Urolithiasis in cats is common and an important cause of lower urinary tract disease.
Urolithiasis in cats occurs with equal frequency in both sexes. However, male cats are at increased risk of urethral obstruction.
Historically, it was thought that most uroliths in cats were small and resembled sand or were gelatinous plugs that differed from typical uroliths in that they contained a greater amount of organic matrix, giving them the consistency of toothpaste. Matrix-crystalline plugs are most commonly found within the urethra near the urethral orifice, and they are primarily responsible for urethral obstruction.
The prevalence of urolithiasis with grossly observable stones composed primarily of calcium oxalate has increased in cats. The most common urolith types in cats are calcium oxalate, magnesium ammonium phosphate, and urate.
Calcium Oxalate Stones in Cats
In cats, calcium oxalate uroliths account for approximately half of all uroliths and the majority of all nephroliths, and they can occur anywhere in the urinary tract. The underlying cause is not known in most cases in cats. As in dogs, however, factors that can increase supersaturation of calcium, oxalate, or both in the urine can contribute to the formation of calcium oxalate stones in cats.
Longhaired cats are predisposed to calcium oxalate stones, but any breed can be affected.
Hypercalcemia is an important risk factor for calcium oxalate stones in cats. A notable percentage of cats with idiopathic hypercalcemia develop urolithiasis.
Calcium oxalate stones are radiopaque and readily identified on radiographs. However, advanced imaging such as ultrasonography might be indicated to confirm their location within the urinary tract and to identify complications such as ureteral obstruction.
A biochemical profile and urinalysis are indicated as part of the diagnostic workup of cats with calcium oxalate stones to evaluate for evidence of concurrent kidney disease, especially if nephroliths or ureteroliths are present. Results of urinalysis might suggest the stone type (eg, calcium oxalate is favored at acidic pH, whereas struvite is favored at neutral to alkaline pH) and help determine whether concurrent bacterial infection is present.
Medical dissolution is not possible for calcium oxalate stones. Surgical or other less invasive removal methods must be used instead.
In cats with confirmed calcium oxalate stones, ionized hypercalcemia should be screened for and managed if present.
Calcium oxalate stones are expected to recur in cats. Targeting a neutral urine pH (6.6–7.5) and decreasing the urine specific gravity by adding water or feeding a high-moisture diet can help slow recurrence. A prescription diet designed to decrease urine supersaturation of calculogenic substances should be fed.
Struvite Stones in Cats
Struvite stones once made up the majority of stones submitted to urolith centers from cats. Although the incidence has decreased, likely because of changes in commercial diet composition, struvite stones remain one of the two most common stone types in cats, along with calcium oxalate stones.
Cats with struvite stones tend to be young to middle-aged adults; the risk decreases in later life.
Contributing factors to struvite urolithiasis in cats include alkaline urine and decreased urine volume.
Struvite stones are radiopaque and readily identified on radiographs. Urinalysis should be performed in all cases to evaluate characteristics such as specific gravity and pH, as well as to look for evidence of bacterial infection.
Although alkaline urine and the presence of struvite crystals, along with radiographic confirmation of stones, suggest that the stone type is struvite, urinalysis is not a perfect predictor of stone type.
Occasionally, struvite crystalluria occurs in healthy cats without uroliths and without lower urinary tract signs, and it does not necessarily require treatment. However, struvite crystalluria in the absence of stones has been associated with lower urinary tract signs in some cats. Furthermore, struvite is the most common mineral found in urethral plugs. For these reasons, struvite crystalluria in fresh urine samples that is heavy, persistent, or associated with lower urinary tract signs should be treated with dietary management.
Sterile struvite uroliths are effectively dissolved in cats by feeding a prescription struvite dissolution diet that is acidifying and low in struvite precursors. A variety of prescription struvite dissolution diets are available for cats.
While being fed these diets for dissolution, cats should be rechecked by means of radiography and urinalysis approximately every 2 weeks to monitor the efficacy of therapy. Although the time frame might vary with the stone burden and specific diet, dissolution typically occurs on these diets after 2–4 weeks of treatment. If no substantial progress is made in 2–4 weeks, despite dietary compliance, the stones are likely not struvite and should be removed via other means.
Sterile struvite uroliths are likely to recur in cats, and cats with a history of struvite stones should be kept on a high-moisture struvite-preventive diet when possible.
Other Stones in Cats
Ammonium urate, uric acid, calcium phosphate, and cystine uroliths are less common than other types of stones in cats. However, ammonium urate and uric acid account for approximately 6% of uroliths in cats.
Although a renal tubular reabsorptive defect and portovascular anomalies have been incriminated as causes in a few cases of urate uroliths in cats, the cause in most such cases has not been established. Nonetheless, formation of highly acidic and concentrated urine associated with diets high in purine precursors (especially liver) appears to be a risk factor.
Medical protocols that consistently promote dissolution of ammonium urate uroliths in cats have not been developed, and surgery remains the most common method of removal. For small stones, voiding urohydropropulsion might be effective.
Prevention should include feeding a diet low in purine precursors and promoting the formation of less acidic urine that is not highly concentrated. Although allopurinol can decrease the formation of urate in cats, studies of the efficacy and potential toxicity of allopurinol in cats are required before meaningful guidelines can be established.
Key Points
Calcium oxalate and struvite are the two most common urolith types in cats.
In cats, struvite uroliths are almost invariably sterile and can be treated with dietary dissolution.
Struvite crystalluria can be of pathological importance in cats because it is a common component of urethral plugs and can be associated with lower urinary tract signs in some cases.
Calcium oxalate stones in cats are not amenable to medical dissolution, but dietary management strategies can decrease the rate of recurrence.
For More Information
Bartges JW. Feline calcium oxalate urolithiasis: risk factors and rational treatment approaches. J Feline Med Surg. 2016;18(9):712-722.
Lulich JP, Berent AC, Adams LG, Westropp JL, Bartges JW, Osborne CA. ACVIM Small Animal Consensus Recommendations on the Treatment and Prevention of Uroliths in Dogs and Cats. J Vet Intern Med. 2016;30(5):1564-1574.
Queau Y. Nutritional management of urolithiasis. Vet Clin North Am Small Anim Pract. 2019;49(2):175-186.
Also see pet owner content regarding urinary stones in cats.
