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Compendium November 2009 (Vol 31, No 11)

Calcium Oxalate Urolithiasis

by Kelly Gisselman, DVM, DACVIM (Small Animal Internal Medicine), Cathy Langston, DVM, DACVIM (Small Animal Internal Medicine), Douglas Palma, DVM, DACVIM (Small Animal Internal Medicine), John McCue, DVM, DACVIM (Small Animal Internal Medicine, Neurology)

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    Calcium oxalate (CaOx) uroliths are one of the most common urolith types in dogs, cats, and humans. Many factors predispose animals to CaOx uroliths, including breed, old age, obesity, male sex, being neutered, and certain diseases. Because there are no effective strategies for medical dissolution of these uroliths, management should focus on physical removal (by surgical or nonsurgical methods) and prevention, especially in patients with recurring uroliths, high-risk breeds, and animals with predisposing diseases. Prevention focuses on diet modification, primarily to increase water intake, and the appropriate use or avoidance of specific medications.

    Calcium oxalate (CaOx) urolith formation depends on the degree of urine saturation with CaOx and the balance between urinary promoters and inhibitors of CaOx formation.1

    Promoters of Calcium Oxalate Formation

    Hypercalciuria, which predisposes patients to CaOx urolithiasis, may be classified as absorptive, renal leak, or resorptive hypercalciuria. Absorptive hypercalciuria is characterized by increased intestinal absorption and renal excretion of calcium and oxalate and has been described in miniature schnauzers with urolithiasis.2 Renal leak hypercalciuria is characterized by impaired renal tubular reabsorption of calcium. Resorptive hypercalciuria is characterized by parathyroid hormone-mediated mobilization of calcium from bone and is a result of primary hyperparathyroidism.3,4 Conditions associated with hypercalciuria include hypercalcemia; excessive dietary calcium, protein, sodium, vitamin D, or vitamin C intake; acidifying diets; urinary acidifiers; and certain drugs (e.g., glucocorticoids, loop diuretics) and diseases (e.g., hyperadrenocorticism).5,6

    Hyperoxaluria can occur with increased intake of dietary oxalate precursors, such as leafy green vegetables (e.g., spinach, rhubarb) or nuts (especially peanuts).7 Dietary calcium binds oxalic acid and inhibits its absorption; thus, calcium restriction enhances hyperoxaluria.7 Vitamin C (ascorbic acid) is metabolized to oxalate, and vitamin B6 alters oxalate metabolism (FIGURE 1).7

    Inhibitors of Calcium Oxalate Formation

    Increases in serum phosphorus concentration decrease calcitriol production and enhance urinary excretion of a natural inhibitor of oxalate precipitation, pyrophosphate.7 Restricted dietary magnesium has been associated with CaOx formation because dietary magnesium complexes with oxalate and reduces oxalate binding with ionic calcium.8,9 Nephrocalcin is a substance in the urine that naturally inhibits CaOx uroliths and is less effective in patients that form CaOx uroliths.10 Other substances (e.g., Tamm-Horsfall mucoproteins, glycosaminoglycans) may inhibit CaOx formation in people, but their role in canine CaOx urolithiasis is unknown.


    Certain breeds of dogs and cats have been shown to have an increased or decreased incidence of CaOx urolithiasis (TABLE 1 and  TABLE 2  and BOX 1).11-13 Obesity and increasing age are risk factors for CaOx urolith formation in dogs and cats.11-15 Cats are at the highest risk between 4 and 16 years of age, with a mean age of 7.3 years at diagnosis.12,16 In two studies,15,17 the mean age of dogs with CaOx uroliths was 7 and 8.4 years. CaOx uroliths are more common in male dogs (2:1 ratio) and male cats (1.5:1 ratio) than in females.11,15,17,18 Neutered dogs and cats are also at increased risk.11,18


    CaOx is one of the most common substances in uroliths in dogs and cats.19,20 Over the past 21 years, the ratio of CaOx uroliths to struvite uroliths has increased significantly. The percentage of canine CaOx uroliths submitted for analysis increased from 7% in 1986 to 38% in 2000,8,11 and the percentage of feline CaOx uroliths submitted increased from 2% in 1984 to 55% in 2001.21,22 Increases in CaOx urolith formation may be related to dietary acidification and the increasing longevity of dogs and cats, while decreases in struvite urolith submissions may be due to improved medical management.18,23,24 More recent information shows a plateau and possible decline in the incidence of CaOx uroliths.15,25

    Nephroliths and ureteroliths account for only 2% to 3% of urolith submissions from dogs and cats.26,27 In dogs, 31% to 39% of nephroliths and ureteroliths are composed of CaOx; 33% to 38% are composed of struvite.28,29 Most (80% to 99%) feline nephroliths and ureteroliths are composed of CaOx.28,30


    Creation of acidic urine (pH <7.0) can predispose animals to CaOx urolithiasis by inducing increased excretion of calcium in the urine; however, CaOx solubility is not influenced by urine pH. Most CaOx uroliths form in sterile urine, either singly or as multiple stones. CaOx crystals can be found during urinalysis but do not necessarily correlate with urolith composition (FIGURE 2).

    CaOx is the most radiopaque urolith substance (FIGURE 3). In an in vitro study,31 CaOx uroliths could be distinguished from struvite uroliths with 75% to 88% accuracy when computed tomography was used and uroliths were composed of a single mineral type. CaOx calculi are usually white and hard (FIGURE 4). Surface characteristics may vary, but they often have sharp, jagged edges. The gross appearance of any stone is never a replacement for stone analysis.

    When radiopaque urinary calculi are found, a serum biochemistry panel, complete blood count, urinalysis, and urine culture should be evaluated for diseases associated with urolithiasis. Testing for hyperadrenocorticism should be considered if clinical criteria for this condition are met. A complete evaluation for hypercalcemia should be conducted if the ionized calcium level is elevated.


    The only effective treatment for CaOx stones is physical removal by any of the surgical or nonsurgical techniques that have been described in the literature.32 Guidelines on when to use surgical versus nonsurgical techniques have been published elsewhere.32

    Compared with other mineral types, canine CaOx uroliths are relatively easy to fragment with extracorporeal shock wave lithotripsy.33 Feline CaOx uroliths are less susceptible to fragmentation with extracorporeal shock wave lithotripsy than canine CaOx uroliths.34



    Diet is an important factor in the prevention of CaOx stones (TABLE 3). Canned diets high in fat, phosphorus, magnesium, potassium, chloride, and moisture have been associated with decreased risk of CaOx formation.9,24,35 Reducing dietary carbohydrate has reduced the risk of CaOx urolithiasis in dogs but not in cats.8,9 Dry diets with a high fiber content have been shown to be associated with an increased risk of urolithiasis in dogs.9 Diets formulated to maximize urine acidity are not proven to cause CaOx formation in dogs and cats.8 Acidemia may promote hypercalciuria and hypocitraturia, increasing risk of urolith formation.36 The source of acidification plays a greater role than the urine pH achieved.

    In one study in dogs,35 Waltham Canine S/O Lower Urinary Tract Support Diet (Royal Canin USA) reduced CaOx relative supersaturation, a measure of the risk of CaOx formation, by 63%.For cats, Prescription Diet x/d Feline (Hill's Pet Nutrition) showed a 59% reduction in CaOx relative supersaturation in one study37; this diet has since been replaced by Prescription Diet c/d Multicare Feline (Hill's Pet Nutrition).

    Increasing water consumption (either through the use of canned food or through adding water to the diet) to achieve a urine specific gravity <1.020 in dogs and <1.025 in cats dilutes the urine concentration and decreases risk of stone recurrence.38 In cats, the risk of CaOx urolith formation was one-third less when canned diets were fed than when other dietary formulations were consumed.7


    If dietary therapy is not effective at preventing CaOx crystalluria, or if uroliths recur, potassium citrate can be administered orally. Citrate may form soluble salts with calcium, thereby decreasing the formation of less soluble CaOx crystals; however, an increase in the urine pH and metabolic alkalosis with resultant reduction in calciuria are the main proposed mechanisms of action.39 The potassium citrate dose (50 to 75 mg/kg mixed with food q12h) is titrated to maintain urine pH between 7.0 and 7.5. If dietary therapy alone maintains the urine pH above 7.0, potassium citrate should not be added. Serum potassium should initially be monitored monthly and the dose reduced if hyperkalemia occurs.

    Thiazide diuretics promote natriuresis in the distal tubules, which results in calcium reabsorption.40 In dogs, urine calcium concentration was reduced by 34% with hydrochlorothiazide (2 mg/kg PO q12h).41 Patients should be monitored for dehydration, hypokalemia, and hypercalcemia.28

    Vitamin B6 supplementation (2 to 4 mg/kg PO q24-48h) could be considered in patients consuming homemade diets.42

    Vitamins D and C enhance intestinal calcium absorption; therefore, excessive supplementation should be avoided. Steroids (e.g., prednisone) and furosemide also lead to calciuresis and should be avoided.

    Monitoring Schedule

    About 50% to 60% of CaOx uroliths recur within 3 years of surgical or nonsurgical removal; this incidence may be exaggerated by incomplete removal of calculi at the initial surgery.38 Recommended monitoring to decrease recurrence after stone removal includes urinalysis, electrolyte evaluation if using thiazide diuretics, radiography, and/or ultrasonography at 2 to 4 weeks, 3 months, and every 6 months thereafter. If recurrent uroliths are identified while they are still small, nonsurgical removal techniques should be considered, along with reevaluation for underlying causes.38


    More research is necessary to understand how to prevent CaOx uroliths. Early screening and detection of uroliths (especially in high-risk breeds) are important to allow use of nonsurgical removal techniques and to start preventive measures. Increased water consumption is the best strategy to maintain good urinary tract health.39

    To Learn More

    Read here for a partial list of human foods to avoid feeding to dogs with CaOx urolithiasis, as well as a list of foods that can be fed to dogs with CaOx urolithiasis.

    Downloadable PDF

    1. Stevenson AE, Robertson WG, Markwell PJ. Risk factor analysis and relative supersaturation as tools for identifying calcium oxalate stone-forming dogs. J Small Anim Pract 2003;44:491-496.

    2. Lulich JP, Osborne CA, Nagode LA, et al. Evaluation of urine and serum metabolites in miniature schnauzers with calcium oxalate urolithiasis. Am J Vet Res 1991;52:1583-1590.

    3. Klausner JS, Fernandez FR, O'Leary TP, et al. Canine primary hyperparathyroidism and its association with urolithiasis. Vet Clin North Am Small Animal Pract 1986;15:227-239.

    4. Marquez GA, Klausner JS, Osborne CA. Calcium oxalate urolithiasis in a cat with a functional parathyroid adenocarcinoma. JAVMA 1995;5:817-819.

    5. Bartges JW. Diagnosis of urinary tract infections. Vet Clin North Am Small Animal Pract 2004;34:923-933.

    6. Hess RC, Kass PH, Ward CR. Association between hyperadrenocorticism and development of calcium-containing uroliths in dogs with urolithiasis. JAVMA 1998;212:1889-1891.

    7. Lulich JP, Osborne CA, Thumchai R, et al. Epidemiology of canine calcium oxalate uroliths: identifying risk factors. Vet Clin North Am Small Animal Pract 1999;29:113-122.

    8. Lekcharoensuk C, Osborne CA, Lulich JP, et al. Associations between dry dietary factors and canine calcium oxalate uroliths. Am J Vet Res 2002;63:330-337.

    9. Lekcharoensuk C, Osborne CA, Lulich JP, et al. Associations between dietary factors in canned food and formation of calcium oxalate uroliths in dogs. Am J Vet Res 2002;63:163-169.

    10. Carvalho M, Lulich JP, Osborne CA, et al. Defective urinary crystallization inhibition and urinary stone formation. Int Braz J Urol 2006;32:324-348.

    11. Lekcharoensuk C, Lulich JP, Osborne CA, et al. Patient and environmental factors associated with calcium oxalate urolithiasis in dogs. JAVMA 2000;217:515-519.

    12. 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. JAVMA 2000;217:520-525.

    13. Kirk CA, Ling GV, Franti CE, et al. Evaluation of factors associated with development of calcium oxalate urolithiasis in cats. JAVMA 1995;207:1429-1434.

    14. Thumchai R, Lulich JP, Osborne CA, et al. Epizootiologic evaluation of urolithiasis in cats: 3,798 cases (1982-1992). JAVMA 1995;219:547-551.

    15. Ling GV, Thurmond MC, Choi YK, et al. Changes in proportion of canine urinary calculi composed of calcium oxalate or struvite in specimens analyzed from 1981 through 2001. J Vet Intern Med 2003;17:817-823.

    16. Osborne CA, Lulich JP, Thumchai R, et al. Diagnosis, medical treatment, and prognosis of feline urolithiasis. Vet Clin North Am Small Animal Pract 1996;26:589-627.

    17. Houston DM, Moore AEP, Favrin MG, et al. Canine urolithiasis: a look at over 17,000 urolith submissions to the Canadian Veterinary Urolith Centre from February 1998 to April 2003. Can Vet J 2004;45:225-230.

    18. Houston DM, Moore AEP, Favrin MG, et al. Feline urethral plugs and bladder uroliths: a review of 5484 submissions 1998-2003. Can Vet J 2003;44:974-977.

    19. Ling GV, Thurmond MC, Choi YK, et al. Changes in proportion of canine urinary calculi composed of calcium oxalate or struvite in specimens analyzed from 1981 through 2001. J Vet Intern Med 2003;17:817-823.

    20. Picavet P, Detilleux J, Verschuren S, et al. Analysis of 4495 canine and feline uroliths in the Benelux. A retrospective study: 1994-2004. J Anim Physiol Anim Nutr (Berl) 2007;91:247-251.

    21. Osborne CA, Lulich JP, Thumchai R, et al. Feline urolithiasis. Etiology and pathophysiology. Vet Clin North Am Small Animal Pract 1996;26:217-232.

    22. Lulich JP, Osborne CA. Epidemiological shifts in feline urolith type, in state of the stone. Hill's Symp Lower Urinary Tract Dis 2007:13-18.

    23. Osborne CA, Lulich JP, Polzin DJ, et al. Analysis of 77,000 canine uroliths: perspectives from the Minnesota Urolith Center. Vet Clin North Am Small Animal Pract 1999;29:17-38.

    24. Lekcharoensuk C, Osborne CA, Lulich JP, et al. Association between dietary factors and calcium oxalate and magnesium ammonium phosphate urolithiasis in cats. JAVMA 2001;219:1228-1237.

    25. Cannon AB, Westropp JL, Ruby AL, et al. Evaluation of trends in urolith composition in cats: 5,230 cases (1985-2004). JAVMA 2007;231:570-576.

    26. Ling GV, Ruby AL, Johnson DL, et al. Renal calculi in dogs and cats: prevalence, mineral type, breed, age, and gender interrelationships (1981-1993). J Vet Intern Med 1998;12:11-21.

    27. Rubin SI. Chronic renal failure and its management and nephrolithiasis. Vet Clin North Am Small Animal Pract 1997;27:1331-1354.

    28. Ross SJ, Osborne CA, Lulich JP, et al. Canine and feline nephrolithiasis: epidemiology, detection, and management. Vet Clin North Am Small Animal Pract 1999;29:231-250.

    29. Snyder DM, Steffey MA, Mehler SJ, et al. Diagnosis and surgical management of ureteral calculi in dogs: 17 cases (1990-2003). N Z Vet J 2005;53:19-25.

    30. Kyles AE, Stone EA, Gookin JL, et al. Diagnosis and surgical management of obstructive ureteral calculi in cats: 11 cases (1993-1996). JAVMA 1998;213:1150-1156.

    31. Pressler BM, Mohammadian LA, Li E, et al. In vitro prediction of canine urolith mineral composition using computed tomographic mean beam attenuation measurements. Vet Radiol Ultrasound 2004;45:189-197.

    32. Langston CE, Gisselman KM, Palma D, et al. Methods of urolith removal. Compend Contin Educ Pract Vet. In press.

    33. Adams LG, Senior DF. Electrohydraulic and extracorporeal shock-wave lithotripsy. Vet Clin North Am Small Animal Pract 1999;29:293-302.

    34. Adams LG, Williams JC Jr, McAteer JA, et al. In vitro evaluation of canine and feline calcium oxalate urolith fragility via shock wave lithotripsy. Am J Vet Res 2005;66:1651-1654.

    35. Stevenson AE, Blackburn JM, Markwell PJ, et al. Nutrient intake and urine composition in calcium oxalate stone-forming dogs: comparison with healthy dogs and impact of dietary modification. Vet Ther 2004;

    36. Fettman MJ, Coble JM, Hamar DW, et al. Effect of dietary phosphoric acid supplementation on acid-base balance and mineral and bone metabolism in adult cats. Am J Vet Res 1992;53:2125-2135.

    37. Lulich JP, Osborne CA, Lekcharoensuk C, et al. Effects of diet on urine composition of cats with calcium oxalate urolithiasis. JAAHA 2004;40:185-191.

    38. Lulich JP, Osborne CA, Lekcharoensuk C, et al. Canine calcium oxalate urolithiasis: case-based applications of therapeutic principles. Vet Clin North Am Small Animal Pract 1999;29:123-139.

    39. Stevenson AE, Wrigglesworth DJ, Smith BH, et al. Effects of dietary potassium citrate supplementation on urine pH and urinary relative supersaturation of calcium oxalate and struvite in healthy dogs. Am J Vet Res 2000;61:430-435.

    40. Lulich JP, Osborne CA, Lekcharoensuk C, et al. Effects of hydrochlorothiazide and diet in dogs with calcium oxalate urolithiasis. JAVMA 2001;218:1583-1586.

    41. Lulich JP, Osborne CA. Management of canine calcium oxalate urolith recurrence. Compend Contin Educ Pract Vet 1998;20:178-189.

    42. Lulich JP. Feline lower urinary tract disorders: making the correct diagnosis. Proc 2007 FLUTD Hill's Symp 2007:36-41.

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