URINARY stones (uroliths) occur when urinary crystals concentrate and solidify in the bladder. Even if they do not form stones, crystals may cause signs of urinary tract disease.
Urinary crystals occur when urine is saturated with minerals and this may be influenced by metabolic disorders, bacterial infections or the diet that is being fed, all of which can create a urinary environment favourable to crystal formation.
There are a number of high-quality diets in dry and wet forms that have been formulated to help support an animal’s urinary tract and the general features of these diets are discussed below.
There are several factors involved in stone formation including the pH of the urine, the volume of the urine and the concentration of minerals within the urine. The breed and lifestyle of cats and dogs can also have an impact on stone formation. Burmese and Persian cats are thought to be at a greater risk of developing oxalate stones and small breed dogs are known to be more prone to developing stones as well.
Supersaturation
A very useful tool for evaluating the risk of crystal and stone formation is a measure known as “relative supersaturation” (RSS). The formation, growth and dissolution of urinary crystals are affected by the concentration of the minerals within the urine that are free to react with each other (i.e. calcium and oxalate for calcium oxalate crystals).
The product of the concentrations of those free fractions is called the activity product. The RSS for a given salt is defined as the ratio of the activity product divided by the thermodynamic solubility product for that salt.
The thermodynamic solubility product is the maximum amount of a given salt that can be dissolved in a solvent (i.e. water) for a given temperature and pH. An RSS <1 means that the urine is undersaturated and that crystals will not form but can be dissolved. An RSS >1 means that the urine is supersaturated and that crystals might form but will not dissolve.
In a complex medium such as urine, it is possible to have an RSS for calcium oxalate or struvite above 1 without spontaneous precipitation of crystals. This level of supersaturation is known as metastable supersaturation.
At this level of saturation, calcium oxalate crystals will not form spontaneously but might occur in the presence of a nucleus. At higher levels of minerals in the urine, crystals will form spontaneously within minutes to hours. This is the labile supersaturation.
The limit between metastable and labile supersaturation is called the formation product. Kinetic precipitation studies in urine have shown that the RSS for the formation product is 2.5 for struvite and 12 for calcium oxalate.
Acidifying the urine
Calculation of the RSS from the urine of cats or dogs fed a specific diet can be used to study the effects of that diet on the crystallisation potential of urine. Royal Canin’s range of Urinary S/O diets has been formulated to help dissolve struvite stones and crystals by acidifying the urine and by creating a very low struvite RSS.
The diets have also been formulated to help increase urine volume, limit the precursors available for stone formation and create a low RSS for both struvite and calcium oxalate.
Together this leads to an unfavourable environment for further formation of both struvite and calcium oxalate stones or crystals. Moderate calorie versions of these diets are also available for patients at a greater risk of weight gain.
The use of RSS as a tool has also resulted in other interesting findings. For example, the urine of small dogs is more saturated than the urine of large dogs.
This could explain the higher incidence of urinary stones in small breeds compared to large breeds. As a result, manufacturers such as Royal Canin have developed a small dog version of their standard urinary diet, with other additional benefits including an adapted kibble and tartar control.
In summary, there are several diets available that can be used to prevent the formation of struvite and calcium oxalate stones, as well as the dissolution of struvite stones. These diets can be a useful tool in the management of pets with urinary stones.
