Bladder stones or uroliths are a common occurrence in animals, especially in domestic animals such as dogs and cats. Occurrence in other species, including tortoises, has been reported as well. The stones form in the urinary bladder in varying size and numbers secondary to infection, dietary influences, and genetics. Stones can form in any part of the urinary tract in dogs and cats, but unlike in humans, stones of the kidney are less common and do not often cause significant disease, although they can contribute to pyelonephritis and chronic kidney disease. Types of stones include struvite, calcium oxalate, urate, cystine, calcium phosphate, and silicate. Struvite and calcium oxalate stones are by far the most common. Bladder stones are not the same as bladder crystals but if the crystals coalesce unchecked in the bladder they can become stones. Signs and symptoms Bladder stones may cause blood in the urine (hematuria) but sometimes there may be no signs at all. Painful urination or straining to urinate are other signs. Urinary tract infections are commonly associated with bladder stones. Smaller stones may become lodged in the urethra, especially in male animals, causing urinary tract obstruction and the inability to urinate. This condition causes acute kidney failure, hyperkalemia, sepsis, and death within a few days. Mechanism Oversaturation of urine with crystals is by far the biggest factor in stone formation in dogs and cats. This oversaturation can be caused by increased excretion of crystals by the kidneys, water reabsorption by the renal tubules resulting in concentration of the urine, and changes in urine pH that influence crystallization. Other contributing factors include diet, frequency of urination, genetics, current medications, and the presence of a urinary tract infection. The stones form around a nidus, which can consist of white blood cells, bacteria, and organic matrix mixed with crystals, or crystals alone. The nidus makes up about two to ten percent of the mass of the stone. It is possible for the nidus to be made of a different type of crystal than the rest of the stone, also known as epitaxial growth. Nutrition often plays a major role in the development of bladder stones. Sodium, calcium, phosphorus and potassium ratios and quantities play a large role in urinary health of animals. Research indicates that low dietary inclusion of potassium is associated with increased renal calcium excretion, which lowers urinary pH. By lowering urinary pH, the risk