Urolithiasis accounts for around 22% of urinary complaints in cats (second only to idiopathic cystitis) (1). Uroliths occur in the bladder and urethra and can be life threatening if urethral obstruction occurs (2). Common signs of urinary disease include dysuria: difficult or painful urination, abnormal frequency of urination or urinating in unusual places.
Common Uroliths
20 years ago, the most common stone encountered in cats was struvite. This has altered in recent years; calcium oxalate is now the most common urolith submitted for analysis (3). This change is likely owing to more acidifying diets being fed, improved diagnostic methods and more effective medical dissolution meaning fewer struvite stones are submitted for identification. Together struvite and oxalate account for 90% uroliths in cats (4).
Struvite
Crystals form most rapidly in alkaline urine (pH greater than 6.5) and are composed of magnesium, ammonium and phosphate; they dissolve easily with medical dietary intervention. Typically, the crystals are rectangular in shape and macroscopically the stones are smooth and pale coloured. Around 38% of stones in cats are now struvite and are most common in young females (4, 5). They do not show a strong breed disposition. Struvite stones in cats tend to be sterile and not associated with infection, so it is unclear why females are much more at risk than males (6). Dissolution can take 2-4 weeks if uncomplicated with infection (8).
Oxalates
Oxalates account for 56% of the stones now seen in cats- overtaking struvite in 1999 (4). They are most typically seen in older, neutered, male animals (5). Tonkinese, Burmese, Himalayan, Devon rex and Persian appear to be at greater risk than DSH (4). Typically, these crystals are square and macroscopically roughened and beige in colour. Oxalate stones can be associated with increased calcinuria so assessment of ionised calcium should form part of the initial investigation. The majority of nephroliths in cats are calcium oxalate in origin.
Urates
5% of uroliths in cats are ammonium urate (4). Purine metabolism dysfunction will result in the development of ammonium urate in dogs; in cats, the mechanism is not fully determined (8). Egyptian Maus and Ocicats are also overrepresented compared to DSH (4) suggesting a potential genetic mutation as seen in dalmatian dogs leading to an increase in urate accumulation (7).
Risk Factors
There are many risk factors for the development of urinary crystals and stones. Age, sex, breed, neutering, body condition and lifestyle are all implicated. As these risk factors change, so the prevalence of the various types of urolith will change, as seen in the last 20 years (4). With all types of stones, diuresis is key- dilution of urine is the most effective way to prevent stone formation. If a fully wet diet is possible by finance and compliance, this is the most effective way to manage urolithiasis (9, 10).
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- varies according to pH
• Concentration of electrolytes- lower concentrations in more dilute urine.
• Duration of contact between electrolytes- time is reduced if the bladder is emptied more frequently.
These factors can be directly influenced by the diet the cat 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 cat receives exerts huge influence over the urine composition from several perspectives:
• Mineral content
• Urine pH (struvite forms much more readily in alkaline urine, oxalate is unaffected by pH)
• Urine volume
From this it is easy to understand how dietary manipulation influences urine composition and hence the risk of crystal and stone formation.
RSS
These above 3 factors have been mathematically combined in a computer algorithm (SuperSat) to produce a value known as the Relative Super- Saturation or RSS value. 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 crystallization occurs producing rapid crystal growth.
2. Metastable supersaturation; there is neither spontaneous crystal formation nor dissolution.
3. Undersaturation; no crystallization 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 range; meaning there will be no further crystallization and indeed dissolution of existing struvite crystals. Oxalate crystals cannot effectively be dissolved. Diets that sport the S/O Index symbol fall into the metastable supersaturation band, meaning they produce urine that will not support spontaneous crystallization.
Royal Canin Urinary S/O diets have several indications
• Dissolution of struvite uroliths
• Management of recurrent struvite and oxalate urolithiasis to help prevent recurrence
• Feline idiopathic cystitis (WET DIET)
This is achieved through acidification of the urine (no impact on oxalate); increase in urine volume and dilution; low RSS values (undersaturation) for both struvite and oxalate; limited precursors of stone constituents (e.g. Magnesium).
Urinary S/O wet – primary indication is feline idiopathic/ recurrent cystitis. If it can be fed as sole food in cases of struvite or oxalate urolithiasis, wet food is considered the most effective way to promote urine dilution. It can be also be used in any cases of struvite or oxalate urolithiasis where mixed feeding is preferred by the cat.
Urinary S/O – this is the first choice when uroliths have been identified in the first occurrence. The diet will effectively dissolve struvite uroliths and delay recurrence of oxalate following surgical removal.
Urinary S/O moderate calorie – first choice when uroliths have been identified in the first occurrence in cats that are overweight or have a tendency to gain weight (eg indoor, older or neutered animals)
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 either dogs or cats have shown any changes in systemic hypertension (9).
Location of the urolith will also affect success- 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 cats with nephroliths before nutritional management is finalised.
S/O diets are required to dissolve struvite stones. In cats to help prevent recurrence the diet should be continued for 6 months post dissolution.
No diets are available that effectively dissolve oxalate uroliths. However following removal, a Urinary S/O diet to help delay recurrence should be fed to cats for at least 6 months and then subject to veterinary review. It is vital that owners understand the need for continual surveillance 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 the Urinary diets are balanced for long-term feeding and can be fed lifelong if required. Alternatively, a diet bearing the S/O Index symbol will produce urine unfavourable to crystal formation for long term maintenance- falling within the metastable RSS range.
References
1. Gerber B, Boretti FS, Kley S, Laluha P, Muller C, Sieber N, et al. Evaluation of clinical signs and causes of lower urinary tract disease in European cats. J Small Anim Pract. 2005;46(12):571-7. Epub 2005/12/17. doi: 10.1111/j.1748-5827.2005.tb00288.x. PubMed PMID: 16355731.
2. Bartges JW, Callens AJ. Urolithiasis. Vet Clin North Am Small Anim Pract. 2015;45(4):747-68. Epub 2015/05/24. doi: 10.1016/j.cvsm.2015.03.001. PubMed PMID: 26002797.
3. Cannon AB, Westropp JL, Ruby AL, Kass PH. Evaluation of trends in urolith composition in cats: 5,230 cases (1985-2004). J Am Vet Med Assoc. 2007;231(4):570-6. Epub 2007/08/19. doi: 10.2460/javma.231.4.570. PubMed PMID: 17696857.
4. Houston DM, Vanstone NP, Moore AE, Weese HE, Weese JS. Evaluation of 21 426 feline bladder urolith submissions to the Canadian Veterinary Urolith Centre (1998-2014). Can Vet J. 2016;57(2):196-201. Epub 2016/02/03. PubMed PMID: 26834273; PubMed Central PMCID: PMCPMC4713001.
5. Lekcharoensuk C, Osborne CA, Lulich JP. Epidemiologic study of risk factors for lower urinary tract diseases in cats. J Am Vet Med Assoc. 2001;218(9):1429-35. Epub 2001/05/10. doi: 10.2460/javma.2001.218.1429. PubMed PMID: 11345305.
6. Houston DM, Moore AE. Canine and feline urolithiasis: examination of over 50 000 urolith submissions to the Canadian veterinary urolith centre from 1998 to 2008. Can Vet J. 2009;50(12):1263-8. Epub 2010/03/02. PubMed PMID: 20190975; PubMed Central PMCID: PMCPMC2777289.
7. Sorenson JL, Ling GV. Metabolic and genetic aspects of urate urolithiasis in Dalmatians. J Am Vet Med Assoc. 1993;203(6):857-62. Epub 1993/09/15. PubMed PMID: 8226242.
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. doi:10.1016/j.cvsm.2018.10.004.
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