Introduction

The two most common urolith types identified in dogs and cats in North America, and many other parts of the world, are calcium oxalate (monohydrate and dihydrate) and struvite (magnesium ammonium phosphate hexahydrate), with the relative proportions of these two types varying over time.

Animal-related and dietary factors contribute in various ways to the development and management of these uroliths and will be the focus of this talk. Other urolith types, including cysteine and urate, occur much less frequently. The ACVIM Small Animal Consensus Recommendations on the Treatment and Prevention of Uroliths in Dogs and Cats, available online, addresses their management.¹

Calcium Oxalate

Canine Calcium Oxalate (CaOx)

Approximately 75% occur in older, small breed dogs.² Many of these at-risk dogs are overweight, a factor we must remember when calculating caloric intake from food including treats.³ Although a genetic basis has not been proven, there are distinct differences from non-stone-forming dogs, in the gastrointestinal absorption of calcium, mineral metabolism and urine composition in certain breeds of dogs including the miniature schnauzer and bichon frise.^4-9 Hypercalciuria rather than hyperoxaluria appears to be a significant predisposing factor of CaOx urolith formation in both dogs and cats.^7,8 The enteric microbial flora of calcium oxalate stone-forming dogs is altered and distinct from non-stone-forming dogs, and Oxalobacter formigenes, an anaerobic organism that metabolizes intestinal sources of oxalate, is absent in some stone-forming dogs.^10,11

Feline Calcium Oxalate

Occurs with increased risk in older, male cats and in particular, Tonkinese, Burmese, Devon rex, Himalayan, Persian and ragdoll cats.¹² Many cats with urolithiasis are overweight. Hypercalcemia and concomitant hypercalciuria may need to be addressed

Management of Calcium Oxalate Urolithiasis in Dogs and Cats

Medical dissolution of calcium oxalate urolithiasis is not yet possible. Minimally invasive procedures (MIP) such as lithotripsy are recommended over surgery for urolith removal.¹ As risk of calcium oxalate urolith recurrence is high, especially in some breeds of dogs, a change to a therapeutic diet designed not to be overly restricted in calcium, magnesium or phosphorus is recommended.^1,5,13-15 There are a number of diets, with variable nutrient profiles and some with either relative supersaturation testing (RSS) or activity product ratio (APR) testing, available to help in prevention of calcium oxalate urolithiasis. Multiple small meals throughout the day may help stimulate water intake and urine output. Treats are generally not recommended unless they follow the exact profile of the therapeutic diet. Unfortunately, more than 50% of owners give human food as treats to their dogs and this behaviour is unlikely to change.³ Consequently, it is important a diet history including treats and supplements is filled out—a short diet history form is available at wsava.org. Any human foods offered must be safe and appropriate, that is to say, low in calories, low in oxalate, moderate in calcium, low in collagen/hydroxyproline, and provide no more than 10% of the pet’s maintenance energy requirements (MER). Oxalate levels are highly variable in human foods depending on the source cited.¹⁶

For the purpose of this talk, a discussion on oxalate and caloric content of human food utilizes the Harvard web site (https://regepi.bwh.harvard.edu/health/Oxalate/files) and the USDA site (www.nal.usda.gov/fnic/foodcomp/search) respectively. The reader is reminded that only a small portion of oxalate comes from the diet with a greater amount endogenously produced from hydroxyproline in the liver.¹⁷ Studies in dogs and cats are limited on this topic, but it is clear that the amount of hydroxyproline in the diet or in treats should be limited.^18-21 In a recent study, rawhide bones were reported to have the highest hydroxyproline content of examined chews.²¹ Rawhide chews and other animal body parts (e.g., pigs’ ears) are potential sources of contamination from bacterial organisms, and they often exceed the 10% of daily caloric intake suggested for treat ingestion.

In addition to diet, additional therapy may be needed: Potassium citrate (50–75 mg/kg q 12–24 h) is commonly recommended for dogs and cats, although citrate deficiencies have not been identified. One study did document an increase in urinary citrate levels and in three miniature schnauzers, a significantly lower urinary relative calcium oxalate supersaturation when fed a diet supplemented with potassium citrate, compared with control diet.²²

Vitamin B6 (pyridoxine) increases the transamination of glycosylate, an important precursor of oxalic acid, to glycine and while naturally occurring deficiencies have not been reported in dogs and cats, supplementation is often recommended at a dosage of 2 mg/kg PO q 12 h. Hydrochlorothiazide diuretics may be used in dogs and cats (with caution).

Probiotics: Given the absence of Oxalobacter degrading bacteria in some dogs (probably cats) with oxalate uroliths, probiotics containing Oxalobacter-degrading bacteria such as Lactobacillus spp., Bifidobacterium spp., and Enterococcus among others may be of value.^23-25

Monitoring

Follow-up is critical. Urinalysis (pH and USG in particular) is recommended every 3–6 months; imaging for stone recurrence is recommended at least every 6 months.

Struvite in Dogs

Struvite in dogs is almost always in female dogs and due to urinary tract infection (UTI) with ascending urea-splitting bacteria, most commonly Staphylococcus pseudintermedius, or less commonly Proteus mirabilis (uncommonly Ureaplasma, Corynebacterium). These bacteria result in hydrolysis of urea to form ammonia and carbon dioxide which increases the urine pH and availability of ammonium and phosphate ions for struvite formation; it is not usual for a struvite urolith to contain smaller amounts of calcium phosphate and urate. Houston et al. determined that medium and larger breeds (in particular, the Saint Bernard, Labrador retriever, and Golden retriever) are more likely to have a struvite urolith than any other type of stone.²

Management

Medical dissolution of struvite uroliths is possible providing the diet maintains an average urine pH <6.5, ideally under saturates the urine in minerals contributing to stone formation, and appropriate antibiotics, based on culture and sensitivity results (from a cystocentesis collected urine sample) are given to treat the UTI.²⁶ Previous protocols for antibiotic therapy recommended treatment throughout the dissolution period and up to one month beyond due to the concern for bacteria trapped within the matrix of a urolith being slowly released from the inner portions of dissolving uroliths re-establishing an active infection leading to formation of more stones.²⁶ Given concerns for antimicrobial resistance, costs/adverse effects associated with longer-term antibiotic use, and limited scientific data available regarding the need for antibiotics in asymptomatic individuals for this duration, new ISCAID guidelines suggest treatment for only 7 days at the time of initial clinical presentation.²⁶

Monitoring

Following dissolution or mechanical removal of uroliths, monitoring for UTIs and management of any predisposing factors to development of UTIs is critical. Struvite stones can form quickly in the presence of a UTI. No diet can prevent an infection, although diets marketed for struvite urolithiasis may delay or minimize urolith burden in the presence of an unrecognized UTI.¹²

Struvite in Cats

Houston et al. reported an association between domestic long-haired cats and struvite urolithiasis with females overrepresented.¹² Unlike the dog, infection is not generally a risk factor for struvite formation in healthy 1–10-year-old cats. Most affected cats are young to mid-age and often overweight, a risk factor for all forms of lower urinary tract disorders. High urine pH coupled with a high urine specific gravity predisposes. In a retrospective case-control study, diets high in magnesium, phosphorus, calcium, chloride and fiber, moderate in protein and low in fat content were associated with increased risk.²⁷

Management

Feline struvite uroliths are readily amenable to medical dissolution. There are four published studies confirming the efficacy of feline struvite dissolution diets.^28-31 On average, sterile struvite uroliths dissolve within a month. The current recommendation is to feed the calculolytic diet alone for 2–4 weeks beyond radiographic resolution to ensure complete dissolution of calculi <3 mm, which are not radiographically visible.

Where a UTI is confirmed by culture, antibiotics are indicated.

For prevention, a diet promoting aciduria (pH <6.5 but above 6.1), restricted in magnesium and phosphorus, under saturating the urine for struvite, and promoting less concentrated urine, is recommended. A weight-loss diet may be needed. Multiple small meals throughout the day will help mitigate the degree of the postprandial alkaline tide and should help stimulate water intake and urine output.

Water consumption is to be encouraged to help in prevention of all urolith types (excluding infection-induced struvite uroliths), with the goal to maintaining a urine specific gravity <1.020 in dogs and <1.030 in cats.¹

References

1.  Lulich JP, Berent AC, Adams LG, Westropp JL, Bartges JW, Osborne CA. ACVIM small animal consensus recommendations on the treatment and prevention of uroliths in dogs and cats. J Vet Intern Med. 2016;30:1564–1574. PMID 27611724.

2.  Houston DM, Weese HE, Vanstone NP, Moore AE, Weese JS. Analysis of canine urolith submissions to the Canadian Veterinary Urolith Centre, 1998–2014. Can Vet J. 2017;58:45–50. PMID 28042154.

3.  Heuberger R, Wakshlag J. The relationship of feeding patterns and obesity in dogs. J Anim Physiol Anim Nutr (Berl). 2011;95:98–105. PMID 20662965.

4.  Stevenson AE, Markwell PJ. Comparison of urine composition of healthy Labrador retrievers and miniature schnauzers. Am J Vet Res. 2001:62:1782–1786. PMID 11703024.

5.  Stevenson AE, Blackburn JM, Markwell PJ, Robertson WG. Nutrient intake and urine composition in calcium oxalate stone-forming dogs: comparison with healthy dogs and impact of dietary modification. Vet Ther. 2004;5:218–231. PMID 15578454.

6.  Lulich JP, Osborne CA, Nagode LA, Polzin DJ, Parker ML. Evaluation of urine and serum analytes in Miniature Schnauzers with calcium oxalate urolithiasis. Am J Vet Res. 1991;52:1583–1590. PMID: 1767976.

7.  Furrow E, Pattersen EE, Armstrong JP, Osborne CA, Lulich JP. Fasting urinary calcium-to-creatinine and oxalate-to-creatinine ratios in dogs with calcium oxalate urolithiasis and breed-matched controls. J Vet Intern Med. 2015;29:113–119. PMID 25581880.

8.  Dijcker JC, Hagen-Plantinga EA, Everts H, Bosch G, Kema IP, Hendriks WH. Dietary and animal-related factors associated with the rate of urinary oxalate and calcium excretion in dogs and cats. Vet Rec. 2012;171(2):46–52. PMID 22735988.

9.  Carvalho M, Lulich JP, Osborne CA, Nakagawa Y. Defective urinary crystallization inhibition and urinary stone formation. Int Braz J Urol. 2006;32:342–348.