Stalking stones: An overview of canine and feline urolithiasis
Did you know that a new type of urolith has been found in cats? Should you institute preventive therapy if you identify only crystalluria? Is antibiotic therapy automatically warranted in animals with indwelling urinary catheters? This internist revamps your knowledge on diagnosing, treating, and preventing urolithiasis.
Table 2. Quick Reference Guide for Uroliths in Dogs and Cats
Struvite (magnesium ammonium phosphate hexahydrate) uroliths are one of the most common uroliths in dogs and cats. Although
struvite was the most prevalent urolith composition overall in dogs from 1981 to 2005 (43.5%) according to one study,21 the occurrence of struvite uroliths in dogs steadily declined during that same period and struvite became the second most
prevalent urolith composition in dogs beginning in 2004 (39%).21
A study at the Minnesota Urolith Center analyzing uroliths in cats from 1981 to 2002 also revealed a change in struvite urolith
prevalence.22 In 1981, struvite uroliths represented most uroliths in cats (78%), but a steady decline was seen over several years, with
struvite decreasing to an overall prevalence of about 33% from 1994 to 2002. Beginning in 2003, the frequency of struvite
uroliths in cats began to notably increase, reaching 48.1% in 2005 and again establishing struvite as the most prevalent urolith
composition identified in cats at the Minnesota Urolith Center.22 A separate California study analyzing a large number of uroliths in cats over a similar period supports the trend in recent
increases in struvite uroliths.23
The shifts in urolith frequency in both dogs and cats have been theorized to be the result of commercial maintenance diet
reformulation and the increased use of therapeutic diets designed to dissolve or prevent uroliths.22,23 However, this theory does not seem to explain the fact that during the same 25-year period, the mineral composition of urethral
plugs in cats has remained consistently and primarily struvite (87% in 2005).22
Figure 4A. A ventrodorsal abdominal radiograph of a 7-year-old, castrated male basset hound reveals a large radiopaque nephrolith
conforming to the shape of the left renal pelvis.
Characteristics. Struvite uroliths vary greatly in size, ranging from sandlike particles to specimens more than 1 cm in diameter. They are
generally sufficiently radiopaque to be seen easily on survey films. These uroliths assume a variety of shapes but are often
spherical or pyramidal (flat-sided). Uroliths that are pyramidal or > 1 cm in diameter are most often of struvite origin.14,24,25 In some cases, a struvite urolith will take on the shape of the urinary tract surrounding it (Figures 4A-4C). Struvite crystals more readily precipitate in alkaline urine and have an angular, prismlike appearance.
Figure 4B. The same nephrolith seen in Figure 4A as seen during the nephrogram stage of an excretory urogram. Note the remnant
of the left renal pelvis seen as a dark zone adjacent to the nephrolith.
Struvite uroliths can be either sterile or bacterial in origin. Infection-related uroliths occur more commonly in dogs between
2 and 9 years old and in cats < 1 year old and > 10 years old.13,14,24,25 Most struvite uroliths in dogs are caused by a urinary tract infection, occur predominantly in females, and have a breed
predisposition for miniature schnauzers, Shih Tzus, bichon frises, miniature poodles, cocker spaniels, and Lhasa apsos.24 Most struvite uroliths in cats are sterile and not associated with a known sex predisposition. Cat breeds predisposed to
struvite formation include the foreign shorthair, Ragdoll, Chartreux, Oriental shorthair, domestic shorthaired cat, and Himalayan.25 The age predisposition for sterile struvite urolith formation in cats is between 1 and 10 years old with the incidence decreasing
after about 7 years of age.
Figure 4C. The same nephrolith seen in Figures 4A and 4B, exposed after nephrectomy. The composition of the body of the nephrolith
was 95% magnesium ammonium phosphate hexahydrate and 5% calcium phosphate and of the shell was 80% magnesium ammonium phosphate
hexahydrate and 20% calcium phosphate.
Sterile struvite uroliths may be associated with increased to normal magnesium concentrations in the diet, but it is still
unclear whether magnesium triggers struvite urolith formation.13,14 Alkaluria and low urine volume also appear to put cats at risk for developing sterile struvite uroliths.