The sustained increase in occurrence of struvite uroliths from 2003 to 2007 may be associated with decreased use of diets
designed to dissolve sterile struvite uroliths as a consequence of the significant increase in occurrence of calcium oxalate
uroliths in the 1980s and 1990s. However, it is likely that most of the 5,432 sterile struvite uroliths obtained from cats
and submitted to the urolith center in 2007 could have been readily dissolved in two to four weeks by feeding a diet designed
to promote formation of urine that is undersaturated with struvite.
Photo 2: Low-power transmission photomicrograph of matrix-struvite urethral plug depicting struvite crystals (clear spaces)
surrounded by matrix.
Epidemiology of feline urethral plugs
Of 506 urethral plugs submitted to the urolith center by veterinarians in 2007, the mineral composition of approximately 92
percent was primarily struvite, with only 1 percent composed of calcium oxalate (Figure 3 and Table 1). Since 1981, struvite
has consistently been the most common mineral in feline urethral plugs (Figure 4), and the prevalence of calcium oxalate in
urethral plugs always has been infrequent.
The explanation as to why there have been significant shifts in the prevalence of calcium oxalate and struvite in feline uroliths
during the past 25 years, while the prevalence of struvite and calcium oxalate in feline urethral plugs has not significantly
changed, is not obvious to us.
Whatever the reason(s), the high prevalence of struvite in urethral plugs is of clinical significance in terms of the design
of dietary strategies to prevent their formation.
Dr. Osborne, a diplomate of the American College of Veterinary Internal Medicine, is professor of medicine in the Department
of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.