New Postgraduate Certificate Programmes New programmes starting in 2021 Find out more

Urinary stones: it’s all in the flow

01 July 2016, at 1:00am

Lee Danks in the 6th in his series for Royal Canin looks at managing stones in cats and dogs

ANYONE WHO LISTENED IN ON CECILIA VILLAVERDE’S WEBINAR series13 on the management of cat and dog urolithiasis a few weeks back will now have a complete appreciation for the way in which diet can be used to manipulate the urinary environment... and realise it’s not always easy to make the “right” recommendation. 

Five key factors (including qualities of the stones and characteristics of the urine) should be taken into account when selecting an appropriate food: solubility, the presence of precursors, the inhibitors present, urinary pH and urine-specific gravity. For me, this really points out the importance of knowing which stone type, or mix of stone types, you’re dealing with.

Cecilia reminds us that the relative importance of diet is stone-dependent. The formulation of a diet alters its influence on the urinary environment and of course, what we’re all searching for in practice is proven effect. Let’s understand each of the above five factors so we can confidently affect the most appropriate nutritional solution.

Solubility in water

Reflecting on the calls to our nutritional helpline, some don’t realise that only one stone type (namely struvite) is consistently proven to be soluble, and therefore able to be dissolved medically in the bladder.

Solubility is of course advantageous for the patient, the owner and often the clinician when our urinary case doesn’t present as an acute blockage and therefore surgical emergency. A non-invasive approach is less traumatic for all involved. 

As an exception to the solubility rule, certain protocols have been written for urate stones but these are fruitful in about 30% of canine cases only, with a significantly longer dissolution period than for struvite. Other stone types, including calcium oxalate, calcium phosphate, cystine and silica aren’t soluble.

Precursors and inhibitors 

When considering diet (and communicating its importance to owners), it’s easy to take the reductionist route and recommend a “low mineral” or even “low ash” diet.

While this may have been considered an appropriate approach a few decades ago, we now know things aren’t so simple in the real world. Urinary diet formulation relies on a meticulous balance which factors in the interactivity of minerals presented to the pet’s body. Whether an element is considered a nutrient (essential or non-essential), levels of dietary inclusion, bioavailability (influenced by digestibility) and the passage of any precursor or inhibitor to the urinary space all need considering.

Thereafter the interaction between electrolytes is key. We should be mindful that the promotors of some crystal and stone types are claimed inhibitors of others (depending on which reference you read).

This again highlights the importance of knowing which stone type we’re dealing with and for me, increases my awareness of the complexity of urinary diet formulation. 

The take-home message here is that a patient with recurrent calcium oxalate stones shouldn’t be simply asked to restrict dietary calcium. We need to recognise not only the essential nutritional role of calcium, but the many interactions which Cecilia called out in her webinar.

Urinary pH

This factor is well recognised to be of critical importance in the management of struvite stones, given that magnesium and ammonium phosphate are considerably less soluble at an alkaline pH.7 This relates of course to the strength of molecular bonds between our precursors and their interactivity with free hydrogen ions. 

As a general rule, other stones are reported at either acidic pHs or in neutral urinary solutions. Let’s remember, however, that calcium oxalate, a stone type equally prevalent to struvite in many regions, can form at any physiological pH.13 

Some epidemiological studies describe urinary acidity as a risk factor, related to the buffering action of the body. After endogenous bicarbonate stores are exhausted to counter this acidity, phosphate is drawn from bone stores and calcium is mobilised with it, predisposing calciuria. 

Finding the “best pH” for minimising calcium oxalate remains a controversial topic in the eld of veterinary nutrition, but considering calcium oxalate is an “idiopathic disease”13, the focus is on manipulating as many factors as possible to extend the “stone-free” periods between bouts. 

Urine-specific gravity (USG)

We’re now acutely aware that in uencing USG is the most important strategy when it comes to the management of all uroliths, hence: it’s all in the ow!

By diluting urine the likelihood of contact between crystal precursors is minimised, simply meaning precipitation is less likely. In diuresing the cat or dog, we lower USG and increase the frequency of urination, offering a further advantage; with regular bladder “flush-out” there is less time for these lasting precursor interactions to occur.

Regardless of the crystal or stone type, achieving a low USG is the name of the game. Cecilia’s targets are <1.030 for the cat and <1.020 for the dog13, which in effect is an intentional polydipsic-polyuric state.

Remember to communicate this intent to pet owners who by re ex will need not only to have multiple, well-placed fresh water sources about the house but clean the cat’s litter trays more often, or take their dog out for more frequent toileting breaks.

Lowering USG can occur by means of simply increasing water intake (feeding wet diets or soaking dry kibbles in water to create a soup-like meal), adding salt to a diet (within safe levels and being mindful of recognising patients with salt-sensitive hypertension once again, according to reference) or less commonly, lowering protein may be trialled. The rst two strategies are more commonly employed in newer formulations.

What’s the solution?

Let’s remember that not all urinary claims are created equal. While foods which are available on supermarket shelves can often have a “urinary health” message on the bag, there are many manners in which this can be justified to the owner and verified in either a laboratory or in animals themselves.

One newer approach is to consider a diet’s relative super-saturation (RSS) measure. This figure (a mathematical ratio calculated as a result of pooled urine analyses) tells us how likely a diet is to create a urinary environment in favour or ideally unlikely to precipitate a particular crystal type, when fed exclusively.

RSS measures are diet-specific and crystal/stone-specific, but in using this technology and incorporating all of the above five factors, a diet can be formulated to help support patients with multiple urolith types, namely struvite and calcium oxalate – our most common presentations in clinical practice.

Clinical diets formulated with the RSS method of verifcation in mind are classed as “formulated for a particular nutritional purpose” (that is, covered by what we call a PARNUT5).

Once again, it’s important that our clients realise this; that a veterinary diet requires an accompanying professional recommendation from the practice team, and that the after-care monitoring and follow-up is key to maintaining their pet’s urinary health. These diets should be fed exclusively just like an elimination diet to render their desired effect and never, ever forget to fill up that water bowl!

  • With thanks to Erica Moorhouse, RVN, for proof reading.

References and further reading 

  1. Albasan, H., Lulich, J., Osborne, C., Lekcharoensuk, C., Urlich L. and Carpenter, K. (2003) Effects of storage time and temperature on pH, specific gravity, and crystal formation in urine samples from dogs and cats. Journal of the American Veterinary Medical Association 222 (2): 176-179.
  2. Bartges, J. and Kirk, C. (2012) Nutritional management of lower urinary tract disease. In: Fascetti, A. and Delaney, S. (eds), Applied Veterinary Clinical Nutrition, 1st edition Wiley- Blackwell, Chichester, pp269-288.
  3. Buranakarl, C., Mathur, S. and Brown, S. A. (2004) Effects of dietary sodium chloride intake on renal function and blood pressure in cats with normal and reduced renal function. American Journal of Veterinary Research 65: 620-627.
  4. Caney, S. Cortadellas, O. Dhumeaux, M. and Nickel, R. (2014) Practical Management of Urinary Tract Disease. Veterinary Focus special edition, Royal Canin, Aimargues, France.
  5. F.E.D.I.A.F. (2011) Code of Good Labelling Practice for Pet Food. [Online] Available from: food/food/animalnutrition/labelling/ docs/pet_food_code_20102011_en.pdf [accessed 16/06/16].
  6. Greco, D. S., Lees, G. E., Dzendzel, G. et al (1994) Effects of dietary sodium intake on blood pressure measurements in partially nephrectomized dogs. American Journal of Veterinary Research 55: 160-165.
  7. Hesse, A. and Neiger, R. (2009) A Colour Handbook of Urinary Stones in Small Animal Medicine. Manson Publishing, London.
  8. Koehler, L. Osborne, C., Buettner, M., Lulich, J. and Behnke, R. (2015) Canine Uroliths: Frequently Asked Questions and Their Answers. [Online] Available at: minnesotaurolithcenter/prod/groups/ cvm/@pub/@cvm/@urolith/ documents/asset/cvm_asset_388871. pdf [accessed 9th May 2015].
  9. Krieger, J. E., Liard, J. F. and Cowley, A. W. (1990) Hemodynamics, uid volume, and hormonal responses to chronic high-salt intake in dogs. Am J Physiol 259: H1,629-H1,636. 
  10. Lulich, J. Osborne, C. and Sanderson, S. (2005) Effects of dietary supplementation with sodium chloride on urinary relative supersaturation with calcium oxalate in healthy dogs. American Journal of Veterinary Research 66 (2): 319- 324.
  11. McNeill (ed) (2014) Veterinary Focus: Lower Urinary Tract Disease, Buena Media Plus, Boulogne, France. 
  12. Moore, A. (2007) Quantitative analysis of urinary calculi in dogs and cats. Veterinary Focus 17 (1): 22-27. 
  13. Villaverde, C. (2016) Deconstructing Urolithiasis: One stone at a time [two-part webinar series], Royal Canin UK, 24th and 31st May 2016, accessible online: https://attendee.gotowebinar.c... register/8836271750277033986 and https://attendee.gotowebinar.c... register/909459218058925570.