | 2.99–240.97† | |
| Norwegian Forest Cat | 1 | 2 | 93.89‡ | 7.80–1,130.47† | |
| Persian | 1 | 201 | 0.93 | 0.12–7.41† | |
| Struvite | |||||
| First recurrence | Mixed-breed | 45 | 1,559 | 1.00 | Reference category |
| European Shorthair | 1 | 18 | 1.92 | 0.25–14.73† | |
| Himalayan | 1 | 38 | 0.91 | 0.12–6.79† | |
| Persian | 2 | 66 | 1.05 | 0.25–4.42† | |
Represents 242 case cats originally identified by search of the MUC database and 5 additional case cats with initial ammonium urate uroliths identified by use of a questionnaire.
95% CI is not significant (P ≥ 0.05; Fisher exact test).
Value is significant (P < 0.05; Fisher exact test if expected cell value was < 5).
NA = Not applicable.
Crude ORs and 95% CIs for age of 247 case cats* with urolith recurrence and 4,193 control cats without urolith recurrence.
| Initial urolith type | Age (y) | Recurrence (No. of cats) | No recurrence (No. of cats) | OR | 95% CI |
|---|---|---|---|---|---|
| Ammonium urate | |||||
| First recurrence | < 4 | 5 | 53 | 1.00 | Reference category |
| 4 to < 7 | 13 | 56 | 2.46 | 0.82–7.38 | |
| 7 to < 10 | 6 | 53 | 1.20 | 0.34–4.17 | |
| 10 to < 15 | 5 | 23 | 2.30 | 0.61–8.74 | |
| ≥ 15 | 0 | 0 | NA | NA | |
| Unknown | 0 | 12 | NA | NA | |
| Calcium oxalate | |||||
| First recurrence | < 4 | 10 | 311 | 1.00 | Reference category |
| 4 to < 7 | 40 | 626 | 1.99 | 0.98–4.03 | |
| 7 to < 10 | 46 | 571 | 2.51 | 1.25–5.03 | |
| 10 to < 15 | 57 | 537 | 3.30 | 1.66–6.56 | |
| ≥ 15 | 9 | 44 | 6.36 | 2.45–16.52 | |
| Unknown | 7 | 135 | 1.61 | 0.60–4.33 | |
| Struvite | |||||
| First recurrence | < 4 | 2 | 537 | 1.00 | Reference category |
| 4 to < 7 | 22 | 552 | 10.70‡ | 2.50–45.73† | |
| 7 to < 10 | 15 | 364 | 11.07‡ | 2.52–48.68† | |
| 10 to < 15 | 9 | 164 | 14.74‡ | 3.15–68.88† | |
| ≥ 15 | 0 | 11 | NA | NA | |
| Unknown | 1 | 154 | 1.74 | 0.16–19.36† | |
See Table 2 for key.
Crude ORs and 95% CIs for sex of 247 case cats* with urolith recurrence and 4,193 control cats without urolith recurrence.
| Initial urolith type | Sex | Recurrence (No. of cats) | No recurrence (No. of cats) | OR | 95% CI |
|---|---|---|---|---|---|
| Ammonium urate | |||||
| First recurrence | Male | 11 | 110 | 1.00 | Reference category |
| Female | 17 | 79 | 2.15 | 0.96–4.85 | |
| Second recurrence | Male | 4 | 110 | 1.00 | Reference category |
| Female | 5 | 79 | 1.74 | 0.45–6.69† | |
| Calcium oxalate | |||||
| First recurrence | Male | 107 | 1,315 | 1.00 | Reference category |
| Female | 62 | 817 | 0.93 | 0.67–1.29 | |
| Second recurrence | Male | 8 | 1,315 | 1.00 | Reference category |
| Female | 7 | 817 | 1.41 | 0.51–3.90 | |
| Struvite | |||||
| First recurrence | Male | 16 | 728 | 1.00 | Reference category |
| Female | 33 | 980 | 1.53 | 0.84–2.81 | |
See Table 2 for key.
Nine of 221 (4.1%) cats with an initial ammonium urate urolith had a second recurrence of a urolith (Table 1). Of the 9 uroliths for the second recurrence, 7 were composed of ammonium urate, and 2 were composed of struvite. Mean ± SD interval between submission of the initial ammonium urate urolith and submission of the urolith for the second recurrence was 3.7 ± 0.7 years. The interval between recurrent episodes of uroliths was also calculated (Table 5). Only mixed-breed (n = 8) and Turkish Angora (1) cats had a second recurrent episode. Risk for a second recurrent episode on the basis of age was not determined because none of the cats < 4 years old (reference group) had a second recurrent episode. Female cats were 1.7 times as likely as male cats to have a second recurrent episode of a urolith; however, this difference was not significant (P = 0.499; Table 4). All of the uroliths for the second recurrent episodes were retrieved from the lower urinary tract.
Mean ± SD interval between recurrent episodes of uroliths in cats.
| Initial urolith type | Interval between recurrent episodes (y) | ||
|---|---|---|---|
| Initial and first | First and second | Second and third | |
| Ammonium urate | 1.8 ± 1.0 (29) | 2.4 ± 0.6 (9) | NA |
| Calcium oxalate | 2.1 ± 1.2 (169) | 1.3 ± 0.7 (15) | 1.4 ± 0.2 (2) |
| Struvite | 2.4 ± 1.3 (49) | 1.7 ± 0.5 (3) | NA |
Values in parentheses are the number of cats.
Recurrence for calcium oxalate uroliths—Of 2,393 cats with calcium oxalate uroliths during the initial episode, 169 (7.1%) had a first recurrence of a urolith (Table 1). Of the uroliths in 169 cats with a first recurrence, 163 were composed of calcium oxalate, and 6 were composed of struvite. Mean ± SD age of cats at the time of the initial urolith was 8.0 ± 2.0 years. Mean interval between submission of the initial urolith and first recurrence of a urolith was 2.1 ± 1.2 years. Compared with the reference group, no breed was at an increased risk for recurrent episodes of uroliths (Table 2). Cats in 3 age groups (7 to < 10, 10 to < 15, and ≥ 15 years old) had a significantly increased risk for a first recurrence (Table 3). Cats ≥ 15 years old had the highest risk of a first recurrent episode; they were 6 times as likely to develop a first recurrence as were cats < 4 years old. The recurrence rate between males and females did not differ significantly (P = 0.742; Table 4). Of the 2,393 initial calcium oxalate uroliths, 2,245 (93.8%) were retrieved from the lower urinary tract, and 81 (3.4%) were retrieved from the upper urinary tract; 19 (0.8%) uroliths were retrieved from both the upper and lower urinary tracts. Location of the uroliths was not specified in 48 (2.0%) cats. Of the first recurrence for 169 uroliths, 164 (97.0%) were retrieved from the lower urinary tract, and 3 (1.8%) were retrieved from the upper urinary tract; 2 (1.2%) uroliths were retrieved from both the upper and lower urinary tracts.
Fifteen (0.6%) cats had a second recurrence of uroliths (Table 1). Of the uroliths for the 15 second recurrent episodes, 14 were composed of calcium oxalate, and 1 was composed of struvite. Mean ± SD interval between submission of the initial calcium oxalate urolith and submission of uroliths for the second recurrence was 3.2 ± 1.0 years. The mean interval between recurrent episodes for uroliths was also calculated (Table 5). Compared with the reference group, only Manx and Norwegian Forest Cats had a significantly higher risk of a second recurrence of a urolith (Table 2). Risk for age groups with second recurrent episodes was not determined because cats < 4 years old (reference group) did not have any second recurrent episodes. Female cats were 1.4 times as likely as males to have a second recurrence of uroliths; however, this difference was not significant (P = 0.596; Table 4). All of the uroliths for the second recurrent episodes were retrieved from the lower urinary tract.
Only 2 of 2,393 (0.1%) cats with an initial calcium oxalate urolith had a third recurrence of uroliths. Uroliths from both cats were composed of calcium oxalate (Table 1). Mean ± SD interval between submission of the initial urolith and submission of the urolith for the third recurrence was 4.0 ± 1.1 years. Only mixed-breed male cats had a third recurrent episode (ie, 4 separate episodes of uroliths). For the third recurrent episode, 1 urolith was retrieved from the lower urinary tract and 1 urolith was retrieved from the upper urinary tract.
Recurrence for struvite uroliths—Of 1,821 case cats with an initial struvite urolith, 49 (2.7%) had a first recurrence of a urolith (Table 1). Of 49 uroliths for the first recurrence, 41 were composed of struvite, 5 were composed of calcium oxalate, and 3 were composed of ammonium urate. Mean ± SD age of cats at the time of the initial urolith was 5.6 ± 1.1 years. Mean interval between submission of the initial urolith and the first recurrence was 2.4 ± 1.3 years. Compared with the reference group, no breed was at an increased risk for a first recurrence (Table 2). Cats in 3 age groups (4 to < 7, 7 to < 10, and 10 to < 15 years old) had a significantly increased risk for a first recurrence (Table 3). Cats ≥ 10 but < 15 years old had the highest risk of a first recurrent episode; they were 15 times as likely to have first recurrence of a urolith as were cats < 4 years old. Female cats were 1.5 times as likely as male cats to have a first recurrence of a urolith; however, this difference was not significant (P = 0.188; Table 4). Of the 1,821 initial uroliths, 1,778 (97.6%) were retrieved from the lower urinary tract, and 9 (0.5%) were retrieved from the upper urinary tract; 3 (0.2%) uroliths were retrieved from both the upper and lower urinary tracts. Location of the uroliths was not specified in 31 (1.7%) cats. For the first recurrence, 47 (95.9%) uroliths were retrieved from the lower urinary tract; the location of the remaining 2 (4.1%) uroliths was not specified.
Only 3 of 1,821 (0.2%) cats had a second recurrence of a urolith (Table 1). Of these 3 uroliths for the second recurrence, 2 were composed of struvite, and 1 was composed of calcium oxalate. Mean ± SD interval between submission of the initial struvite urolith and submission of the urolith for the second recurrence was 3.3 ± 1.7 years. The interval between recurrent episodes for uroliths was also calculated (Table 5). Only mixed-breed female cats had a second recurrent episode. Risk for age groups with a second recurrence was not determined because cats < 4 years old (reference group) did not have a second recurrent episode. All of the uroliths for the second recurrence were retrieved from the lower urinary tract.
Uroliths without true recurrence—Subsequent to the original search of the database, 15 case cats were censored because they probably did not represent a true recurrence of a urolith. Of those 15 initial uroliths, 11 were composed of calcium oxalate, 3 were composed of struvite, and 1 was composed of ammonium urate. Mean ± SD age of cats at the time of the initial urolith was 7.3 ± 3.7 years. Mean interval between submission of the initial urolith and the presumed recurrence was 3.6 ± 0.9 years. All 15 cats had the same urolith type for the recurrent episode as for the initial urolith, which suggested that they were persistent uroliths or not true recurrent uroliths. Only 2 uroliths (both composed of calcium oxalate) were retrieved from the upper urinary tract; the remaining 13 uroliths were retrieved from the lower urinary tract.
Discussion
The interval between recurrent uroliths in cats has apparently not been evaluated in controlled studies. This may be related, in part, to the biological variability associated with the etiopathogenesis of underlying diseases, comorbidity of concurrent diseases, inappropriate selection and interpretation of diagnostic tests leading to inappropriate treatment, errors in owner observations and interpretations, lack of owner or patient compliance with treatment protocols, or difficulty in retrieving uroliths from the upper urinary tract.
Analysis of our data indicated that recurrent episodes of uroliths after an initial ammonium urate, calcium oxalate, and struvite urolith in cats were less common than recurrent episodes after an initial calcium oxalate urolith in dogs. In 1 studyc of 50 Bichon Frises, the 2-year recurrence rate after an intial calcium oxalate urolith was 57%. In another studyd of 33 dogs with initial calcium oxalate uroliths, recurrence rates were reported to be 36% after the first year for the initial urolith formation, 42% after the second year, 48% after the third year, and 52% after the sixth year. In our study, the rate of recurrence after an initial calcium oxalate urolith in cats was 169 of 2,393 (7.1%).
The low frequency of recurrence after struvite uroliths in cats was surprising. One factor that may have played a role in the low frequency of recurrence was the common use of effective dietary preventative strategies.8 Data did not consistently include dietary changes after the initial episode of uroliths in the cats reported here. In 1 study,9 dietary changes had a substantial effect on initial development or recurrence of struvite urethral plugs in cats. Another factor that may have reduced the frequency of recurrence was error associated with the questionnaire. We attempted to verify the number of recurrent uroliths after an initial ammonium urate urolith in cats; only 121 of 221 (55%) questionnaires were returned, which did yield an additional 5 cats with recurrent uroliths. Assuming the recurrence rate after an initial ammonium urate urolith in the remaining 100 (45%) cats of our survey group was similar, we would have expected to identify an additional 4 cats with recurrent uroliths. Initially, the recurrence rate was 24 of 221 (10.9%), which increased to 29 of 221 (13.1%) on the basis of results of the questionnaire. But assuming our assumption is valid, the projected recurrence rate for initial ammonium urate uroliths would increase to 33 of 221 (14.9%). These assumptions indicated that our study underestimated recurrence after initial ammonium urate, calcium oxalate, and struvite uroliths.
Additional uroliths submitted within 6 months after the preceding submission were designated as incomplete urolith removal and were not counted as a recurrence.a They were excluded from the study because we could not verify that they represented a true recurrence of a urolith. On the basis of our empirical evaluation of > 10,000 uroliths from cats and dogs, it is likely that calcium oxalate and purine uroliths usually require at least 6 months to recur.10 Our consultations with colleagues in general practices and specialty practices located throughout North America indicate that clinical signs initially attributed to episodes of recurrent uroliths often were detected in patients for which radiography or ultrasonography was not used immediately after a procedure to verify that all of the uroliths had been removed. In 1 study,a uroliths were detected in 4 of 20 cats radiographically evaluated 14 days after cystotomy. Also, in the study reported here, uroliths that were not truly a recurrence and that were removed during a second procedure were composed of the same mineral type as the initial uroliths that were removed. This observation, combined with the short interval between the initial and so-called first recurrent episodes, supports our concern that these cats may not have had recurrent uroliths.11 Infection-induced struvite uroliths have been detected within 2 weeks after urolith removal in Beagles.12 However, infection-induced struvite uroliths constitute an estimated 1% to 2% of the uroliths retrieved from cats and submitted to the MUC. Controlled studies of the biological behavior of naturally developing sterile struvite and calcium oxalate uroliths in cats are being conducted.
In this study, there was an association between recurrent episodes after initial calcium oxalate uroliths and struvite uroliths and older cats. These results are consistent with those of another study13 in which age was reported as a risk factor for urolith formation. Our study results also indicated that there may be an association between Persians and ammonium urate uroliths. Persians have been reported13 to be at an increased risk for formation of both calcium oxalate and struvite uroliths. Although results of our study indicated that Norwegian Forest Cats and Manx cats were at an increased risk for development of calcium oxalate uroliths, the number of these cats was small. In that aforementioned study,13 these 2 breeds were not affected by formation of calcium oxalate or struvite uroliths. Additional epidemiological studies are required to resolve this discrepancy.
In 232 of 247 (94.0%) of the first recurrent episodes, composition of ammonium urate, calcium oxalate, and struvite uroliths was identical to that of the initial urolith. Therefore, we conclude that the composition of an initial urolith may be used as an estimate of the composition of subsequent uroliths.
ABBREVIATIONS
| CI | Confidence interval |
| MUC | Minnesota Urolith Center |
| OR | Odds ratio |
Lulich JP, Osborne CA, Polzin D, et al. Incomplete removal of canine and feline urocystoliths by cycstotomy (abstr), in Proceedings. 11th Annu Am Coll Vet Intern Med Forum 1993;937.
PROC logistic, SAS, version 6.12, SAS Institute Inc, Cary, NC.
Lulich JP, Osborne CA, Daubs B, et al. Biological behavior of calcium oxalate uroliths in Bichon Frise dogs (abstr), in Proceedings. 22nd Annu Am Coll Vet Intern Med Forum 2004;861.
Lulich JP, Perrine L, Osborne CA, et al. Postsurgical recurrence of calcium oxalate uroliths in dogs (abstr), in Proceedings. 10th Annu Am Coll Vet Intern Med Forum 1992;802.
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