Stalking stones: An overview of canine and feline urolithiasis - Veterinary Medicine
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Stalking stones: An overview of canine and feline urolithiasis
Did you know that a new type of urolith has been found in cats? Should you institute preventive therapy if you identify only crystalluria? Is antibiotic therapy automatically warranted in animals with indwelling urinary catheters? This internist revamps your knowledge on diagnosing, treating, and preventing urolithiasis.


If diet or diet plus potassium citrate fails to prevent calcium oxalate urolith recurrence, consider adding thiazide diuretics (hydrochlorothiazide 2 to 4 mg/kg orally twice a day)33 to decrease the calcium excretion in the urine.36,37 Although unproven in cats, thiazide diuretics decrease urinary calcium excretion in people and dogs with calcium oxalate urolithiasis. A study in dogs indicated that feeding them Prescription Diet u/d (Hill's Pet Nutrition) in conjunction with hydrochlorothiazide administration enhanced reduction of urine calcium.36 Thiazide diuretics are contraindicated in hypercalcemic patients because they may aggravate the hypercalcemia.

Use loop diuretics, such as furosemide, with caution in patients at risk for calcium oxalate urolithiasis since their mechanism of action may increase the formation of such uroliths by augmenting calcium urine excretion. Avoid supplementation of vitamin C, an oxalate precursor, and vitamin D, an aid to the gastrointestinal absorption of calcium, in patients with calcium oxalate urolithiasis.31

Urate (purine)

The third most common noncompound type of urolith reported in dogs and cats is purine. Most purine uroliths are composed of urates (uric acid and uric acid salts).24,34 Unlike struvite and calcium oxalate uroliths, urate uroliths have varied relatively little in incidence over the last 20 years, accounting for about 4.6% of uroliths in cats and 8% in dogs.14,22

Characteristics. Ammonium urate crystals are large and irregular in shape, and uric acid crystals are slender and hexagonal. These crystals form small; smooth; round; light-yellow, light-brown, or light-green radiolucent uroliths that are found most frequently in the bladder or urethra. Positive or double-contrast cystography or urethrography or high-frequency ultrasonography is usually needed for detection.24,39

Dalmatians and English bulldogs are the classic breeds associated with this urolith because of inherent differences in the way that uric acid is metabolized and transported in these dogs. Male Dalmatians and English bulldogs are affected more commonly than females. Dalmatians are thought to have impaired transport of uric acid into the liver and to have increased excretion of uric acid into the urine. All Dalmatians appear to have these alterations in uric acid metabolism, but only a small percentage actually develop urate uroliths. The Dalmatians developing these uroliths are most commonly affected at 1 to 4 years of age, with a decrease in incidence after 6 years of age.14,24 Although the cause of the English bulldog's predisposition to urate urolith formation is not clearly defined, it has been theorized that a renal proximal tubular defect and alterations in purine liver metabolism cause increased urate urolith formation in this breed.38

Dogs other than Dalmatians and bulldogs and some cats can be at risk for developing urate uroliths because of changes in metabolism related to hepatic disease, especially portovascular anomalies, including hepatic microvascular dysplasia.39 Miniature schnauzers, Yorkshire terriers, and Shih Tzus are at increased risk.24 Urate uroliths can be found in both male and female dogs with portovascular anomalies and are commonly diagnosed when the patient is < 3 years old.39

In many cats, urate urolith formation is idiopathic but may occur as a result of portovascular anomalies.25,34 No known sex or breed predisposition has been established for cats.34 Although urate uroliths occur most commonly in cats that are < 4 years old, urate uroliths associated with portovascular anomalies are frequently found in cats < 1 year of age.34


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