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?
Common types 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.
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:
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).
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.
Mineral composition 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.
Management considerations How would you manage the urocystoliths in this puppy?
The objectives of management of infection-induced uroliths in immature or mature dogs include:
In that dog, the urocystoliths dissolved within nine days of initiation of calculolytic diet and antimicrobial (amoxicillin and clavulanic acid) treatment; the diet was discontinued on day 10. A slight reduction in serum albumin concentration (from approx. 3.2 to 2.7 g/dl) was observed during the 10-day interval of dietary treatment. The serum albumin concentration returned to reference values soon after the pup resumed eating a normal growth diet. The vesicourachal diverticulum and urethral stricture spontaneously resolved over the next few months.
When considering modified diets designed for use in adult dogs to treat young growing dogs, expected benefits must be evaluated in light of potential risks. Because it is not formulated to meet the nutrient requirements of growing dogs, we do not recommend feeding the canine calculolytic diet to immature dogs for more than a few weeks! If the calculolytic diet is used, we recommend serially monitoring body weight, serum albumin concentration and PCV for evidence of protein/calorie malnutrition. If substantial reductions in these parameters are observed, appropriate adjustments in dietary management should be made. If dietary therapy is associated with sufficient reduction in the size of urolith urocystoliths so that they could pass through a distended urethra, they may be removed by voiding urohydropropulsion.
Controlling UTI How would you prevent urolith recurrence in this pup?
With appropriate medical and/or surgical therapy to eliminate the uroliths, the short-term prognosis for this puppy is good. Eradication and/or control of UTI caused by urease-producing bacteria are the most important factors in preventing recurrence of struvite uroliths in this patient. This type of stone cannot recur in the absence of infection with urease-producing microbes.
By eliminating the infection, the puppy can be fed a balanced diet designed for growth. However, if the UTI persists or recurs following therapy, further evaluation of the puppy with the goal of identifying and correcting factors that predispose to infection should be recommended.
What about the association of the Shih Tzu breed and calcium oxalate uroliths? When considering prognoses, it is important to keep the significance of risk factors in perspective. When used in a qualitative (rather than a quantitative) way, the significance of risk factors should not be assigned an "all-or-none" or "always-or-never" interpretation.
In many situations, specific risk factors only contribute a limited role to the development of urolithiasis. Furthermore, identification of one event in a chain of etiological events is not the same as identifying the entire etiological chain. Other than stating that the Shih Tzu breed has been identified as a risk factor for calcium oxalate uroliths, it would not be reasonable to forecast that this puppy is likely to develop calcium oxalate uroliths. In this context, the observations that calcium oxalate uroliths tend to be associated with advancing age and gender (higher risk in males) are relevant.