Page 862 - Small Animal Clinical Nutrition 5th Edition

Canine Struvite Urolithiasis 893

Table 43-2. Some potential risk factors for canine infection-induced struvite uroliths.
VetBooks.ir Diet Urine Patient/metabolic Drugs
High protein content
Breeds
associated bacterial
(source of urea) Urease-positive UTI Females Glucocorticoid-
High urea concentration
Urine alkalinizing potential Hyperammonuria Miniature schnauzers UTI
High phosphorus content High-ionic phosphorus concentration Bichon Frises
High magnesium content High magnesium levels Shih Tzus
High pH Pugs
Urine retention Dachshunds
Concentration of urine and thus Hyperadrenocorticism associated
lithogenic substances with bacterial UTI
Key: UTI = urinary tract infection.

concentration of urea normally present in urine of individuals Musher, 1975). First, growth of urease-producing Proteus spp.
that consume dietary protein in excess of the daily requirement in urea-free urine, or in urine containing a urease inhibitor, did
for protein anabolism makes urine an environment well suited not cause alkalinization, supersaturation or crystallization of
to support the pathogenic effects of urease-producing mi- struvite and apatite. Second, growth of weak urease-producing
crobes. Because of the importance of urease in the etiopatho- bacteria (Klebsiella spp. and Pseudomonas spp.) and non-urease-
genesis of struvite urolithiasis in people and many other ani- producing bacteria (Escherichia spp.) was not associated with
mals, the name “urease stones” has been proposed (Griffith, alkalinization, supersaturation and subsequent precipitation of
1978). Following a parallel line of reasoning in context of path- struvite and apatite crystals.
ophysiologic events, the name “urea stones” would also be Staphylococcus and Proteus spp. are consistent and potent ure-
appropriate (Osborne et al, 1985). ase producers and have been commonly isolated from animals
Continued production of ammonia and perhaps other toxic and people with infection-induced struvite uroliths (Griffith,
reactants as a consequence of urease-induced ureolysis appears to 1978, 1978a; Osborne et al, 1981). For unexplained reasons,
induce an inflammatory response in the urothelium and adjacent staphylococci have been more commonly associated with stru-
structures (Griffith, 1978a; Krawiec et al, 1984, 1984a). In fact, vite uroliths in dogs than Proteus spp., whereas Proteus spp. are
urease production contributes to the virulence of uropathogens more commonly associated with struvite uroliths in people
that produce this enzyme (Brande and Siemienski, 1960; Mac- (Osborne et al, 1999; Griffith and Klein, 1983; Feit and Fair,
Laren, 1968; Parsons et al, 1984; Rosenstein and Hamilton- 1979; Krajden et al, 1984; Lewis et al, 1984; Stamey, 1980). It
Miller, 1984). The associated increase in urine concentration of appears that some strains of Proteus mirabilis have special affin-
proteinaceous inflammatory products acts as a form of matrix ity for the urinary tract of people (Senior, 1979). In pilot stud-
and contributes to lithogenesis. ies involving dogs at the University of Minnesota, better suc-
Another mechanism that has been hypothesized to predis- cess occurred in inducing struvite uroliths with clinical isolates
pose patients with UTIs to urolithiasis is a bacteria-mediated of staphylococci than with Proteus spp. Results of studies in rats
reduction in the urine concentration of citrate (Conway et al, were interpreted to indicate that different strains of staphylo-
1949; Robertson and Peacock, 1982; Scott et al, 1943). Citrate cocci had different lithogenic potential (Vermeulen and Goetz,
is often called a crystallization inhibitor because it can combine 1954).
with cations such as calcium and magnesium to increase their Although other organisms such as Klebsiella and Pseudomonas
solubility (Schwille et al, 1979). It has also been suggested that spp. have potential to produce varying quantities of urease
bacteria may produce lithogenic matrix substances (Stegmayr (Griffith, 1978), they have not been as commonly associated
and Stegmayr, 1983). with initiation of struvite urolith formation in people or dogs.
Likewise, E. coli and other non-urease-producing microbes
Bacterial UTIs have not been linked to naturally occurring struvite uroliths,
Clinical and non-clinical studies involving dogs have repeat- presumably because they infrequently produce urease (Griffith,
edly demonstrated a close relationship between formation of 1978; Lesher and Jones, 1978). However, it has been reported
struvite uroliths and UTIs caused by urease-producing bacte- that urease activity may be transferred by bacterial plasmids
ria (Osborne et al, 1999). Bacterial UTIs have been such a (Grant et al, 1981).
common finding in dogs with struvite uroliths that they are The bacterial flora of urine may change after formation of
sometimes called infection stones (Griffith, 1978; Osborne et struvite uroliths in dogs as a result of staphylococcal UTI.The
al, 1981). change in bacterial flora may be associated with damage to
Several in vitro observations indicate that bacterial urease- local host defense mechanisms by uroliths, iatrogenic infec-
induced supersaturation of urine with MAP is the primary (but tion induced by urinary catheters or administration of antimi-
not necessarily the only) cause of infection-induced struvite crobial agents.
uroliths (Griffith, 1978; Griffith et al, 1976; Griffith and A very small percentage of dogs with struvite uroliths have