Stone Age? Not quite! But whether it's struvite, calcium oxalate, urate or cystine stones, all of these urinary stones are a common complaint in animal patients with urinary tract disorders. Did you know that cats and dogs can be equally affected? We give you an insight into the formation, symptoms and treatment options of struvite stones.
Getting those stones rolling...
Urinary stones are concretions that can occur in animals' urinary tract due to precipitation of mineral constituents as part of urolithiasis. Struvite stones are a type of urinary stones that mainly consist of magnesium-ammonium-phosphate. In the urine, struvite stones appear colorless and sharp-edged, predominantly in the shape of a coffin lid. Crazy, isn't it? But they all have one thing in common: They put our furry noses in a really tricky position.
Several factors can contribute to the formation of struvite stones. For example, they may form in conjunction with urinary tract infections. Bacteria possessing the enzyme urease break down the urea contained in the urine into ammonia and carbon dioxide. The increased concentration of bicarbonate and ammonium leads to an increase in the pH of the urine. The alkalinization of urine can cause precipitation of minerals, such as magnesium, ammonium and phosphate. Consequently, a change in the pH of the urine plays a crucial role in the formation of struvite stones. However, factors such as an increased concentration of magnesium, ammonium and phosphate in the urine, as well as insufficient water intake, may also play a role. Overall, urinary stone formation can occur promptly and, in addition, there is the possibility that bacteria can become trapped and persist within the stones.
Under the microscope: struvite stones in urine sediment
The pH of urine indicates how acidic or basic the urine is. More precisely, pH describes the hydrogen ion concentration in a liquid. The pH value is decisive for the inhibition or activation of crystallization. An alkaline pH, greater than 7 to 7.5, influences the precipitation of struvite and calcium apatite crystals. Thus, an alkaline pH favors the formation of these stones. An acidic urine, with a pH of 5.5 to 6, can promote the precipitation of calcium oxalate, cystine and urate crystals.
The pH can be measured using a pH stick or determined mathematically using the cation-anion ratio. In healthy cats and dogs, the pH should be approximately 6.5. Determination times should be taken into account, depending on the patient's feeding. After feeding, the urine becomes alkaline and during prolonged obstruction, the pH value is more in the acidic range.
There are certain factors that can increase the risk of struvite stone formation in small animals. In cats, some breeds are predisposed, such as Persian cats for example, who are more prone to struvite stones due to their altered urinary composition. In dogs, dwarf schnauzers, dachshunds and retrievers, among others, are frequently affected. Obesity, lack of exercise, and an exclusive dry food diet with insufficient water intake also play a role in the development of urinary stones. In addition, certain underlying diseases such as urinary tract infections or metabolic disorders can increase the risk of struvite stone formation.
Treatment for struvite stones aims to dissolve the stones and prevent them from forming again. The first method of dissolving struvite stones is dietary measures. This involves the use of special dietary feeds or supplements that lower the pH of the urine and reduce the levels of magnesium, ammonium and phosphate. A pH of 6.0 to 6.5 is aimed at for therapy, this can promote the dissolution of the stones. In addition, it is important to provide the animal with sufficient fresh water to dilute the urine concentration and flush the urinary tract. Dietary measures are not sufficient to treat urinary stones in all cases. In these cases, surgical procedures are necessary to remove the stones. The type of treatment depends on the composition, size, location as well as symptoms triggered by the urinary stones.
There are also things that can be done to prevent struvite stones: A balanced diet, tailored to the individual needs of the animal, can help optimize the urinary composition and thus prevent the formation of struvite stones. Dietetics in dogs and cats can be used prophylactically and therapeutically. The aim of dietary measures is to lower the pH and reduce the intake of stone-forming substances, such as magnesium, calcium and phosphate. During prophylaxis of struvite stones, a pH of 6.6 to 6.8 should be aimed for. High-quality wet food options that have a higher water content, compared to dry food, are important and can contribute to adequate hydration. The provision of fresh, clean water is also of great importance. In addition, the solubility of renally excreted substances should be improved by a high urinary flow, an optimal pH and a solubilizer, or inhibitor of concretion formation. In addition, unnecessarily high levels of alkalizing ingredients in feeds can be avoided.
Targeted acidification of the urine, such as by the amino acid methionine, can prevent the formation of struvite stones and even partially lead to their dissolution. Sufficient intake of the sulfur-containing amino acid results in the formation of sulfate, which is excreted renally. Proton formation and secretion into the urine results in lowering the pH. In addition, methionine reduces the attachment of bacteria to the urothelial cells.
The next time you encounter a patient with struvite stones in the office, the treatment may differ depending on the size of the urinary stone. Acidification of the urine can already dissolve small stones, but if this is not possible, there is a need for surgical intervention followed by prophylaxis against the formation of new struvite stones. Lowering the pH, whether for therapy or metaphylaxis of struvite stones, are of great importance.
