Urolithiasis accounts for 18% of urinary complaints in dogs (behind bacterial cystitis and incontinence). Uroliths occur in the bladder and urethra and can be life threatening if urethral obstruction occurs [1]. Common signs of urinary disease include dysuria: difficult or painful urination, abnormal frequency of urination or urinating in unusual places.

Epidemiology[1]

There are many risk factors for the development of urinary stones. Age, sex, neutering, body condition, activity and infection are all indicated. 20 years ago, the most common stone encountered in dogs was struvite. Calcium oxalate and struvite uroliths remain the most common and account for over 80% of all stones analysed in dogs [2, 3]. Increasingly cystine uroliths are diagnosed [2,10].
These changes is likely due to a combination of improved diagnostics with radiolucent stones and more effective medical dissolution with acidifying diets.

Struvite
Historically struvite stones had been associated with small and miniature breeds [6]. More recent studies find medium to large breeds to be more at risk such as the Golden retriever and Labrador retriever [3].

Crystals form most rapidly in alkaline urine (pH>6.5) and are composed of magnesium, ammonium and phosphate; they dissolve easily with dietary intervention if the urine pH can be reduced to 6.2 or lower [9]. In dogs, struvite uroliths are strongly associated with infection with urease-producing microbes and as such occur much more commonly in females. Unless the infection is managed concurrently, medical dissolution is likely to be unsuccessful as the urine will remain at an elevated pH [9].
Typically, the crystals are rectangular in shape and macroscopically the stones are smooth and pale coloured.

Oxalate
Oxalates are one of the most common uroliths submitted for analysis in dogs, respectively 47% of all submissions over the mentioned period [2]. Breed predispositions are observed; Fox Terriers, Cairn Terriers, Chihuahua, Maltese, Pomeranian and Miniature Schnauzers are 3.5 times more likely to suffer with oxalate stones compared with mixed breed dogs [3, 5]. The recurrence rate is high- up to 57% of dogs will experience repeat stone formation 3 years following initial diagnosis [9].

Using nutritional management to adjust the urine pH is considered ineffective for prevention of recurrence- nutritional focus is on decreasing the Relative Super Saturation (RSS)- see below mainly by encouraging urine dilution via increased hydration and by the addition of sodium or potassium chloride to support diuresis [9].

Microscopically these crystals are square; macroscopically roughened and beige in colour.

Urate
Ammonium urate stones accounts for only 3.2% of all submissions. 38% of all urate reports are found in Dalmatians, of which 98% are from male dogs [3]. 94% of uroliths seen in Dalmatians are urate; Bulldogs of all descriptionsare also overrepresented [3]. Dalmatians have the predisposing factor of a genetic mutation (SLC2A9) resulting in uric acid. Metabolism dysfunction and an increase in urate accumulation [6,10]. Only in Dalmatians has medical dissolution with diet been reported [9].

Ammonium urate stones can also be found in patients with hepatic disease, including portosystemic shunts; these cases have not been resolved with nutritional management alone [9].

Uric acid is a waste product produced when purines are metabolised. Purines are nitrogenous compounds found in high levels in fish and organ meats [9]. Nutritional management relies on a purine restriction- either via moderate protein restriction or use of low purine containing protein sources such as egg and soy [9]. Urate stones are less soluble in acidic urine- acidifying diets such as Urinary S/O are contraindicated [9]. 

RC Hypoallergenic and Renal range can used as a wet food alternative due to the moderate protein restriction, soy-protein base and alkalinising properties.   In cases of portosystemic shunt and hepatic insufficiency, the RC Hepatic diet may be most appropriate for management.

Cystine

These uroliths equate to approximately 2.7% of submissions, and this proportion has been noted to be on the increase over the last decade [2,10]. 70% of submissions are from intact males and other characteristics are non-specified.  It is suspected that an androgen-dependent mutation preventing renal re-uptake is responsible for the increased cystine found in urine [10]. Neutering is recommended in these cases to limit recurrence and also to prevent further transfer of the mutation to the next generation [10].

Cystine is less soluble and more likely to crytallise in acidic urine – occasionally cystine crystals can be confused with struvite crystals [7,9]. 

Treatment will focus on neutering if appropriate, promoting hydration and urine dilution, and limiting cystine and methionine intake. This can be done using plant-based proteins which tend to be lower in the sulphur amino acids than animal sources and by using alkalinising diets [9]. Urinary U/C and RC Hypoallergenic and Renal range are recommended for use.

Crystal Formation

Urine contains charged electrolyte particles that interact with each other, bind together and form crystals. These then aggregate together to form stones that are visible to the naked eye. The degree to which the particles interact determine the rate of crystal formation and stone growth.

Factors that affect the interaction include:
• Electric charge- which varies according to pH
• Concentration of electrolytes- which will be lower in dilute urine
• Duration of contact between electrolytes- time will be reduced if the bladder is emptied more frequently

These factors can be directly influenced by the diet the animal is being fed. Therefore, through dietary manipulation we can positively or negatively affect the chance of crystal and stone formation.

Dietary Influences
The diet a pet receives exerts huge influence over the urine composition from several perspectives:
• Mineral content
• Urine pH
• Urine volume

From this it is possible to understand how dietary manipulation influences urine composition and hence the risk of crystal and stone formation.

RSS

These 3 factors have been mathematically combined to produce a value known as the Relative Super- Saturation or RSS. This relative value tells us how likely crystals or stones are to form in the urine produced when an animal is fed a particular diet. The RSS is particular to the diet and to the crystal type (e.g. struvite or oxalate).

There are 3 bands that the RSS value can fall into:
1. Labile supersaturation, spontaneous crystallisation occurs producing rapid crystal growth
2. Metastable supersaturation, there is neither spontaneous crystal formation nor dissolution
3. Undersaturation, no crystallisation occurs, spontaneous crystal dissolution occurs.

Royal Canin Urinary diet range

Urinary S/O diets have RSS values for both Struvite and Oxalate that fall into the undersaturation band. Meaning there will be no further crystallisation and indeed dissolution of existing struvite crystals. Diets that sport the S/O Index symbol fall into the metastable supersaturation band, meaning they produce urine that will not support spontaneous crystallisation.

Royal Canin Urinary S/O Diets have several indications;
• Dissolution of struvite uroliths
• Management of recurrent struvite and oxalate urolithiasis to help prevent recurrence
• Canine bacterial cystitis

• Urinary S/O – this diet will effectively dissolve struvite uroliths and prevent recurrence of oxalate following surgical removal.
• Urinary S/O small dog – as for Urinary S/O but formulated for dogs under 10kg.
• Urinary S/O wet – suitable for mix feeding with the dry S/O diet when the dog prefers this. Useful in cases of recurrence to increase urine dilution.
• Urinary U/C – Low purine. This is the diet of choice when urate or cystine stones have been identified. It should be used as a long-term maintenance diet for cases in Dalmatians to help prevent recurrence.
• RC Hypoallergenic and Renal range can used as a wet food alternative due to the moderate protein restriction, soy-protein base and alkalinising properties. 
• In cases of portosystemic shunt and hepatic insufficiency, the RC Hepatic diet may be most appropriate for management.

Contraindications and Limitations of Urinary diets

In humans, adjustment of salt is contraindicated in cases of hypertension, chronic renal failure and cardiac disease. Studies in cats monitored over the longer term (6-24 months) found no negative effects on glomerular filtration rate or serum creatinine [9]. No studies in dogs or cats have shown any changes in systemic hypertension [9].The location of the urolith will also affect success at medical dissolution- the stone needs to be ‘bathed’ in affected urine in order to dissolve, so those located within the kidney, ureter and urethra will likely require urohydropropulsion or surgical intervention [8].

Pregnancy, lactation, growth, metabolic acidosis and concurrent use of urine acidifying drugs are contraindications for use. Renal function should be checked in older dogs before prescription if nephroliths are present.

2-12 weeks of dietary therapy with a Urinary S/O diet is required to dissolve struvite stones. In dogs they are often associated with infection; if this is the case, it is imperative that appropriate antibacterial therapy be concurrent and recommended to maintain for at least one month after a negative urine culture is obtained. No diets are available that effectively dissolve oxalate uroliths. However following removal, a Urinary S/O diet to help delay recurrence and should be fed for at least 6 months and then under veterinary recommendation.

It is vital that owners understand the need for routine surveillance for oxalate management and that the goal is to step in quickly to manage small stones with minimally invasive techniques such as urohydropulsion (10). Complete resolution is unlikely and regular veterinary care will be needed to optimise outcome regardless of management (8).

All of the diets are balanced for long-term feeding and can be fed lifelong as maintenance if required. Alternatively, a diet bearing the S/O Index symbol will produce urine unfavourable to crystal formation for long term maintenance.

References

1. Bartges, J.W. and A.J. Callens, Urolithiasis. Vet Clin North Am Small Anim Pract, 2015. 45(4): p. 747-68.
2. Kopecny, L. et al., Urolithiasis in dogs: Evaluation of trends in urolith composition and risk factors (2006-2018). J Vet Intern Med, 2021(35). p. 1406-1415.
3. Houston, D.M., et al., Analysis of canine urolith submissions to the Canadian Veterinary Urolith Centre, 1998-2014. Can Vet J, 2017. 58(1): p. 45-50.
4. Rogers, K.D., et al., Composition of uroliths in small domestic animals in the United Kingdom. Vet J, 2011. 188(2): p. 228-30.
5. Allen, H.S., et al., Associations of diet and breed with recurrence of calcium oxalate cystic calculi in dogs. J Am Vet Med Assoc, 2015. 246(10): p. 1098-103.
6. Sorenson, J.L. and G.V. Ling, Metabolic and genetic aspects of urate urolithiasis in Dalmatians. J Am Vet Med Assoc, 1993. 203(6): p. 857-62.
7. Fielder, S. et al., Urine crystals in dogs & cats. NAVC Clinician’s Brief, 2010. p. 72-74
8. Trehy, M. (2022) ‘Nutritional management of urolithiasis in dogs and cats’, In Practice, 44(6), pp. 316–327. doi:10.1002/inpr.91.
9. Queau, Y. (2019) ‘Nutritional Management of Urolithiasis’, Veterinary Clinics of North America: Small Animal Practice, 49(2), pp. 175–186.
10. Byron, J. (2021) ‘Nutritional management of urolithiasis in dogs and cats: Explore the crucial role nutrition plays in treating this frustrating condition and how to determine which diet is best for your patients’, DVM360, 52(9), pp. 62–63.

Post updated: 4th October 2023