Calcium oxalate

Calcium oxalate uroliths have had an inverse relationship with struvite uroliths over the last several years in both dogs and cats.^21,23 In dogs, as the incidence of struvite uroliths has gradually decreased, the incidence of canine calcium oxalate uroliths has steadily increased, surpassing struvite in frequency of occurrence.²¹ The incidence of struvite and calcium oxalate uroliths in cats has followed a similar course except for the last few years, which have seen a decline in the incidence of calcium oxalate and an increase in struvite uroliths. In 2005, the Minnesota Urolith Center reported calcium oxalate was the second most frequent type of urolith in cats.²²

Characteristics. Calcium oxalate urolith formation has been reported to have a male predisposition in both cats and dogs.^14,23 Small-breed dogs are predisposed, including miniature schnauzers, Lhasa apsos, Yorkshire terriers, bichon frises, Shih Tzus, and miniature poodles.²⁹ Dogs 8 to 12 years old and older cats (bimodal peaks at 5 and 12 years of age) have the greatest risk of developing calcium oxalate uroliths.^14,23 Cat breeds predisposed to calcium oxalate formation include Ragdolls, British shorthairs, foreign shorthairs, Himalayans, Havana browns, Scottish folds, Persians, and exotic shorthairs.²⁵ Cats housed indoors may also be at a greater risk for developing calcium oxalate uroliths, which could be a result of a decrease in water consumption and voiding.³⁰

Hypercalcemia is a predisposing factor for calcium oxalate urolith formation and has been reported in 35% of cats with uroliths and in about 4% of dogs with uroliths.^14,30,31 The hypercalcemia observed in this cat population is usually idiopathic, while dogs most often have primary hyperparathyroidism. In dogs, hyperadrenocorticism, which has been associated with hypercalciuria, also increases the risk of developing calcium oxalate uroliths.³² Hypercalciuria contributes to calcium oxalate formation in some dogs and cats as a result of hyperabsorption of calcium from the intestines or renal leak hypercalciuria, which is associated with decreased renal tubular calcium reabsorption.³³ When possible, treat these underlying conditions through diet, medication, or surgery to lessen the risk of calcium oxalate urolithiasis.

Treatment. Treating calcium oxalate uroliths is limited to surgical removal, voiding urohydropropulsion, and lithotripsy since no method of medical dissolution is available. Leaving the uroliths in place may be a reasonable option in some asymptomatic patients. Continue monitoring the patient for the development of clinical signs and employing measures to prevent further calcium oxalate formation.

Prevention. In dogs, recurrence rates of up to 50% within three years of initial diagnosis have been reported,²⁴ so preventing initial urolithiasis is key. Although many conflicting factors have been reported regarding the best diet for calcium oxalate urolith prevention, protein and oxalate restriction and urine alkalinization are recommended.³¹ Diets formulated for oxalate urolith prevention in dogs and cats are listed in Table 1.

In addition, in dogs Prescription Diet Canine w/d and potassium citrate (e.g. Urocit-K—Mission Pharmacal, 75 mg/kg orally twice a day) are also options, especially in obese patients or those prone to developing pancreatitis.²⁴ And in some hypercalcemic cats with calcium oxalate uroliths, Prescription Diet Feline w/d (Hill's Pet Nutrition) and potassium citrate (75 mg/kg orally twice a day) normalizes calcium concentrations.^34,35

In people, potassium citrate's alkalinizing effect helps decrease bone calcium release and promote calcium excretion in a more soluble form, calcium citrate.^24,25 These same beneficial effects may occur with potassium citrate administration in dogs and cats. The desired urine pH range for preventing calcium oxalate urolith formation is 6.5 to 7.5.