Introduction
Urolithiasis in ferrets (Mustela putorius furo) may cause stranguria, hematuria, vocalization on urination, urinary incontinence, and life-threatening urethral obstruction.1,2 Postrenal obstruction is especially common in male ferrets because of the os penis and narrow diameter of the penile portion of the urethra.1 A study3 of ferrets, including 408 with uroliths analyzed between 1981 and 2007, shows that struvite uroliths were identified most commonly (272 [67%]), followed by cystine (61 [15%]) or calcium oxalate (43 [11%]) uroliths.3 Although urolithiasis in ferrets has been reported2 as becoming less common owing to improved commercial diets, the incidence of cystine uroliths in ferrets has been reporteda as increasing in North America, with grain-free diets as a putative underlying cause.
Cystinuria has been related to variants of the genes SLC3A1 and SLC7A9 in dogs,4,5 cats,6,7 mice,8 and humans.9,10 Similarly, because ferret populations and breeding practices vary around the world, genetic differences may contribute to differences in the proportions of urolith types identified among ferret populations from separate geographic regions. Therefore, a first step in elucidating the underlying cause of potential discrepancies for cystine urolithiasis versus other types of urolithiasis in ferrets from separate regions is to establish a current global epidemiological profile for urolith mineral composition in ferrets. The objectives of the study reported here were to compare mineral types of naturally occurring uroliths in ferrets from North America, Europe, and Asia and to identify potential risk factors associated with cystine urolithiasis in ferrets.
Materials and Methods
Power analysis
For the primary outcome, we were interested in whether the period prevalence of cystine urolithiasis versus other types of urolithiasis differed for ferrets from North America versus Europe or Asia. A sample size calculation was tailored on the primary outcome. To our knowledge, there was no current data on the difference in the prevalence of cystine urolithiasis in ferrets at low versus high risk of developing cystine uroliths. Thus, on the basis of published literature on carnivores at low risk (eg, mixed-breed dogs)11 and high risk (eg, Bulldogs and servals)12,13 of developing cystine uroliths, we expected a 0.5% to 8%4,11,14,15 prevalence rate of cystine uroliths in ferrets at low risk of cystinuria and a 27% to 30%12,13 prevalence rate of cystine uroliths in ferrets at high risk. The power calculation was tailored on the small possible difference of 19% (ie, 8% to 27%) to ensure proper power. Additionally, we expected fewer submissions from Europe and Asia, compared with North America, given the workloads of the participating laboratories. This anticipated difference in submissions was accounted for with a Europe and Asia-to-North America submission ratio of 1:20. Therefore, accounting for a 1:20 submission ratio, the minimal anticipated sample size that would provide 80% power to detect a significant (< 0.05) difference was the inclusion of 35 urolith submissions in the smaller population and 700 urolith submissions in the larger population.
Samples and data collection
Records of the Minnesota Urolith Center (Saint Paul, Minn), Canadian Veterinary Urolith Centre (Guelph, ON, Canada), Urolithiasis Laboratory Inc (Houston, Tex), and Laboklin Veterinary Diagnostic Laboratory (Bad Kissingen, Germany) were searched for submissions of uroliths obtained from ferrets and dated between January 1, 2010, and December 31, 2018. Data collection included submission date; ferret sex, neuter status, age, and diet (when available); submission country; and urolith type. All data were compiled in an electronic spreadsheetb by 1 investigator (EBH). Submissions without a submission date were excluded. Submissions with missing patient-related data (ie, ferret sex, neuter status, or age) were included.
Urolith analysis
Minnesota Urolith Center—Mineral composition of uroliths was determined by polarization microscopy and infrared spectroscopy. A urolith was classified as the primary mineral type if it comprised > 70% of the urolith. A urolith was designated as mixed if the urolith nuclei and shell were of different mineral types or no single mineral type comprised > 70% of the urolith.
Canadian Veterinary Urolith Centre—Mineral composition of uroliths was quantitatively determined for each layer by use of optical crystallography with polarized light microscopy. As necessary, radiographic microanalysis in a scanning electron microscope and Fourier transform infrared spectroscopy with an infrared microscope were also used.
Laboklin Veterinary Diagnostic Laboratory—Mineral composition of uroliths was determined chemically between 2010 and 2014, and the primary mineral detected was used to classify the type of urolith. After 2014, determinations were made with a spectrometer and by comparison of the light transmission of the analytic substance with a reference database.
Urolithiasis Laboratory Inc—Mineral composition was determined for the nidus, stone, shell, and surface of uroliths. This was accomplished by use of quantitative petrographic crystallography of each layered section and a standard reference oil with a specific refractive index.
Statistical analysis
Continuous variables were summarized as either mean ± SD or median and IQR depending on their distribution. The Shapiro-Wilk test was used to analyze continuous data for normality. Results for categorical variables were reported as numbers and percentages. Statistical analysis was performed with the use of available software.c,d A 2-tailed value of P < 0.05 was considered significant. Data are freely available upon request.
Logistic regression—Bivariable binary logistic regression analyses with 1 predictor variable were used to identify any other relevant predictors of cystine urolithiasis. The dependent variable was the urolith mineral composition (cystine vs others). Predictors tested were continent of submission (ie, North America vs Europe or Asia), patient-related variables (age, sex, and neuter status [yes vs no or unknown]), and year of submission (2010 to 2018). Crude ORs and aORs with 95% CIs were used to quantify these associations. Multivariable logistic regression models were then developed to evaluate whether the association between cystine urolithiasis and continent of submission persisted after adjustment for other patient-related variables and potential confounders. All patient-related characteristics and covariates demonstrated in the bivariable analysis to be prognostically significant (P < 0.05) were entered into the final model. To avoid overfitting the model, ≤ 1 predictor variable was included for 10 events (ie, cystine uroliths).16 The Hosmer-Lemeshow test and the Nagelkerke R2 were used to assess the model’s goodness-of-fit.
Secondary analysis—To assess the impact of missing data on the model, linear interpolation was used for the imputation of missing data for variables most frequently missing in the data set, and results of the analyses were compared. The linearity assumption of the variables ferret age and submission year were assessed as previously described17: variables were binned in quartiles; the multivariable model was fitted, replacing the continuous variables with the 4-level binned temperature variable; log-transformed odds of the upper 3 quartiles (the lower quartile was used as an indicator) were plotted versus the respective quartile midpoints; the 4 plotted points were connected with straight lines; and then the plot was visually inspected for linearity.
Next, 2 sensitivity analyses were conducted. First, to reduce the potential bias caused by the use of different urolith analysis methods among the laboratories, a sensitivity analysis including only submissions from the laboratory with the most submissions was performed. The objective was to evaluate whether the main effects observed in the complete data set would persist in a data set from a single laboratory. The logistic regression model was identical to the main model in all regards with the exceptions that only submissions from 1 laboratory were included and that the variables used as predictors were continent of submission and submission year (categorical). Second, owing to the concerns about potential overfitting of the main model, a logistic regression model identical to the main model but including only variables for ferret age, submission continent, and submission year was built to decrease the number of predictor variables included. Results for crude ORs, aORs, 95% CIs, and significance of the included predictors of these 2 additional models were compared with those for the main model.
Results
Samples
The participating laboratories received 1,054 submissions of uroliths obtained from ferrets and dated between January 1, 2010, and December 31, 2018, with 1,013 submissions in North America and 41 in Europe and Asia (Table 1). Overall, cystine uroliths were the most common (949/1,054 [90.0%]), followed by struvite uroliths (65/1,054 [6.2%]).
Urolith mineral types identified for 1,054 submissions of uroliths obtained from ferrets and submission dates for analysis between January 1, 2010, and December 31, 2018, stratified by participating laboratory.
| Urolith type | Canadian Veterinary Urolith Centre (n = 109) | Minnesota Urolith Center (n = 892) | Urolithiasis Laboratory Inc (n = 35) | Laboklin Veterinary Diagnostic Laboratory (n = 18) | All submissions (n = 1,054) | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| No. of submissions (%) | 95% CI (%) | No. of submissions (%) | 95% CI (%) | No. of submissions (%) | 95% CI (%) | No. of submissions (%) | 95% CI (%) | No. of submissions (%) | 95% CI (%) | |
| Calcium oxalate | 0 (0.0) | 0.0–3.3 | 14 (1.6) | 0.9–2.6 | 1 (2.9) | 0.1–14.9 | 4 (22.2) | 6.4–47.6 | 19 (1.8) | 1.1–2.8 |
| Cystine | 104 (95.4) | 89.6–98.5 | 809 (90.7) | 88.6–92.5 | 33 (94.3) | 80.8–99.3 | 3 (16.7) | 0.4–41.4 | 949 (90.0) | 88.1–91.8 |
| Purine | 0 (0.0) | 0.0–3.3 | 3 (0.3) | 0.1–1.0 | 0 (0.0) | 0.0–10.0 | 0 (0.0) | 0.0–18.5 | 3 (0.3) | 0.1–0.8 |
| Silica | 0 (0.0) | 0.0–3.3 | 2 (0.2) | 0.0–0.8 | 0 (0.0) | 0.0–10.0 | 0 (0.0) | 0.0–18.5 | 2 (0.2) | 0.0–0.7 |
| Struvite | 2 (1.8) | 0.0–6.5 | 53 (5.9) | 4.4–7.7 | 0 (0.0) | 0.0–10.0 | 10 (55.6) | 32.6–78.5 | 65 (6.2) | 4.8–7.8 |
| Uric acid | 0 (0.0) | 0.0–3.3 | 0 (0.0) | 0.0–0.4 | 1 (2.9) | 0.1–14.9 | 0 (0.0) | 0.0–18.5 | 1 (0.1) | 0.0–0.5 |
| Mixed or other | 3 (2.8) | 0.0–7.8 | 11 (1.2) | 0.6–2.2 | 0 (0.0) | 0.0–10.0 | 1 (5.6) | 0.1–27.3 | 15 (1.4) | 0.8–2.3 |
Patient-related variables
Sex and neuter status of ferrets—Sex of the affected ferret was unknown for 44 of the 1,054 submissions and known for 1,010, consisting of 797 (78.9%; 95% CI, 76.3% to 81.4%) submissions for male ferrets and 213 (21.1%; 95% CI, 18.6% to 23.7%) for female ferrets. Of these 1,010 ferrets, 829 (82.1%; 95% CI, 79.6 to 84.4) were neutered (651 castrated males and 178 spayed females), and 181 (17.9%; 95% CI, 15.6% to 20.4%) were sexually intact (146 males and 35 females).
Of the 1,013 submissions from North America, 776 (76.6%; 95% CI, 73.8% to 79.2%) were of uroliths from male ferrets, 198 (19.5%; 95% CI, 17.2% to 22.1%) were from female ferrets, and the remaining 39 (3.8%; 95% CI, 2.8% to 5.2%) were from ferrets for which the sex was not reported. On the basis of neuter status, there were 811 (80.1%; 95% CI, 77.5% to 82.4%) submissions of uroliths in North America for neutered ferrets (640 castrated males, 141 spayed females, and 30 for which sex was not reported).
Of the 41 submissions from Europe and Asia, 21 (51.2%; 95% CI, 36.5% to 65.7%) were of uroliths from males, 15 (36.6%; 95% CI, 23.6% to 51.9%) were from females, and the remaining 5 (12.2%; 95% CI, 5.3% to 25.6%) were from ferrets for which the sex was not reported. On the basis of neuter status, there were 18 (43.9%; 95% CI, 29.9% to 59.0%) submissions from Europe and Asia for neutered ferrets (11 castrated males and 7 spayed females).
Age and diet of ferrets—The age of the affected ferret was reported for 894 submissions, and ferret age was nonnormally distributed (Shapiro-Wilk test, P < 0.001). Overall, the median age was 2.0 years (IQR, 1.3 to 3.2 years; range, 0.2 to 12.2 years). When considered on the basis of sex, the median age was 2.0 years (IQR, 1.2 to 3.0 years; range, 0.2 to 12.2 years; n = 797) for male ferrets and 2.4 years (IQR, 1.5 to 4.0 years, range, 0.4 to 11.0 years; 213) for female ferrets. When ferrets were grouped on the basis of urolith type, the median age was 2.0 years (IQR, 1.3 to 3.0 years; range, 0.2 to 12.2 years) for ferrets with cystine urolithiasis but 4.0 years (IQR, 2.0 to 5.1 years; range, 0.2 to 9.9 years) for ferrets with uroliths of other mineral compositions. Diet as a variable was not evaluated because dietary data were not consistently recorded by each laboratory.
Period prevalences of cystine uroliths
When submissions were grouped on the basis of cysteine versus struvite or all other types of uroliths, the yearly period prevalence increased for cystine uroliths and decreased for struvite and other urolith types over the years of the study period for both continents (Figure 1). Of the 14 submissions received in 2010, 8 (57.1%; 95% CI, 28.9% to 82.3%) were cystine uroliths, 1 (7.1%; 95% CI, 0.18% to 33.9%) was a struvite urolith, and the remaining 5 (35.7%; 95%, 12.8% to 64.9%) were other types of uroliths. Of the 256 submissions received in 2018, 243 (94.9%; 95% CI, 91.5% to 97.3%) were cystine uroliths, 5 (2.0%; 95% CI, 0.6% to 4.5%) were struvite uroliths, and 8 (3.1%; 95% CI, 1.4% to 6.1%) were other types of uroliths.
Period prevelances of types of uroliths (cystine [dark gray], struvite [light gray], or other [white]) obtained from ferrets (Mustelo putorius furo) and submitted for analysis at participating laboratories in Europe and Asia (A; n = 41) or North America (B; 1,013) between January 1, 2010, and December 31, 2018.
Citation: Journal of the American Veterinary Medical Association 259, 7; 10.2460/javma.259.7.757
Period prevelances of types of uroliths (cystine [dark gray], struvite [light gray], or other [white]) obtained from ferrets (Mustelo putorius furo) and submitted for analysis at participating laboratories in Europe and Asia (A; n = 41) or North America (B; 1,013) between January 1, 2010, and December 31, 2018.
Citation: Journal of the American Veterinary Medical Association 259, 7; 10.2460/javma.259.7.757
Period prevelances of types of uroliths (cystine [dark gray], struvite [light gray], or other [white]) obtained from ferrets (Mustelo putorius furo) and submitted for analysis at participating laboratories in Europe and Asia (A; n = 41) or North America (B; 1,013) between January 1, 2010, and December 31, 2018.
Citation: Journal of the American Veterinary Medical Association 259, 7; 10.2460/javma.259.7.757
Geographic location
There were 1,013 urolith submissions in North America: cystine, 938 (92.6%; 95% CI, 90.8% to 94.1%); struvite, 44 (4.3%; 95% CI, 3.3% to 5.8%); and all other types, 31 (3.1%; 95% CI, 2.1% to 4.3%; Figure 1). There were 41 uroliths submissions in Europe and Asia: struvite, 17 (41%; 95% CI, 26% to 58%); cystine, 11 (27%; 95% CI, 14% to 43%); and all other types, 13 (32%; 95% CI, 18% to 47%).
Regression analysis
The assumption of linearity was fulfilled for the variable of ferret age; therefore, ferret age was included as a continuous variable in the final model. In contrast, the assumption of linearity was not fulfilled for the variable submission year; therefore, submission year was included as a categorical variable with 9 categories, 1 for each year from 2010 to 2018.
In the unadjusted analyses, submission continent and year and ferret age and neutered status were all associated with an increased risk of cystine urolithiasis; however, sex was not (Table 2). The final multivariable model included age, sex, neuter status, continent, and submission year, and after multivariable adjustment, the associations of submission continent, ferret age, and submission year with cystine urolithiasis persisted. Specifically, the odds of a submission being a cystine urolith were 59.5 times (aOR, 59.5; 95% CI, 21.4 to 165.6) the odds for those from ferrets in North America versus Europe or Asia and 21.1 times (aOR, 21.1; 95% CI, 5.1 to 87.9) the odds for those submitted in 2018 versus 2010. Further, the odds of a submission being a cystine urolith decreased with ferret age in that for every 1 year increase in ferret age, and the odds of cystine urolith decreased 49% (aOR, 0.67; 95% CI, 0.58 to 0.77). The final logistic regression model was well fitted (Hosmer-Lemeshow test, P = 0.62; Nagelkerke R2 = 0.36).
Results of bivariable and multivariable regression analysis to determine variables associated with cystine versus other types of uroliths among the 1,054 submissions described in Table 1.
| Variable | No. of submissions | Cystine uroliths* | Other uroliths* | Bivariable analysis | Multivariable analysis | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| Crude OR | 95% CI of the crude OR | P value | aOR | 95% CI of the aOR | P value | |||||
| Ferret age (y) | 894 | 2 (1.3–3.0)† | 4 (2.0–5.1)† | 0.65 | 0.57–0.74 | < 0.001 | 0.667 | 0.578–0.770 | < 0.001 | |
| Ferret sex | 0.170 | 0.315 | ||||||||
| Male | 797 | 725 (91.0) | 72 (9.0) | Referent | Referent | |||||
| Female | 213 | 187 (87.8) | 26 (12.2) | 1.40 | 0.87–2.25 | 1.415 | 0.719–2.784 | |||
| Ferret neutered | 0.004 | 0.159 | ||||||||
| Yes | 829 | 758 (91.4) | 71 (8.6) | Referent | Referent | |||||
| No or unknown | 225 | 191 (84.9) | 34 (15.1) | 0.53 | 0.34–0.82 | 0.611 | 0.308–1.212 | |||
| Submission continent | < 0.001 | < 0.001 | ||||||||
| Europe or Asia | 41 | 11 (26.8) | 30 (73.2) | Referent | Referent | |||||
| North America | 1,013 | 939 (92.6) | 75 (7.4) | 34.11 | 16.44–70.76 | 59.528 | 21.404–165.557 | |||
| Submission year | ||||||||||
| 2010 | 14 | 8 (57.1) | 6 (42.9) | Referent | Referent | |||||
| 2011 | 18 | 10 (55.6) | 8 (44.4) | 0.94 | 0.23–3.83 | 0.930 | 0.743 | 0.148–3.724 | 0.718 | |
| 2012 | 64 | 51 (79.7) | 13 (20.3) | 2.94 | 0.87–9.98 | 0.080 | 2.955 | 0.731–11.946 | 0.129 | |
| 2013 | 98 | 86 (87.8) | 12 (12.2) | 5.37 | 1.59–18.18 | 0.007 | 5.700 | 1.359–23.907 | 0.017 | |
| 2014 | 178 | 166 (93.3) | 12 (6.7) | 0.37 | 3.09–34.79 | < 0.001 | 12.404 | 2.937–52.380 | < 0.001 | |
| 2015 | 139 | 126 (90.6) | 13 (9.4) | 7.27 | 2.18–24.20 | < 0.001 | 8.901 | 2.228–35.561 | 0.002 | |
| 2016 | 125 | 117 (93.6) | 8 (6.4) | 0.97 | 3.06–39.37 | < 0.001 | 18.279 | 3.534–94.548 | < 0.001 | |
| 2017 | 162 | 142 (87.7) | 20 (12.3) | 5.32 | 1.67–6.94 | 0.005 | 8.055 | 1.986–32.682 | 0.004 | |
| 2018 | 256 | 243 (94.9) | 13 (5.1) | 4.02 | 4.24–46.38 | < 0.001 | 21.134 | 5.078–87.945 | < 0.001 | |
Data are reported as number and percentage of observed events, except where indicated.
Data are reported as median (IQR).
Missing data and sensitivity analyses
There were 886 submissions that included all data needed for analyses and inclusion in the final logistic regression model. The remaining 168 submissions were missing data for ≥ 1 variable, with ferret age missing most commonly (n = 160). On the basis of ages in the data set, linear interpolation was used to impute missing data for age, and the assumption for imputing was uninformative missingness. The final logistic regression model was repeated after the inclusion of the variable for ferret age with the imputed missing data, and results did not differ substantially before versus after imputation of missing age data.
Sensitivity analysis
The first sensitivity analysis, performed on results only from the laboratory with the most submissions (Minnesota Urolith Center, with 892 submissions), demonstrated the persistence of association of continent (aOR, 36.1) and submission year (eg, greater odds of cystine urolith in 2013 [aOR, 6.7] through 2018 [aOR, 27.4] than in 2010; Supplementary Table S1).
The second sensitivity analysis, which included variables only for submission continent, submission year, and ferret age, was performed to reduce the number of predictors in the model. Results demonstrated the persistence of association among all 3 of these variable as predictors, with the odds of a cystine urolith that were greater for submissions from North America versus Europe or Asia (aOR, 68.7) and for submissions in 2013 or later versus in 2010 (aORs ≥ 5.1) but that decreased with with increasing age of affected ferrets (aOR, 0.97; Supplementary Table S2).
Discussion
The present study provided a longitudinal view of uroliths from ferrets submitted to North American and European laboratories during the previous decade, and results highlighted a dramatic increase in the yearly period prevalence of cystine uroliths among those submitted for analysis, compared with findings during the 2 previous decades (1992 to 2009).3 However, in dogs, the proportion of submissions identified as cystine uroliths has decreased from 40% submission in 1979 to approximately 5% in 2013.4 The yearly numbers of submissions of uroliths obtained from ferrets also substantially increased during the study period, from just 14 in 2010 to 257 in 2018. This increase could be explained by increases in the pet ferret population, owners seeking veterinary care for ferrets, owner compliance or awareness regarding prevention of urolithiasis, rate of submissions by veterinarians, or incidence rate of urolithiasis in ferrets, alone or in combination. However, on the basis of a comparison of AVMA data from 201218 and 2017,19 there was no increase in the number of ferrets owned by individual households or in the total population of ferrets; therefore, an increase in pet ferret population could not explain the increased proportion of cystine uroliths identified in the present study.
Cystinuria occurs when the amino acid cystine is excreted in the urine, typically owing to a defect in the active resorption of cystine by the proximal renal tubules.20 Although cystine, ornithine, lysine, and arginine amino acid transport may be similarly affected by such a defect, cystine is least soluble in urine, and a high concentration of cystine in urine is the primary risk factor for cystine urolithiasis in various species.5–10 Currently, the most likely cross-species underlying cause for cystinuria is genetic related. Studies show variants in the genes SLC3A1 and SLC7A9 are associated with cystinuria in dogs,4,5 cats,6,7 and humans,9,10,20,21 and 3 authors (ND, UD, and JLB) of the present study are involved in ongoing research to evaluate variability in the SLC3A1 and SLC7A9 genes of ferrets with cystinuria.
The 3 main contributing urinary factors for urolithiasis in veterinary patients are the presence of a protein matrix core that may instigate urolith formation, a lack of crystallization inhibitors, and the presence of existing precipitation factors, which are complex relationships between urine solutes and other urine components.22 An unproven theory regarding precipitation factors and the development of cystine uroliths in ferrets is related to ferret diet and the diet’s effect on mineral solubility. The lower solubility of cystine is more pronounced in acidic or concentrated urine,23 and healthy ferrets have concentrated urine (specific gravity, 1.026 to 1.070).24 In dogs, high-protein dry diets, especially those rich in methionine (a precursor of cysteine), that promote acidic and concentrated urine are risk factors for cystine urolithiasis in susceptible animals.23,25 However, the effect of diet on aminoaciduria, urine pH, and urine specific gravity of ferrets is unknown. In the present study, dietary information was recorded at only 1 of the 4 participating laboratories, precluding further analysis.
On the basis of a literature search, the present study was the first to our knowledge to evaluate the period prevalence of cystine urolith among submissions of uroliths obtained from ferrets on the basis of global location, revealing a dramatic difference between those submitted in North America (938/1,013 [92.6%]) versus Europe and Asia (11/41 [27%]). In dogs, geographic variability in relative frequency of cystine urolithiasis occurs; however, the variation between continents was < 10% (ie, 0.3% to 0.8% in North America vs 3.0% to 5.6% in Europe).4 Several factors could explain the difference we detected in submissions on the basis of continent. For instance, breeding practices differ in that most pet ferrets in North America originate from a single commercial breedere with scattered private breeders and sellers. This is drastically different than practices in Europe, where most pet ferrets are privately bred.26 In North America, there are limited numbers of stakeholders responsible for the breeding of ferrets and thus it would be relatively easy to introduce, control, and remove genetic mutations. Thus, an association between such a small genetic pool and an emergence of a genetic anomaly that increases the incidence of cystinuria is plausible. If such a genetic mutation could be identified, then genetic selection could limit the occurrence of cystine urolithiasis in ferrets. Other potential factors that could account for the differences in yearly period prevelences of cystine uroliths could have been related to differences in owner compliance and decisions (eg, owners electing euthanasia rather than surgical intervention), veterinary practice (eg, urolith submission rates and geographic availability of laboratories for urolith analysis), and ferret diet, all of which are difficult to quantify. For instance, diet information was not included with submissions to most of the participating laboratories in the present study, and systematic data on diets fed to ferrets in different geographic locations are lacking.
The median age of ferrets reported on submissions of uroliths in the present study was 2 years, which was younger than previous reports of 3.6 years.3 However, our findings also indicated that the median age varied for ferrets grouped on the basis of urolith type: 2.0 years for ferrets with cystine uroliths and 4.0 years for ferrets with other types of uroliths. This was similar to a finding in a study4 of cystinuria in which dogs that had cystine versus other types of uroliths were younger.
Results of bivariable regression analysis indicated an association between neutered status and cystine urolithiasis; however, there was no such association on multivariable regression analysis. In our data set, most of the ferrets in North America were recorded as neutered (811/1,013[80%]), whereas only 18 of the 41 (44%) ferrets in Europe and Asia were neutered. The fact that the association between neuter status and cystine urolithiasis discontinued from bivariable to multivariable analysis indicated that the association was spurious and driven by the different proportions of neutered ferrets when grouped by continent.
Although there was no association between sex and neuter status with cystine urolithiasis, most submissions were of uroliths from castrated male ferrets, similar to findings in previous studies.3,27 This overrepresentation of castrated males could be due to male ferret anatomic factors, such as the J-shaped os penis and narrow urethra, that predispose male ferrets to urinary obstruction and therefore potential surgical intervention and urolith analysis. In genetic knockout models of cystinuria in mice, both sexes have comparable levels of crystalluria; however, few females form uroliths, whereas males form cystine uroliths as early as 6 to 8 weeks of age.8 The mechanism for this difference between sexes is unknown but may relate to urinary crystal aggregation or solubility differences.8 In a recent report,7 cats affected with cystine urolithiasis were prepubertal or neutered before calculi formation occurred. In dogs, one of the mechanisms of inheritance of cystinuria is androgen dependent; therefore, castrating affected males leads to markedly decreased cystine excretion and prevents urolith formation.28 Similarly, in a recent study29 that included a population of client-owned dogs, being a sexually intact male, regardless of breed, was associated with an increased risk of cystine urolithiasis. In the present study, the fact that most submissions were of uroliths from castrated male ferrets suggested that castration likely provides no protective effect in ferrets.
Veterinary species tend to form cystine uroliths in the bladder rather than the kidneys,24 and treatment does not often result in a resolution. However, because cystinuria is androgen responsive in dogs, castration is recommended when sexually intact males are affected.23 In humans, the mainstay of prevention and treatment consists of urine dilution and alkalinization and the reduction of cystine to its more soluble metabolite, cysteine, with the use of thiol-binding agents.30 For dogs and cats, feeding high-moisture foods (> 75% moisture) and adding water to dry kibble are recommended for cystine urolithiasis.23 Treatment with tiopronin, dietary protein restriction, and other treatments may be recommended for affected dogs23; however, protein restriction may not be appropriate in ferrets because they are obligate carnivores.31 In fact, carnitine deficiency and associated dilated cardiomyopathy were reported in 5 cystinuric dogs fed low-protein diets.32 Because thiol-binding drugs, such as tiopronin, can result in adverse effects including fever, anemia, and lymphadenopathy, the American College of Veterinary Internal Medicine also recommends that their use should be restricted to animals with recurrent or severe disease.23
The present study had strengths and limitations. On the basis of a literature search, the present study was the first to our knowledge to evaluate ferret urolith submissions from 4 separate laboratories on 2 continents. The inclusion of multiple laboratories had the advantage of increasing the generalizability of the findings, the large number of included submissions allowed proper statistical modeling, and the results were significant with a large effect magnitude. The main limitation was that submissions of uroliths from ferrets with recurrent urolithiasis could not be identified in the data retrieved from the laboratories because the data were anonymized. The inclusion of multiple laboratories was also a potential limitation because differences in techniques used to determine urolith types could have yielded inconsistent results among the laboratories; however, the techniques used by the participating laboratories were considered adequate to diagnose cystine uroliths. Furthermore, results of the sensitivity analysis performed to test the multivariable regression model on data from 1 laboratory detected no change in the overall results for the model. Another potential limitation was our use of single, linear interpolation to impute missing data for ferret age because single imputation may lead to potential underestimation of SEs. However, the primary analysis reported for the present study was the analysis without interpolation of missing data.
Results of the present study indicated a dramatic increase in the proportion of cystine urolith among submissions of uroliths from ferrets, and the magnitude of this effect has been much larger in North America than in Europe and Asia. Our findings also supported the potential for underlying genetic contributions to cystine urolithiasis in ferrets and a compelling need to identify prevention and treatment measures to mitigate cystine urolithiasis in ferrets.
Supplementary Materials
Supplementary materials are available online at: avmajournals.avma.org/doi/suppl/10.2460/javma.259.7.757.
Acknowledgments
No external funding was used in this study. The authors declare that there were no conflicts of interest.
Presented in abstract form at the Annual Conference of the Association of Exotic Mammal Veterinarians, St Louis, September 2019; and at the 2020 Yaboumba World Congress, Paris, February 2020.
Abbreviations
| aOR | Adjusted odds ratio |
| IQR | Interquartile (25th to 75th percentile) range |
Footnotes
Fisher P. Cystine urolithiasis in the ferret (Mustela putorius furo) (abstr), in Proceedings. 13th Annu Conf Assoc Exotic Mammal Vet 2014.
Microsoft Excel, version 16.16.15, Microsoft Corp, Redmond, Wash.
SPSS Statistics, version 24, IBM Corp, Armonk, NY.
Sample Size Calculators 2021, Kohn MA, Senyak J, Clinical & Translational Science Institute, University of California, San Francisco, Calif. Available at: sample-size.net. Accessed Mar 11, 2021.
Marshall BioResources, North Rose, NY.
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