Formation of stones in the kidney or urinary tract is known as Nephrolithiasis or more commonly, kidney stones. This condition occurs as a result of accumulation of salt or other substances in the kidneys. Stones of the urinary tract begin as microscopic crystals that aggregate to form stones of variable size and shape anywhere within the urinary system, including the kidneys, ureters, and urethra.
Basic structural and functional unit of kidney is nephron. Thousands of nephrones make up the kidney. Each of these nephrons consists of blood capillaries and series of tubules through which filtered fluid flows as urine is produced. These tubules of nephron drain into urinary ducts which eventually enter the renal pelvis and a tube through which urine follows into the ureter. Kidney stones typically leave the body by passage in the urine stream, and many stones are formed and passed without causing symptoms. If stones grow to sufficient size they can cause obstruction of the ureter.
Dogs naturally have minerals in their bodies, such as calcium, magnesium, ammonia or phosphorus. Where there is too much of these mineral salts in the urine, they are filtered out through the kidneys. Because it can be difficult for these minerals to be soluble in the naturally acidic urine of dogs, these minerals can form stones. While they may not pose a health risk while in the kidneys, they may grow very large and be quite painful if they pass through the rest of the urinary tract, even blocking the tract in the worst cases.
Development of the stones can be caused by a decreased in urine volume or increased excretion of stone-forming components such as struvite or oxalate. Kidney stones can be caused by urinary tract infections (UTIs), kidney infections, medications and more.
Nephrolithiasis is the most common kidney disease in dogs. While it can occur in any dog, some breeds are at a higher risk of forming certain types of stones. Lhasa apso, Yorkshire terrier and miniature poodle breeds are more prone to calcium oxalate nephroliths (stones containing calcium and oxalic acid) while Dalmatians, Yorkshire terriers and English bulldogs are more prone to kidney stones containing uric acid (known as urate nephroliths). The kidney stones are classified into four different types including Calcium stones, Uric acid stones, Struvite stones and Cystine stones.
Not all dogs with stones show signs of nephrolithiasis. Often, the condition is 'accidentally' detected when diagnostic testing is done for other medical reasons. Signs, if present, include blood in urine (hematuria), vomiting, recurrent urinary tract infections, painful difficult urination (dysuria), and frequent urination with small volume of production (polyuria). Clinical signs depend on the exact location, size, shape and number of kidney stones.