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Today's VeTerinary PracTice | september/october 2015 | tvpjournal.com Feline sTruViTe & calcium oxalaTe uroliThiasis Peer reviewed 14 Uroliths are concretions composed primarily of highly organized crystalloids and a small amount of organic matrix. They are identifed based on their mineral composition: calcium oxalate and struvite (magnesium ammonium phosphate) are the 2 most common feline uroliths. Feline urine is a complex solution in which salts, such as calcium oxalate and magnesium ammonium phosphate, can remain in solution under conditions of supersaturation. This urine does, however, have a potential energy for precipitation, or the tendency to form crystals from dissolved salts. When crystals aggregate and grow into calculi before being voided, uroliths form. Feline uroliths form in the urinary bladder (most commonly) or renal pelvis. They are most commonly found in the urinary bladder and urethra but may also be located in the kidneys and ureters. Urolithiasis may affect as many as 25% of cats with lower urinary tract disease. 1 PATHOPHYSIOLOGY Struvite Uroliths In cats, approximately 45% of uroliths consist entirely or predominantly of struvite, and most feline struvite uroliths form in sterile urine (Figure 1). 1 The pathophysiology of struvite urolith formation in sterile urine is poorly understood; however, dietary and metabolic factors that result in alkaline urine and increased concentrations of magnesium, ammonium, and phosphate in urine have been implicated. Dietary Factors. Diets high in magnesium, phosphorus, calcium, sodium, chloride, and fber, with moderate protein content have been associated with increased risk. 2 A greater ability to concentrate urine (compared with dogs, in which struvite uroliths are commonly associated with infection) and, therefore, greater urine supersaturation, may be partially responsible for urolith formation in cats without urinary tract infections (UTIs). Metabolic Factors. In addition, formation of sterile struvite uroliths may be twice as likely if urine pH is consistently elevated (6.5–6.9 versus 6–6.2). 2 Alkaline urine pH is commonly affected by diet but may also be associated with drug therapy and renal tubular disorders. In 1981, more than 80% of the feline uroliths analyzed at the Minnesota Urolith Center were struvite. 3 A major factor contributing to the decline in feline struvite uroliths over the last 35 years is the widespread use of magnesium- restricted, acidifying diets. Infection. Struvite uroliths in cats may be associated with UTI, especially when caused by bacteria that produce urease, such as Staphylococcus and Proteus species. In such cases, urease increases the ammonium concentration in urine, resulting in an increase in urine pH and a change in the ionization state of phosphorus. Interestingly, struvite uroliths associated with UTI tend to be more radiodense than struvite uroliths that form in sterile urine. 4 Calcium Oxalate Uroliths Factors involved in the pathogenesis of calcium oxalate urolithiasis (Figure 2, page 16) in cats are not completely understood but involve urine supersaturation with calcium and oxalate: • Increased dietary intake or endogenous production of oxalate may result in hyperoxaluria. • Metabolism of vitamin C, glycine, and glyoxylate can increase production and excretion of oxalate. • Enteric oxalate absorption may actually increase if (1) dietary intake of calcium is low and (2) less oxalate in the gut is complexed with calcium. • Hypercalciuria may result from overt hypercalcemia (eg, idiopathic hypercalcemia, parathyroid adenocarcinoma). 5 Ionized calcium concentrations should be measured in cats with calcium oxalate uroliths to rule out hypercalcemia. • Decreased concentrations of calcium oxalate crystallization inhibitors (eg, citrate, magnesium, nephrocalcin, and osteopontin) may contribute to calcium oxalate urolith formation. Dietary & Metabolic Factors. As with struvite uroliths, diet can also infuence the development Feline Struvite & Calcium Oxalate Urolithiasis Gregory F. Grauer, DVM, MS, Diplomate ACVIM (Small Animal Internal Medicine) Kansas State University