Research Updates: How often do uroliths really recur in cats?

The recurrence of uroliths in cats is presumptively a common clinical situation; however, until now, no study has examined recurrence rates or the approximate time until recurrence for the main types of calculi found in cats. This study was performed using the database of the Minnesota Urolith Center at the University of Minnesota. The authors searched the database for cats that had their first ammonium urate, calcium oxalate, or struvite urolith submitted for analysis in 1998 as well as any subsequent calculi from these patients from one or more recurrences between 1998 and 2003. The authors excluded urolith analyses submitted within a six-month window based on the theory that de novo calcium oxalate and ammonium urate urolith formation requires a minimum of six months. Information recorded included the cat's breed, age at time of urolith removal, and sex; the location within the urinary tract and the composition of the uroliths at each removal; and the time elapsed between calculi submissions.

A total of 4,435 cats were included in the study. Regarding the first uroliths analyzed, 54% of the identified cats (n = 2,393) had calcium oxalate uroliths, 41.1% ( n= 1,821) had magnesium ammonium phosphate uroliths, and < 1% (n = 221) had ammonium urate uroliths. Almost all calculi (95.5%) were removed from the lower urinary tract (bladder or urethra).

Urolith recurrence was determined based on repeat urolith analysis in 5.5% of cats (n = 242) during the six-year study period. The frequency of recurrence was 7.1% (first recurrence), 0.6% (second recurrence), and 0.1% (third recurrence) for calcium oxalate uroliths; 2.7% (first recurrence) and 0.2% (second recurrence) for magnesium ammonium phosphate uroliths; and 13.1% (first recurrence) and 4.1% (second recurrence) for ammonium urate uroliths. Urolith composition at the time of recurrence was the same as the initial uroliths in almost all cases, ranging from 83% to 96% depending on the mineral type. Mean interval to first recurrence was between 1.8 and 2.4 years. Older cats initially diagnosed with calcium oxalate or magnesium ammonium phosphate calculi were more likely to have recurrent uroliths identified than younger cats were.

COMMENTARY

Urolithiasis is a common diagnosis in general small-animal practice. Previous studies estimated that calcium oxalate uroliths occur most frequently in cats (46% of cases), followed by magnesium ammonium phosphate (43%) and ammonium urate (5%) uroliths; however, the relative frequency of these mineral types has changed over the previous decades and may change again in the future as new diets are introduced.1 Unfortunately, urolith recurrence is a common event, and pet owners should be prepared for this eventuality.

This study suggests that cats have less frequent urolith recurrence than do dogs, particularly of calcium oxalate uroliths,1 and that no specific cat breed is at increased risk for recurrence. However, true recurrence must be distinguished from urolith persistence (incomplete removal of all uroliths). Abdominal radiography or ultrasonography immediately after urolith removal is recommended to confirm removal success and to help you determine what short- and long-term management may be needed. Because these imaging tests are not systematically performed by all clinicians, results in this study may have been skewed by persistent calculi being incorrectly counted as recurrent. In addition, the true recurrence rate may have been underestimated in this study because subsequently removed stones may not have been sent in for analysis under the assumption that the urolith mineral type was likely the same as the first occurrence. This hypothesis was tested by sending questionnaires to veterinarians who had diagnosed ammonium urate uroliths in cats in 1998. The results showed that in five of 29 cases, recurrent uroliths were not submitted for mineral analysis, thus increasing the rate of first recurrence from 10.9% to 13.1%. To what extent this study bias may have affected the calculated recurrence rate for all three mineral types of the entire six-year period is unknown.

At this time, the optimal way to decrease the likelihood of urolith recurrence is to prescribe an appropriate diet, manipulate urine pH, and appropriately treat or prevent urinary tract infections, depending on urolith type. Canned food is generally recommended over dry food in order to dilute urine and, thus, decrease the likelihood that urine will be supersaturated by the implicated mineral type. Other dietary management strategies may include increasing the frequency of feedings throughout the day to increase water consumption and providing a highly digestible diet that reduces fecal water loss and promotes diuresis through increased solute load excretion. Some feline therapeutic diets are formulated to maintain a neutral urine pH and may contain urolith formation inhibitors.

Albasan H, Osborne CA, Lulich JP, et al. Rate and frequency of recurrence of uroliths after an initial ammonium urate, calcium oxalate, or struvite urolith in cats. J Am Vet Med Assoc 2009;235(12):1450-1455.

The information in "Research Updates" was provided by Erika Meler, DVM, MS, and Barrak Pressler, DVM, PhD, DACVIM, Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.

REFERENCE

1. Osborne CA, Lulich JP, Kruger JM, et al. Analysis of 451,891 canine uroliths, feline uroliths, and feline urethral plugs from 1981 to 2007: perspectives from the Minnesota Urolith Center. Vet Clin North Am Small Anim Pract 2009;39(1):183-197.