Uroliths (Bladder Stones) in Cats
Most bladder stones in cats cause symptoms of FLUTD. The same process that causes small crystals or crystalline material to plug the urethra also causes stones to form in the bladder. Stones are more likely to form in a persistently infected bladder and in a bladder that is partially obstructed. Stones irritate the bladder wall, prolong infection, and produce symptoms of FLUTD. Cats with bladder stones may not have any crystals present in urine samples. Some affected cats show no clinical signs.
The two most common types of uroliths are struvite (magnesium phosphate) and calcium oxalate. There are also other types of uroliths. Two factors of primary importance in struvite stone formation are high concentrations of magnesium in the urine and a urine with an alkaline pH (over 6.8). Factors of primary importance in calcium oxalate urolith formation include urine with an acidic pH and lower levels of magnesium in the diet.
Female cats have a higher risk of developing struvite uroliths, especially female cats 1 to 2 years of age. However, struvite uroliths have been found in kittens as young as 1 month and cats as old as 20 years. Calcium oxalate uroliths tend to occur more often in male cats, particularly 10- to 15-year-old neutered males.
General Feline Urolith Facts
The Minnesota Urolith Center at the University of Minnesota College of Veterinary Medicine, established by Dr. Carl A. Osborne, is well known for its work on uroliths in cats. Among other things, the center analyzes uroliths removed from cats and sent in for study. From 1981 until about 2002, the percent of struvite uroliths dropped from close to 98 percent to just 33 percent. This is undoubtedly due to changes in feline diets that led to acidic urine.
Meanwhile, the percentage of calcium oxalate uroliths rose from 2 percent to 55 percent. For a few years since 2002, the two types of uroliths were about equal in number, as judged by samples sent to the lab. Struvite uroliths are now on the rise again.
In evaluating the content of urethral plugs submitted to the lab, struvite has continued to dominate-often 85 percent or higher. It is not known why there is the discrepancy between the composition of urethral plugs and that of uroliths.
Treatment: Struvite stones will usually dissolve in one to three months under the same treatment protocol described for FLUTD. Abdominal X-rays are taken periodically to monitor the progress of dissolution. Any associated bacterial infection is treated as described for Cystitis.
Stones that do not dissolve must be removed surgically. Calcium oxalate stones almost always require surgical removal. Following treatment, a cat should be placed on the same protocol as described for Preventing FLUTD. The dietary regimen will vary with the type of urolith. Stones should be sent for analysis, to determine the proper course of treatment.
This article is excerpted from “Cat Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc
Copyright © 2008 by Delbert Carlson, DVM, and James M. Giffin, MD. All rights reserved.