Treatment. In dogs, urate uroliths can be medically dissolved either completely or partially in about two-thirds of the cases in which an appropriate calculolytic diet is given.^14,24 The calculolytic diet of choice for dogs is Prescription Diet u/d, which is low in dietary purines, alkalinizes urine, and forms a more dilute urine. Because of nutrient restrictions, Prescription Diet u/d should not be given to pregnant or lactating bitches or immature dogs and should be given with caution to English bulldogs, which may develop dilated cardiomyopathy because of carnitine deficiency.^14,24 The time required for urolith dissolution averages about three or four months. It may be necessary to add allopurinol (15 mg/kg orally twice a day), a competitive inhibitor of the enzyme xanthine oxidase, to the dissolution protocol.³⁹ In cats, allopurinol (7.5 mg/kg orally twice a day) and Prescription Diet Feline k/d (Hill's Pet Nutrition) can be used in a dissolution protocol.³⁰ Administering allopurinol to treat or prevent urate uroliths may result in xanthine urolith formation because allopurinol inhibits xanthine oxidase. Therefore, always feed a purine-restricted diet (e.g. Prescription Diet Canine u/d, Prescription Diet Feline k/d) to patients receiving allopurinol.

Urate uroliths in cats and urate uroliths associated with portosystemic shunts often cannot be medically dissolved.¹³ Consequently, voiding urohydropropulsion, lithotripsy, or surgical intervention is often required to remove urate uroliths in these patients. Spontaneous dissolution of urate uroliths after correcting portovascular anomalies may occur but has not been documented with published studies.²⁴ Urinary tract infections occur commonly with urate uroliths; treat these with appropriate antibiotics.

Prevention. Preventing urate uroliths in dogs can often be accomplished by feeding Prescription Diet Canine u/d with or without allopurinol (5 to 10 mg/kg orally once or twice a day). Preventing urate uroliths in cats is generally successful by feeding them a kidney disease management diet such as Prescription Diet Feline k/d, which is alkalinizing and protein-restricted.³⁴ Patients with urate uroliths associated with portovascular shunts should undergo surgical correction for the anomaly, if possible, to prevent further urolith formation.

Xanthine is a purine urolith recognized more recently in cats but also reported in Cavalier King Charles spaniels and dachshunds. These two dog breeds are thought to have an inborn error of purine metabolism, which leads to xanthine formation.^24,40 In cats, a familial or congenital defect in the activity of xanthine oxidase is suspected to be the primary cause of xanthine urolith formation.⁴¹

Characteristics. Xanthine crystals resemble uric acid crystals or are amorphous. Xanthine uroliths are usually a few millimeters in diameter, have a smooth surface, and are yellow, tan, or light-brown. The uroliths are radiolucent and often must be detected through positive double-contrast cystography or urethrography or high-frequency ultrasonography. Xanthine uroliths have been found primarily in the lower urinary tract, although they occasionally occur in the upper urinary tract. Affected cats' urine may be a mustard-yellow color. Male cats appear to be predisposed to xanthine formation, and the average age of affected cats at the time of diagnosis is 2.8 years.³⁴ An incidence of 0.1% in cats was reported by the Minnesota Urolith Center in 2006.²² Although allopurinol administration predisposes both cats and dogs to xanthine formation, most affected cats have not received this drug.³⁴

Treatment and prevention. No medical dissolution protocol exists for this urolith type. Treatment consists of removal or lithotripsy. The recommended prevention protocol for dogs and cats consists of feeding them a protein-restricted, alkalinizing diet such as Prescription Diet k/d.⁴¹ Because allopurinol administration without purine restriction can cause xanthine urolith formation, always feed a purine-restricted diet to patients receiving allopurinol for any reason.^23,39,42