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Urinary Stones (Uroliths, Calculi) in Cats


Scott A. Brown

, VMD, PhD, DACVIM, Department of Small Animal Medicine and Surgery, College of Veterinary Medicine, University of Georgia

Reviewed/Revised Aug 2018 | Modified Oct 2022
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Minerals that naturally occur in urine can clump together to form tiny crystals. When crystals clump together, they form uroliths (also known as urinary stones or calculi). These stones can develop anywhere in the urinary system, including the kidney, ureter, bladder, or urethra. Certain types of stones appear to have increased in cats in recent years. The cause is not clear, but researchers are looking at the effects of diet to determine if there is any link.

Veterinary researchers do not completely understand what causes stones to form ( see Why Cats Develop Uroliths (Stones) Why Cats Develop Uroliths (Stones) Why Cats Develop Uroliths (Stones) , below). There are many different types of stones, each formed from a complex mixture of various minerals. The most common types are made of struvite, calcium oxalate, and urate. These and some other types of stones only develop under certain conditions. They can be caused by a problem with the minerals themselves or by a problem with other chemicals that exist in urine and which, under normal circumstances, prevent stones from forming. The environment of the urinary tract may also contribute to stone formation. All of these conditions can be affected by urinary tract infections, diet, digestion, the amount of urine that a cat produces, how frequently a cat urinates, medications, and genetics.

Why Cats Develop Uroliths (Stones)

Cats have highly concentrated, acidic urine that predisposes them to form stones of the urinary tract, especially when certain minerals or other substances are abundant or deficient. This may be a result of diet, inflammation or infection, or possibly the pH level of the individual cat’s urine.

The urolith itself is made up of a hardened center, with various layers that have formed around it. It is unclear what starts the process of formation, but it is likely that an excess of urolith-forming minerals in the urine form small crystals that stick together to form a stone. Once the stone is formed, it gradually increases in size—a process that can take weeks or months.

Cats can also form small stones that resemble sand or toothpaste. Small stones, crystals, and other components of urine can create "plugs" that can block the urinary tract, especially the urethra.

Veterinarians identify stones based on their makeup. In cats, the most common stones are generally composed of either magnesium ammonium phosphate (known as struvite) or calcium oxalate. Calcium oxalate stones are now the most common stone found in the urinary tract and kidney, although their cause is unknown. Some types of diets used to control struvite stones may contribute to the formation of calcium oxalate stones.

Cats with very tiny stones in the urinary system do not usually have any signs. However, larger stones in the lower urinary tract may interfere with urination or irritate the lining of the urethra. In turn, these problems can cause an inability to urinate, blood in the urine, and slow or painful urination. Kidney stones (which are rare in cats) usually cause no signs unless the kidney becomes inflamed or the stones pass into the ureter. If a ureter becomes blocked by a stone, it can cause vomiting, depression, or pain in the abdomen in the area around the kidneys. Such pain is particularly common when both ureters are suddenly and completely blocked; the fluids back up causing the kidneys to become enlarged. Pain is the only sign of stones in the ureter on only one side; however, pain can be difficult to detect in cats. If the blocked ureter is not diagnosed right away, kidney damage occurs. Ultimately, the blocked kidney is destroyed.

Veterinarians can occasionally detect stones in the bladder by pressing on the cat’s abdomen. Stones in the urethra may also be detected during a rectal examination or when attempting to insert a catheter. There may be many stones present at once. If one stone is located, it is important to examine the entire urinary tract to look for others. X-rays can detect stones as small as 3 millimeters in size. Your veterinarian will also perform tests on the cat’s urine (such as a urinalysis and culture) and may need to do ultrasonography or other specialized tests.

Treating stones, and preventing their return, depends on their type and location. Treatment and prevention may include surgery, a special diet, lithotripsy (a procedure that uses sound waves to break apart stones), and medication. When stones are removed, your veterinarian may send them to a laboratory to be analyzed. Knowing what types of minerals are in the stone can provide the information needed to prescribe medication that can prevent the formation of more stones. Cats undergoing treatment for uroliths will need to be monitored closely, returning at regular intervals for additional testing. It is also important to eliminate any urinary tract infections, to avoid certain mineral and vitamin supplements, and to encourage adequate water consumption.

Urethral Obstruction

Uroliths can become lodged in the urethra and block the flow of urine out of the bladder. This is called a urethral obstruction Obstructions of the Urinary Tract Not every disease is caused by infection with bacteria, viruses, or other outside agents. There are a variety of noninfectious disorders that can impair the urinary system. All of these diseases... read more and is common in male cats. It may occur suddenly or may develop slowly across several days or weeks. At first, the cat may frequently try to urinate and produce only a fine stream, a few drops, or nothing. Cats may also have extreme pain and cry out when trying to urinate. Complete obstruction causes toxins to build up within the body in 1 to 2 days, which leads to depression, lack of appetite, vomiting, diarrhea, dehydration, arrhythmias (abnormal heart rhythms), coma, and death within about 3 days. Urethral obstruction is an emergency condition, and your cat needs to be treated by a veterinarian immediately.

Treatment involves relieving the obstruction, either by pushing the stone back into the bladder with a catheter or by removing it surgically. If the stone is flushed back into the bladder, surgery to remove the stone is usually necessary so that it does not pass into the urethra again.

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