Urolithiasis

What it is

Uroliths form when urine becomes supersaturated with stone-forming minerals that precipitate as crystals and grow into stones. The type depends on urine composition and pH, diet, genetics and infection:

• Struvite (magnesium ammonium phosphate): in dogs usually infection-induced, by urease-producing bacteria (Staphylococcus, Proteus) that alkalinise the urine. Can be dissolved medically.
• Calcium oxalate: in older dogs of small breeds, more often males; it cannot be dissolved.
• Urate: in Dalmatians (and some bulldogs and other breeds) with a genetic defect in uric acid transport, and in dogs with portosystemic shunts.
• Cystine: in dogs with inherited cystinuria, mainly intact males of certain breeds.
• Less common: calcium phosphate, silica and xanthine (for example after allopurinol).

Who it affects

Struvite: more often females, because of their predisposition to urinary tract infection. Calcium oxalate: miniature schnauzer, Lhasa apso, Yorkshire terrier, bichon frise, shih tzu, miniature poodle. Urate: Dalmatian, English bulldog, and small breeds with shunts. Cystine: English and French bulldog, Newfoundland, dachshund, mastiffs and others.

Anatomy involved

• Urinary bladder: the most common site (cystolithiasis); stones irritate the mucosa and predispose to infection.
• Urethra: stones lodge where the lumen narrows, typically just caudal to the os penis in males, causing obstruction.
• Kidneys and ureters: nephroliths and ureteroliths are less common in dogs; they can obstruct the ureter and cause hydronephrosis and pyelonephritis.

Clinical signs

Haematuria, pollakiuria, stranguria and dysuria; urinary incontinence or inappropriate urination. Some stones are found incidentally. Urethral obstruction is an emergency: unproductive straining, a large painful bladder and, after 24–48 hours, post-renal azotaemia, hyperkalaemia, vomiting and depression.

Diagnosis

• Abdominal radiographs including the whole urethra: struvite, calcium oxalate and calcium phosphate stones are radiopaque; urate and cystine stones are often radiolucent.
• Ultrasound: detects radiolucent stones and evaluates the kidneys and ureters.
• Urinalysis and urine culture: pH, crystals (suggestive, not definitive of stone type) and infection.
• Quantitative stone analysis of a retrieved urolith is the only way to know its composition.

Treatment

Urethral obstruction is relieved first, by retrograde urohydropropulsion of the stone back into the bladder or catheterisation, while correcting fluid and electrolyte disorders. Following the ACVIM consensus, medical dissolution is attempted for struvite (calculolytic diet plus antibiotics when infection is present), urate and cystine stones. Stones that cannot be dissolved are removed by the least invasive method available: voiding urohydropropulsion, cystoscopic basket retrieval or laser lithotripsy, percutaneous cystolithotomy or, if not available, cystotomy. Prevention depends on stone type: diet, increased water intake, urine pH management and regular monitoring.

In the cat

In the cat the two main types are also struvite and calcium oxalate, but with important differences:

• Struvite: unlike in the dog, in the cat it is usually sterile, related to concentrated, alkaline urine rather than infection. It can be dissolved with a calculolytic diet, usually within a few weeks.
• Calcium oxalate: more common in older cats and in breeds such as the Burmese, Himalayan and Persian; it cannot be dissolved. It is the predominant type in the upper urinary tract.
• Urate, mainly in cats with portosystemic shunts and in some breeds (such as the Egyptian Mau); dried solidified blood calculi are found in the ureters and renal pelvis.

Ureteral obstruction by calcium oxalate ureteroliths is much more common in cats than in dogs: the feline ureter is very narrow and a small stone is enough to block it. It causes acute kidney injury or an acute-on-chronic crisis; often one kidney is already damaged and the other becomes obstructed (big kidney, little kidney). It is diagnosed with ultrasound and radiographs and treated with diuresis and, if it does not resolve, with ureteral stents, a subcutaneous ureteral bypass device or surgery. In male cats, small uroliths and urethral plugs cause urethral obstruction. Prevention relies mainly on increasing water intake (wet food, fountains) and on diets suited to the stone type.

Key studies

• ACVIM consensus on uroliths in dogs and cats (Lulich et al., 2016): recommendations for treating and preventing uroliths by type. It covers medical dissolution of struvite and minimally invasive removal of stones that do not dissolve.
• iCatCare consensus (Taylor et al., 2025): the current guideline for feline lower urinary tract diseases, uroliths included.