The feline kidneys have many important functions.  Among them are: to filter metabolic waste such as urea, mineral salts, and various toxins from circulating blood; to help regulate the volume of body fluids and the blood levels of important chemicals and hormones; to initiate the recirculation of purified blood throughout an animal’s system; and to facilitate the excretion of the filtered-out waste products (mixed with water to form urine) before they reach toxic concentrations in the body. Most cats will go through life without experiencing a serious disruption in these vital processes. Some others, however, will experience urolithiasis—a potentially lethal condition marked by the formation of small stones (uroliths) somewhere within this elaborate system.

The upper tract consists of two kidneys, which handle the biochemical processes; and two slender tubes (ureters)—one leading from each kidney—that deliver waste-containing urine from the kidneys down to the lower tract. The lower tract, whose function is purely excretory, consists of the bladder, a muscular sac that receives the urine delivered to it through the ureters and stores it until it is expelled from the body via the other component of the lower tract, the urethra, a thin tube leading from the bladder to the outside world. Stones can develop anywhere within either the upper or lower tracts of the urinary system.

Bladder stones, says Richard Goldstein, DVM, associate professor of small animal medicine at Cornell University’s College of Veterinary Medicine, are composed of minerals—either struvite or calcium oxalate—while kidney stones are always made of calcium oxalate. “These minerals are present naturally in a cat’s body,” he says, “and the stones form when the minerals exceed a certain threshold of concentration in the urinary system. When the concentration goes over that threshold, they start to form crystals, and the crystals accumulate and may grow into stones. We don’t know why this process takes place, but we’ve observed that it tends to occur frequently in domesticated cats, especially in those that are not very active, don’t take in enough fluids, and don’t urinate enough.”    

In some cases, bladder stones may be tiny and inconsequential, remaining harmlessly lodged somewhere within the urinary tract or passed without notice in a cat’s urine. In other cases, however, they can grow to significant size, painfully irritate the tender tissue lining the tract, and cause internal bleeding. In the worst case, they can slip into a cat’s urethra and interfere with the passage of urine. A complete blockage—one that totally obstructs the flow of urine and prevents the elimination of poisonous waste from a cat’s system—will present a medical emergency that, without immediate veterinary care, may prove fatal.  Blockages are most common in male cats since they have a very narrow and easily obstructed urethra.  Typical early signs include blood in the urine as well as increasingly frequent and painful urination.

Kidney stones, on the other hand, do not typically cause noticeable signs of disease until they become very large. "So most of those cats go undiagnosed until there is a problem,” says Dr. Goldstein. Both kidney stones and bladder stones, he notes, can be readily diagnosed by means of x-rays and ultrasound.

Regarding the treatment of kidney stones, he says: “In most cases, we don’t believe that we should go in, open the kidneys, and remove the stones unless they’re causing significant obstruction or infection. But bladder stones are typically treated either surgically or by going in with a laser and sort of blowing up the stones.”  Unfortunately, he adds, stones are likely to recur unless certain preventive measures are taken, which generally entail a combination of dietary and medicinal therapies. 

To reduce the risk of kidney and bladder stones, Dr. Goldstein makes the following recommendations to cat owners: "Make sure that your cat always has access to water and is drinking it. Cats prefer fresh water, so don’t let it stand around for days. We also believe that at least 50 percent of a cat’s diet should be wet food. And make sure that the cat is getting plenty of exercise.”  Keeping the litter box clean will encourage more more frequent urination and is advisable.