Canine calcium oxalate urolithiasis: Frequency of Whewellite and Weddellite stones from 1979 to 2015

Abstract

This study reports on a retrospective evaluation of epidemiological data from calcium oxalate stones in dogs differentiated into calcium oxalate monohydrate (Whewellite, Wh) and calcium oxalate dihydrate (Weddellite, Wd). Of the 22 456 uroliths submitted from 1979 to 2015, 6690 (29.8%) were composed of > 70% calcium oxalate. During the observation period, the proportion of calcium oxalate stones rose from 4% (1979) to 46% (2015). Of all the calcium oxalate stones, 31.0% were Wh and 49.4% Wd, while 19.6% were a mixture of Wh and Wd. The dogs with Wh stones were significantly older than the dogs with Wd stones. Several breeds have increased odds ratios (OR) for either Wh (5 highest OR: Norwich terrier, keeshond, Norfolk terrier, fox terrier, sheltie) or Wd (Pomeranian, borzoi, Japanese spitz, Finnish lapphund, bichon frise). Analytical differentiation of the calcium oxalate stones into Wh and Wd is important for understanding the cause and possible treatment and prevention of the uroliths.

Introduction

Over time, the relative frequency of calcium oxalate stone submissions has increased in most countries around the world (1–9). Calcium oxalate is reported to be the most frequently submitted urolith in Canada, USA, Switzerland, and France (2,4–6,8). Many of these publications fail to give possible reasons for the increase in calcium oxalate submissions and the etiology remains unclear.

Modern techniques, such as infrared spectroscopy, allow the differentiation of 2 hydrate forms of calcium oxalate stones, namely Whewellite (Wh; calcium oxalate monohydrate) and Weddellite (Wd; calcium oxalate dihydrate) (10). Markedly different urine compositions can be present during the crystallization process of these 2 forms (11,12). Most studies on canine uroliths have omitted to differentiate between the 2 hydrate forms of calcium oxalate stones (1–9,13,14).

This retrospective study reports on epidemiological data on the 2 hydrate forms of calcium oxalate uroliths in dogs analyzed from 1979 to 2015.

Materials and methods

From January 1979 to December 2015, a total of 22 456 uroliths from dogs were analyzed with infrared spectrometry. Uroliths, most commonly obtained during cystotomy, were submitted mainly from Germany (> 99%), but a small percentage was also received from other European countries (Finland, Switzerland, Italy, and France). Epidemiological data, such as breed, age, gender, and location of stone removal, were provided with each submission.

Prior to analysis, the uroliths were dried if necessary at 37°C. Next, the material was homogenized in an agate grinder and a sample was obtained with < 1 mg required. Small stones up to 5-mm in diameter were homogenized in toto, while larger stones were broken up or sawed into smaller pieces and representative samples were obtained; a separate analysis of core, shell, and surface crystals was not performed during routine analysis. Analysis was always performed by infrared spectrometry; an FT-IR spectrometer (Spectrum two; Perkin Elmer, Rodgau, Germany) was used throughout the study period. Evaluation of the graphs was performed with an atlas of infrared spectra for the analysis of uroliths (15) as well as with a computerized library of personally created reference spectra.

As previously reported (2,16), if a urolith contained at least 70% of a single mineral it was classified as that mineral type. Uroliths containing < 70% of a single mineral component and without an obvious nidus or shell were classified as mixed.

The frequency of calcium oxalate urolith submissions, separated into > 70% Wh and > 70% Wd, was calculated for each year. Epidemiological data between Wh and Wd stones were compared by either Pearson Chi-square test/Fisher exact test (discrete values) or t-test (age). For all breeds, odds ratios (OR) were calculated by using breeds other than the considered breed as well as mixed breeds as a reference group. Odds ratios and 95% confidence intervals (CI) were calculated with a statistical software package (StatXact Version 9.0.0; Cytel Corporation, Cambridge, Massachusetts, USA). A P-value < 0.05 was considered significant for all comparisons.

Results

Composition

Of the 22 456 submitted uroliths, 6690 (29.8%) were composed of > 70% calcium oxalate. The percentage of calcium oxalate stones increased from 4% (7 of 167 stones) in 1979 to 46% (356 of 773 stones) in 2015.

Of all calcium oxalate stones, 2076 (31.0%) were of > 70% Wh composition, 3304 (49.4%) were of > 70% Wd composition and 1310 (19.6%) were a > 70% mixture of Wh and Wd. Changes in relative frequency over time of Wh and Wd uroliths are depicted in Figure 1.

Figure 1.

Change in relative frequency of Whewellite and Weddellite uroliths from 1979 to 2015.

Of the 534 mixed stones with < 70% of one single mineral component, 172 were mixed stones with 5% to 65% Wh and 175 were mixed stones with 5% to 65% Wd origin. The remaining content of these stones was mostly carbonate apatite or struvite, rarely also ammonium urate or cystine.

Location

Of all analyzed Wh stones with known location, 7 (0.3%) were located in the kidney and 13 (0.6%) were located in the ureter. The rest were located in the lower urinary tract with 1046 (50.4%) in the bladder, 271 (13.1%) in the urethra and 626 (30.2%) in bladder and urethra combined. Location of 113 (5.4%) Wh uroliths was unknown. Eleven Wd stones were found in the upper urinary tract [kidney = 3 (0.1%), ureter = 8 (0.2%)], 1782 (53.9%) were detected in the bladder, 389 (11.8%) in the urethra, and 952 (28.8%) in both bladder and urethra. Location of 170 (5.1%) Wd uroliths was unknown. There was a significant difference (P = 0.003) between upper and lower urinary tract location of Wh and Wd stones.

Gender

In male dogs, there were 1623 (83.3%) Wh stones and 2589 (82.6%) Wd stones (Table 1), which was significantly (P < 0.001) more frequent than submissions from female dogs. The male to female ratio was 5:1 for Wh and 4.8:1 for Wd stones. There was no significant difference (P = 0.66) in gender distribution between Wh and Wd uroliths.

Table 1.

Gender distribution of dogs with Whewellite, Weddellite, and mixed Whewellite/Weddellite stones compared with non-calcium oxalate stones.

Gender	Whewellite n (%)	Weddellite n (%)	Mixed Whewellite/ Weddellite n (%)	Non-calcium oxalate n (%)
Male	990 (50.8)	1852 (59.3)	820 (64.1)	6294 (41.6)
Male neutered	633 (32.5)	737 (23.6)	266 (20.8)	1449 (9.6)
Female	146 (7.5)	267 (8.5)	123 (9.6)	4794 (31.7)
Female spayed	179 (9.2)	269 (8.6)	70 (5.5)	2584 (17.1)
Total	1948 (100)	3125 (100)	1279 (100)	15 121 (100)
Gender unknown	128	179	31	647

Age

Mean [± standard deviation (SD)] age of dogs with Wh stones was 9.3 ± 2.81 y, which was significantly (P < 0.0001) older than mean age of dogs with Wd stones (8.2 ± 2.68 y). In 123 submissions, age was unknown. There was no significant difference between the age of male and female dogs with either Wh or Wd stones (P = 0.32 and P = 0.07, respectively). Dogs with calcium oxalate stones were significantly older (P < 0.0001) compared with dogs from which non-calcium oxalate stones were submitted (6.8 ± 3.09 y).

Breeds

Whewellite and Wd stones were submitted from 136 and 143 breeds, respectively. The highest odds ratios for Wh stones were found in the Norwich terrier, keeshond, and Norfolk terrier, with 18 breeds being significantly at risk of Wh urolithiasis compared to all other breeds combined (Table 2). The highest odds ratio for Wd stones was seen in the Pomeranian, borzoi, and Japanese spitz with 28 breeds being significantly at risk of Wd urolithiasis compared to all other breeds combined (Table 3).

Table 2.

Odds ratios (95% CI) for 32 breeds from which Whewellite stones were submitted in comparison to all non-Whewellite stones submitted from all breeds combined as well as compared to mixed breeds.

Breeds	Whewellite stones (n)	Non-Whewellite stones (n)	Odds ratio for Whewellite stones relative to all other breeds combined	95% CI for Whewellite stones relative to all other breeds combined	Odds ratio for Whewellite stones relative to mixed breeds	95% CI for Whewellite stones relative to mixed breeds
Norwich terrier	23	35	6.51	(3.67, 11.36)	8.42	(4.68, 14.89)
Keeshond	8	17	4.63	(1.73, 11.34)	6.03	(2.23, 14.90)
Norfolk terrier	29	67	4.30	(2.67, 6.75)	5.55	(3.40, 8.86)
Fox terrier	62	178	3.49	(2.56, 4.71)	4.47	(3.20, 6.16)
Sheltie	8	24	3.28	(1.27, 7.56)	4.27	(1.64, 9.94)
Jack Russell terrier	133	520	2.61	(2.13, 3.19)	3.28	(2.59, 4.13)
Pinscher	19	76	2.47	(1.41, 4.13)	3.21	(1.80, 5.45)
Chihuahua	30	124	2.40	(1.55, 3.60)	3.10	(1.9, 4.75)
Rehpinscher	20	85	2.32	(1.35, 3.82)	3.02	(1.73, 5.04)
Chow chow	62	266	2.33	(1.73, 3.09)	2.99	(2.17, 4.07)
Miniature pinscher	107	488	2.22	(1.77, 2.75)	2.81	(2.18, 3.60)
Bichon frise	27	124	2.15	(1.36, 3.29)	2.79	(1.74, 4.35)
Griffon Brusselois	15	70	2.11	(1.12, 3.73)	2.75	(1.44, 4.92)
Havanese	9	43	2.06	(0.88, 4.29)	2.68	(1.14, 5.65)
WHWT	45	220	2.03	(1.44, 2.82)	2.62	(1.82, 3.72)
Maltese	58	290	1.99	(1.47, 2.66)	2.56	(1.85, 3.51)
Spitz	20	101	1.95	(1.14, 3.19)	2.54	(1.46, 4.21)
Giant schnauzer	10	51	1.93	(0.87, 3.85)	2.51	(1.13, 5.08)
Eurasier	11	57	1.90	(0.90, 3.67)	2.47	(1.16, 4.84)
Lhasa apso	32	171	1.85	(1.22, 2.72)	2.40	(1.56, 3.60)
Australian terrier	12	65	1.82	(0.89, 3.40)	2.37	(1.15, 4.49)
French bulldog	19	110	1.70	(0.99, 2.79)	2.22	(1.26, 3.69)
Cairn terrier	37	218	1.68	(1.15, 2.39)	2.18	(1.46, 3.17)
Welsh terrier	12	72	1.64	(0.81, 3.05)	2.14	(1.04, 4.03)
Beagle	22	160	1.35	(0.82, 2.13)	1.76	(1.06, 2.82)
Yorkshire terrier	174	1470	1.18	(0.99, 1.39)	1.52	(1.24, 1.86)
Mixed breed	275	3526	0.73	(0.64, 0.83)	—	—
Dachshund	127	2359	0.50	(0.41, 0.60)	0.69	(0.55, 0.86)
Pekinese	11	362	0.30	(0.15, 0.54)	0.39	(0.19, 0.72)
Cocker spaniel	10	631	0.15	(0.07, 0.28)	0.20	(0.10, 0.38)
German shepherd	4	266	0.15	(0.04, 0.38)	0.19	(0.05, 0.51)
Dalmatian	2	849	0.02	(0.00, 0.08)	0.03	(0.00, 0.11)

CI — Confidence interval; WHWT — West Highland white terrier.

Table 3.

Odds ratios (95% CI) for 38 breeds from which Weddellite stones were submitted in comparison to all non-Weddellite stones submitted from all breeds combined as well as compared to mixed breeds.

Breeds	Weddellite stones (n)	Non-Weddellite stones (n)	Odds ratio for Weddellite stones relative to all other breeds combined	95% CI for Weddellite stones relative to all other breeds combined	Odds ratio for Weddellite stones relative to mixed breeds	95% CI for Weddellite stones relative to mixed breeds
Pomeranian	6	5	6.97	(1.77, 28.87)	7.92	(2.00, 32.91)
Borzoi	7	6	6.78	(1.95, 24.41)	7.70	(2.20, 27.84)
Japanese spitz	23	34	3.94	(2.21, 6.90)	4.47	(2.49, 7.88)
Finnish lapphund	6	9	3.87	(1.13, 12.18)	4.40	(1.28, 13.9)
Bichon frise	60	91	3.87	(2.74, 5.44)	4.35	(3.04, 6.18)
Lakeland terrier	10	18	3.22	(1.33, 7.38)	3.67	(1.50, 8.43)
Bolognese	6	11	3.17	(0.96, 9.35)	3.60	(1.09, 10.67)
Jack Russell terrier	230	423	3.31	(2.80, 3.91)	3.59	(2.96, 4.34)
Collie	19	35	3.16	(1.71, 5.58)	3.58	(1.92, 6.50)
Schapendoes	11	22	2.91	(1.2, 6.26)	3.30	(1.43, 7.15)
Samoyed	7	14	2.90	(0.99, 7.69)	3.30	(1.21, 8.79)
Griffon	28	57	2.86	(1.75, 4.58)	3.24	(1.95, 5.24)
Fox terrier	76	164	2.73	(2.04, 3.61)	3.06	(2.26, 4.11)
Malinois	13	29	2.61	(1.2, 5.18)	2.96	(1.40, 5.92)
Chow chow	96	232	2.44	(1.90, 3.12)	2.73	(2.09, 3.55)
Coton de Tuléar	9	23	2.27	(0.92, 5.10)	2.58	(1.05, 5.83)
Laika	13	34	2.22	(1.07, 4.32)	2.52	(1.21, 4.95)
Norfolk terrier	26	70	2.16	(1.3, 3.44)	2.45	(1.49, 3.94)
Chihuahua	41	113	2.12	(1.44, 3.09)	2.40	(1.61, 3.50)
Welsh terrier	22	62	2.06	(1.21, 3.41)	2.34	(1.36, 3.91)
Miniature pinscher	27	78	2.02	(1.25, 3.16)	2.29	(1.40, 3.62)
Irish terrier	36	104	2.02	(1.34, 2.98)	2.29	(1.50, 3.41)
German pinscher	24	71	1.97	(1.18, 3.17)	2.23	(1.33, 3.63)
Cairn terrier	64	191	1.96	(1.45, 2.62)	2.21	(1.61, 3.00)
Lhasa apso	49	154	1.86	(1.32, 2.58)	2.10	(1.47, 2.96)
Spitz	29	92	1.84	(1.16, 2.82)	2.08	(1.31, 3.23)
WHWT	62	203	1.79	(1.32, 2.39)	2.02	(1.47, 2.74)
Airedale	17	57	1.73	(0.94, 3.03)	1.97	(1.07, 3.47)
Yorkshire terrier	357	1287	1.68	(1.48, 1.91)	1.83	(1.57, 2.13)
Miniature schnauzer	127	468	1.60	(1.30, 1.95)	1.79	(1.43, 2.24)
Maltese	71	277	1.50	(1.13, 1.95)	1.69	(1.26, 2.24)
Beagle	37	145	1.48	(1.00, 2.15)	1.69	(1.13, 2.47)
Corgie	36	144	1.45	(0.98, 2.11)	1.65	(1.10, 2.43)
Papillon	38	167	1.32	(0.90, 1.90)	1.50	(1.01, 2.18)
Mixed breed	500	3301	0.85	(0.77, 0.95)	—	—
Bernese mountain dog	13	271	0.28	(0.14, 0.48)	0.32	(0.17, 0.56)
Dachshund	113	2373	0.25	(0.2, 0.30)	0.31	(0.25, 0.38)
Dalmatian	9	842	0.06	(0.03, 0.11)	0.07	(0.03, 0.14)

CI — Confidence interval; WHWT — West Highland white terrier.

Discussion

As in previous studies, the current study supports the increasing frequency of calcium oxalate urolith submissions in dogs but few of these publications reported on whether the calcium oxalate was calcium oxalate monohydrate (Wh) or calcium oxalate dihydrate (Wd) (1–9). Osborne et al (16) analyzed 77 191 stones, 31.4% of which were calcium oxalate, predominantly Wh with 21% versus 6.3% Wd stones, while 3.9% were of mixed Wh/Wd composition. In 1990, we analyzed 1731 urinary stones from dogs (10) and found that only 5.03% were calcium oxalate stones, 36.8% of which were composed of Wh and 63.2% of Wd. In the present study, we found 29.8% calcium oxalate stones (1979 to 2015) with a Wh/Wd ratio of 31.0% to 49.4% (19.6% mixed Wh/Wd stones). This shows that in Germany, the greater frequency of Wd stones compared to Wh stones in dogs has remained unchanged for 25 y.

The majority of submitted stones are from the lower urinary tract (6). This was also the case in the present study. While surgical difficulty of nephrotomy for removal or nephroliths compared to cystotomy for cystoliths surely has some weight for this vast difference, it cannot explain why most dogs have uroliths in the bladder but rarely in the kidneys.

Whewellite and Wd stones are formed more frequently in male than in female dogs. However, there is no significant difference in gender regarding Wh and Wd stones. Others also described the greater incidence of calcium oxalate stones in male dogs, however, without differentiation between Wh and Wd (2,13,17).

Earlier publications showed that dogs with calcium oxalate stones are significantly older than dogs with other types of uroliths (2,18,19). This is confirmed by the present data. In addition, we found that dogs with Wh stones were significantly older than dogs with Wd stones. In humans it was shown that there is a direct link between the propensity to develop Wd uroliths and having a hypercalciuria. The decline of Wd uroliths with age may be related to the decrease in urinary calcium excretion (20).

To analyze if a breed is more likely to have a specific stone type, the odds ratio (OR) must be calculated. This is most commonly done using either the mixed breed or all other breeds as reference population, as done in this study. Interestingly, there was no discernible difference in using either reference population with very similar OR found. Odds ratios for breeds having calcium oxalate stones have been calculated previously. Lekcharoensuk et al (17) showed an increased risk in the USA in the schnauzer, Lhasa apso, Yorkshire terrier, bichon frise, shi tzu, and poodle. Ling et al (18) and Low et al (6) found increased OR for the poodle, terrier, schnauzer, bichon frise, pinscher, Pomeranian and Lhasa apso. Similar results were reported from the UK by Roe et al (19). In Hungary (14) and Switzerland (4) pinscher, terrier, schnauzer, and poodle were found to have an increased OR for calcium oxalate stones. Recently, in Canada increased OR for calcium oxalate uroliths were reported for terrier breeds, miniature pinscher, Chihuahua, Pomeranian, Maltese, schnauzer and papillon (2). All mentioned breeds also have a significantly increased OR to have calcium oxalate stones in the present study. Interestingly, there was a noticeable difference between breeds with Wh and breeds with Wd stones. Whewellite stones have mainly been submitted from terriers and pinschers as well as from the keeshond, sheltie, and Chihuahua, while there is an increased risk for Wd stones in the Pomeranian, borzoi, spitz, lapphund, bichon frise as well as in some terrier breeds. Interestingly, many breeds have an increased OR for one calcium oxalate stone type but not for the other. A clear explanation of this oddity cannot be given as a different metabolic status for the production of the 2 calcium oxalate stones has not been described in different breeds.

Calcium oxalate stones are often reported in human medical publications. In earlier publications, 73.1% (USA, 1962) of analyzed stones were composed of calcium oxalate (21), while in more recent reports, 67% (USA), 66% (France, 2004), and 72.7% (Germany) of analyzed stones were composed of calcium oxalate (18,22,23). Human medicine reports on urinary stone analysis often differentiate between the 2 hydrate forms Wh and Wd in calcium oxalate stones. In contrast to our results in dogs, in humans, Wh stones were found more often than Wd stones, e.g., 44% versus 22% (20) and 61% versus 12% (23). As in dogs, humans with Wh stones were also, on average, older than those with Wd stones (20).

The 2 hydrate forms Wh and Wd differ not only in one molecule of water of crystallization, but also in the crystal lattices in which the atoms are arranged. The crystal lattice of Wh is monoclinic and that of Wd tetragonal. Thermodynamically, Wh is the stable hydrate form of calcium oxalate, i.e., under normal temperature and pressure conditions it does not change. In contrast, synthetically produced pure Wd is metastable, i.e., even under normal conditions, Wd precipitates a mole of water of crystallization, the crystal lattice changes and becomes Wh. In vitro studies showed that pure Wd can be manufactured in an aqueous solution at a temperature of 5°C to 10°C, and that a complete transformation of pure Wd to Wh at 20°C is possible in time spans of 5 to 75 h (11,24). In the absence of moisture, Wd stones remain stable for weeks at temperatures up to 110°C (10). If, however, they are treated with aqueous solution at 37°C (body temperature), they slowly transform into stable Wh (12). This transformation from Wd to Wh can also be observed in urinary stones in vivo (25–27). Such crystals are referred to as pseudomorphs of Wh to Wd. In order to produce urinary stones from Wd, certain conditions are required to stabilize this tetragonal crystal structure. Mass spectrometry analysis of Wh and Wd urinary stones has shown that the foreign element content of Wh at 1.39% differed significantly from that of Wd at 2.45% (8,28). This allows the conclusion that foreign elements are incorporated into the crystal lattice of Wd stones which contribute to the stabilization of the tetragonal lattice. In crystallization experiments with dialyzed urine, it was shown that Wh is always formed in the fraction with high molecular weight components (> 5000) and that Wd crystallizes from the dialysate solution with low molecular weight mineral constituents (11). Crystallization studies have also shown that certain amounts of magnesium favor the crystallization of Wd (29). In animal experiments, it has been shown that intraperitoneal administration of sodium glyoxylate strongly increased oxalate excretion in the urine, while the intrarenally formed crystals consisted exclusively of Wh (30). In primary hyperoxaluria type I, a genetic disease in humans, there is also a high renal oxalate excretion with calcium oxalate stone formation. Analysis of stones from these patients always revealed Wh (31).

In human medicine, it has been observed that the risk of recurrence with Wd stones is higher than with Wh stones. However, it remains to be clarified how Wd or Wh stone formation can be predicted or reliably prevented. Experimental investigations have shown that a permanently high oxalate concentration in the urine tends to result in Wh crystallization and that a high concentration of minerals, especially calcium, leads to the rapid formation of Wd. The determination of calcium and oxalate in the urine of human patients with Wh and Wd stones, respectively, revealed that in vivo, Wh is dependent on oxalate concentration, whereas Wd is dependent on calcium concentration (18). Wh stones grow more slowly than Wd concrements. Wh stones have a very compact structure as long as they are formed as primary Wh stones, whereas Wd stones consist of loosely connected single crystals. Thus, Wh stones that have been transformed from Wd also have a loose structure (24,25).

The differentiation between Wh and Wd in urinary stones may point to possible conditions under which the uroliths are formed. However, the above-mentioned experimental investigations on the formation of Wh and Wd are somewhat dated. New experimental methods are required to verify the formation conditions of the calcium oxalates Wh and Wd. This could result in new approaches for therapies in the prophylaxis of relapse of the urinary stone types Wh and Wd. Currently, the distinction between Wd and Wh in urine analysis is a first approach to understanding specific contexts of their formation conditions.

There are several limitations to this study. Due to the retrospective nature, it is unknown how many calcium oxalate uroliths were submitted from the same dog. Multiple stones submitted from the same dog of an uncommon breed might affect the odds ratio for that breed. Unfortunately, these limitations have been present in all epidemiological studies on urolithiasis based on submitted stones. Other limitations are that not all epidemiological data are available for each stone and inaccuracies in data submission might exist. The size of the data pool should alleviate the significance of any errors in data recording from individual dogs. Nevertheless, differentiating calcium oxalate stones submitted from dogs into Wh and Wd uroliths increases the knowledge regarding breed predispositions, age, and gender.