Recently a colleague asked me for advice about stones in the urinary bladder of an 8-week-old female Shih Tzu. The owner,
who had recently acquired the dog, was concerned about whether or not the uroliths were indicative of an inherited disorder.
The sources of information she consulted indicated that Shih Tzus are at increased risk for recurrent calcium oxalate urolithiasis.
This in turn prompted questions related to therapeutic options and short-term and long-term prognosis. How would you respond
to these questions?
What kinds of uroliths occur in immature dogs?
Urolithiasis most commonly causes illness in middle age to older dogs. However, 1.2 percent (2,102 of 181,386) of the canine
uroliths analyzed at the Minnesota Urolith Center from 1981 through 2002 were obtained from dogs that were less than 12 months
old. It would be logical to expect that the type of uroliths most commonly encountered in immature dogs would be those linked
to congenital or familial predispositions.
For example, a predisposition to calcium oxalate uroliths has been identified in several breeds including Shih Tzus, Lhasa
Apsos, Miniature and Standard Schnauzers, Yorkshire Terriers and Bichon Frises. Likewise, some Dachshunds, Newfoundlands,
English Bulldogs, Mastiffs and Staffordshire Bullterriers are predisposed to cystine uroliths. However, in our series of uroliths
retrieved from immature dogs, only 5 percent were calcium oxalate, and only 2 percent were cystine (Table 1). However, 57
percent were composed of magnesium ammonium phosphate (struvite). What is the explanation of these trends?
Table 1: Quantitative mineral composition
There is substantial evidence that most struvite uroliths in immature and mature dogs are acquired secondary to urinary tract
infections with urease producing microbes, especially staphylococci. Staphylococcal UTI's are in turn often associated with
underlying defects in urinary tract defense mechanisms that normally prevent bacterial infections. These abnormalities include:
- interference with normal micturition.
- anatomical defects
- alterations in urothelium
- altered urine volume and/or composition.
When formulating diagnostic, prognostic and therapeutic plans for immature dogs with infection-induced struvite uroliths,
congenital or acquired abnormalities in these defense mechanisms should be considered.
Purines, especially ammonium urate, accounted for 23 percent of the uroliths from immature dogs (Table 1). The relatively
high prevalence of ammonium urate uroliths in young dogs is associated with congenital portovascular anomalies. Although urate
uroliths occasionally occur in immature Dalmatians, this stone type is most commonly retrieved from Dalmatians when they are
1-4 years of age (mean age is 4 ±2.5 years).
Recall that cystine uroliths only comprised 2 percent of the stones retrieved from young dogs (Table 1). However, data compiled
at the Minnesota Urolith Center revealed that the mean age of dogs at the time of cystine urolith retrieval was 4.8 ± 2.5
years (range = 3 months to 14 years). This observation is surprising inasmuch that one might expect an earlier onset of clinical
manifestations of an inherited disorder. The reasons that cystine uroliths often are not recognized in most affected dogs
until after they reach maturity are unknown to us.
Figure 1: Mineral composition of uroliths formed by dogs less than 1 year of age.
Calcium oxalate was encountered in only 5 percent of immature dogs with uroliths (Table 1). However, calcium oxalate was found
in 39 percent of 25,268 canine uroliths submitted to the Minnesota Urolith Center in 2002. This stone type is most commonly
retrieved from dogs when they are 6 to 11 years of age (mean age is 8.5 ± 2.9 years). The reasons that calcium oxalate uroliths
are most often recognized in middle age to older dogs are unknown to us. It is of interest that the infrequent occurrence
of calcium oxalate uroliths in children has been hypothesized to be related to their ability to produce a urinary protein
with stronger calcium crystallization inhibitory properties than adults.
What is the mineral composition of the urolith in this pup?
What is the likelihood that this puppy has calcium oxalate uroliths? Although epidemiologic studies have revealed that adult
Shih Tzus have four to five times greater risk for calcium oxalate uroliths than dogs without urinary tract disorders, it
is inadvisable to extrapolate this data to immature dogs. Evaluation of the puppy described above revealed that she had a
Staphylococcus intermedius urinary tract infection. The pH of her urine was 7.5, and the sediment contained numerous struvite
crystals. Survey radiographs of her abdomen revealed numerous small radiodense urocystoliths. Quantitative analysis of a small
urolith collected in a tropical fish net when she voided revealed that it was composed of 95 percent magnesium ammonium phosphate
and 5 percent calcium phosphate (carbonate apatite; Photo 1, next page). This finding is consistent with the epidemiologic
trends of uroliths formed by young dogs summarized in Table 1, p. 25.