Canine Struvite Urolithiasis

Struvite is the most common canine urolith. Struvite uroliths form when urine is supersaturated with magnesium, ammonium, and phosphate. Saturation is usually associated with a bacterial urinary tract infection (UTI) with urease-producing bacteria such as Staphylococcus species or, less commonly, Proteus species. Infections are particularly common in female and young dogs. Medical dissolution of struvite is best achieved with a combination of a struvitolytic diet and antibiotics chosen by culture and sensitivity. These treatments should continue for at least 1 month past survey radiographic resolution of uroliths. Prevention or early eradication of bacterial UTI is the most important preventive measure for infection-induced struvite uroliths in dogs. Struvite uroliths are composed of magnesium ammonium phosphate hexahydrate. A Swedish geologist coined the term struvite in 1845 to honor H.C.G. Struve, a Russian diplomat and naturalist. Struvite has also been called triple phosphate; however, this is a misnomer, reflecting historical confusion about the mineral composition of the stone based on qualitative instead of quantitative analysis. ETIOPATHOGENESIS An important cause of urolith formation is supersaturation of urine with calculogenic substances. For struvite uroliths to form, urine must contain increased concentrations of magnesium, ammonium, and phosphate ions. In dogs, most struvite uroliths are associated with a bacterial urinary tract infection (UTI) with urease-producing bacteria such as Staphylococcus species or, less commonly, Proteus species (Figure 1). Occasionally, urease may be produced by Streptococcus or Klebsiella species or, rarely, by Escherichia coli; but these organisms have rarely been associated with struvite uroliths. Urease is an enzyme that, in the presence of water, metabolizes urea to ammonia and carbon dioxide, resulting in increased levels of ammonia in urine. Ammonia combines with water or hydrogen ion to produce ammonium ion (NH4). Buffering of hydrogen ion by ammonia results in increased urinary pH and a shift of phosphorus from H3PO4 to PO4. With increased levels of NH4 and PO4 in the presence of Mg and an alkaline pH, the solubility product and formation product of struvite may be exceeded, resulting in precipitation. In addition, damage to bladder epithelium by bacteria and their metabolic byproducts may provide a surface on which struvite can preCE Vol. 23, No. 5 May 2001 407 n Struvite uroliths are usually detected on survey abdominal radiography; however, ultrasonography or contrast radiography may be necessary to confirm their presence. n Medical dissolution of struvite uroliths often requires both a struvitolytic diet and antibiotics. cipitate. In order for infection-induced struvite uroliths to form, a bacterial UTI must be present and the bacteria responsible for the infection must produce urease (Figure 1). Infection-induced struvite uroliths often contain calcium carbonate (also called calcium apatite carbonate form or carbonate apatite). Because of alkaluria and release of carbonate ion during metabolism of urea, carbonate apatite may also precipitate with struvite. Although uncommon, the presence of carbonate apatite may hinder medical dissolution of struvite uroliths. Furthermore, because formation of carbonate apatite is due to a urease-producing bacterial UTI, prevention of carbonate apatite is similar to that described for infection-induced struvite urolithiasis. Struvite uroliths are occasionally formed in dogs in the absence of urease-producing bacteria. The etiopathogenesis and biologic behavior of these sterile struvite uroliths are different from those of infectioninduced struvite uroliths in dogs but similar to those of sterile struvite uroliths in cats. Renal tubular acidosis results in alkaluria and could therefore theoretically contribute to the formation of struvite uroliths. None of the dogs in one study had renal tubular acidosis. However, renal tubular acidosis was found in several related dogs affected with sterile struvite uroliths. In these dogs, uroliths were highly recurrent and successfully prevented by feeding a diet with added acidifiers or by administering a urinary acidifier. PREVALENCE Although the relative prevalence of different types of canine uroliths has changed over the past 10 years, struvite remains the most common canine urolith type submitted to the Minnesota Urolith Center, St. Paul, MN. Struvite constituted 50% of canine uroliths and 33% of canine nephroliths from 1981 to 1997. The majority (95%) of struvite uroliths occur in the lower urinary tract. Stones vary in size, shape, and texture (Figure 2). The size and shape of struvite uroliths may also change rapidly or remain static over a period of years. Struvite uroliths have been documented to begin forming within 2 to 8 weeks following infection with urease-producing Staphylococcus species. Although size and shape vary within each mineral type, there are some similarities. Urocystoliths over 10 mm in diameter are more than 92% likely to be struvite. Urocystoliths that are smooth, faceted, or pyramidal are usually struvite (Figure 2), but smooth uroliths can also be cystine, ammonium urate, or calcium oxalate monohydrate. Jackstone configurations are most consistent with silica, and grape clusters (botryoidal stones) are usually calcium oxalate. The mean age at which struvite stones are removed is 6.0 ± 2.9 years (range, 1 month to 19 years). Female dogs are more commonly affected than males. Of uroliths in females, 88% were struvite versus 38% in males. This may be related to the greater propensity for females to develop bacterial UTIs. Females are also more likely to form stones of mixed mineral type than are males. Infection-induced struvite is also the most common mineral component of uroliths retrieved and analyzed from dogs younger than 1 year of age. Although any breed may be affected, miniature schnauzers, shih tzus, bichon frises, miniature poodles, cocker spaniels, and Lhasa apsos are more frequently affected than other breeds. An increased risk of forming struvite uroliths compared with other mineral types has been documented in cocker spaniels, Labrador retrievers, Scottish terriers, Pekingese, basset hounds, springer spaniels, and German shepherds. Among breeds that frequently form uroliths, several (e.g., dalmatians, Pomeranians, Maltese) have a relatively decreased risk of forming struvite uroliths. Nephroliths and ureteroliths are uncommon in dogs, representing less than 4% of uroliths. In one analysis of 80 canine renal calculi, 29% were struvite; 24% were calcium oxalate; 11% were ammonium urate; 6% were calcium apatite; and 1% each were calcium hydrogen phosphate, sodium urate, silica, and xanthine. The other 25% of these nephroliths were of mixed composition with no one predominant mineral type. More recent reviews of canine and feline renal calculi found that 57% of 317 specimens contained a mixture of mineral types and 41% of 797 nephroliths were calcium oxalate and 33% were struvite. Struvite nephroliths are associated with bacterial UTIs in over 408 Small Animal/Exotics Compendium May 2001