Urolithiasis

Urolithiasis is the medical word for stones in the urinary tract. The stones themselves are called uroliths, and they form when minerals that are normally dissolved in urine clump together into solid concretions, most often in the bladder, sometimes in the kidneys or the narrow tube that carries urine out of the body. They can be a single pebble or dozens of gritty fragments, and they range from grains of sand to stones several centimetres across.

Stones are not the same thing as crystals. Microscopic crystals in a urine sample, called crystalluria, are a common and often harmless finding, particularly in cats, and on their own they usually need no treatment. Stones are the larger, visible version, and they cause trouble by rubbing against the bladder wall or by physically blocking the flow of urine.

The two most common types in dogs and cats are struvite, made of magnesium, ammonium and phosphate, and calcium oxalate. The distinction matters a great deal for treatment: struvite stones can often be dissolved with a therapeutic diet over a few weeks, while calcium oxalate stones cannot be dissolved at all and have to be removed if they are causing problems. Less common types include urate and cystine, both of which have strong breed links, for example urate stones in Dalmatians. In dogs, struvite stones are usually tied to a urinary tract infection with bacteria that change the urine's chemistry, whereas in cats they typically form without any infection. Because appearance alone cannot tell you the type reliably, vets send retrieved stones for mineral analysis, which is what makes accurate long-term prevention possible.

Signs and when it becomes urgent

Stones tend to look like a stubborn urinary tract infection. Watch for:

• Frequent trips to urinate, often producing only a few drops
• Straining, or crying out while trying to go
• Blood in the urine, or urine that looks pink or brown
• Accidents in the house, or a cat urinating outside the litter tray
• Licking at the genital area, low energy, or going off food

A pet who is straining repeatedly and passing nothing at all needs to be seen immediately, not tomorrow. A complete blockage stops the kidneys from clearing waste and potassium from the blood, which can become life threatening within a day or two. This is most common in male cats, whose urethra is longer and narrower, and it is one of the true emergencies in small animal practice. The good news is that treated cats have a high survival rate, though blockages do recur in a meaningful proportion of cases.

Treatment depends on the stone type, where it sits, and how big it is. Current guidance from the American College of Veterinary Internal Medicine favours dissolving struvite stones with diet and treating any infection, and using the least invasive method available for the rest, such as flushing small stones out or retrieving them with a scope, rather than defaulting to open bladder surgery. Stones that cause no signs and are too large to slip into the urethra may simply be monitored. Prevention afterwards centres on keeping urine dilute, usually by increasing water intake and feeding a high-moisture or prescription urinary diet, plus periodic urine checks and imaging, because recurrence rates are high. Your vet can tell you which plan fits your pet's stone type.