Magnesium ammonium phosphate (MAP, also called struvite) comprised 41 percent of 28,629 canine uroliths submitted to the Minnesota
Urolith Center during the year 2003 (Table 1).
Table 1: Quantitative mineral composition of 28,629 canine uroliths -- 2003
Calcium oxalate comprised 40 percent of the canine uroliths submitted. Almost all struvite uroliths encountered in dogs occur
as a consequence of infection of the urinary tract with urease producing microbes (especially staphylococci). Unlike cats,
where >95 percent of the struvite uroliths are linked to urinary excretion of excessive quantities of dietary minerals (so-called
sterile struvite), we estimate that <2 percent of the canine struvite uroliths form under sterile conditions.
The objective of this continuing series is to provide insight into the risks and benefits associated with therapeutic strategies
designed to dissolve and prevent recurrence of canine infection-induced MAP uroliths. Therapeutic caveats associated with
other types of uroliths will be the subject of future Diagnotes.
Carl A. Osborne
DVM, Ph.D., Dipl. ACVIM
1. Infection-induced struvite uroliths may form in any breed of dog. Struvite uroliths formed by more than 160 breeds have
been evaluated at the Minnesota Urolith Center including mixed breeds (25 percent), Miniature Schnauzers (12 percent), Shih
Tzus (9 percent), Bichon Frises (7 percent), Cocker Spaniels (5 percent) and Lhasa Apsos (4 percent).
2. Struvite uroliths are more common in females (85 percent) than males (15 percent). Mean age of affected dogs is 6 years
(range <1 to >19 years).
3. Struvite uroliths are more commonly retrieved from the lower urinary tract (95 percent) than from the upper urinary tract
4. Approximately two-thirds of uroliths detected in immature (<12 months of age) dogs are composed of infection-induced struvite.
5. Urine must be oversaturated with MAP for struvite uroliths to form and supersaturated with MAP for struvite uroliths to
grow. Although oversaturation of urine with MAP may be associated with several factors, the most important are 1) urinary
tract infections (UTI) with urease producing bacteria, and 2) formation of urine with large quantities of urea (Table 2).
Increased excretion of magnesium, ammonium, and phosphorus may increase the risk of stone formation; however, this is not
required for initiation or growth of infection-induced struvite.
6. Urease in vertebrates must be derived from microbes (some bacteria, some yeasts or urea plasmas). Urease produced by microbes
hydrolyzes urea to ammonia. Subsequent formation of ammonium ion reduces hydrogen concentration and results in increased urine
7. The majority of urea in urine originates from dietary protein. In addition, diets high in protein are high in phosphorus
Table 2: Some Potential Risk factors for Canine Infection Induced Struvite
8. UTIs caused by urease producing microbes (frequently staphylococci; infrequently Proteus spp. and ureaplasma) in dogs that
are excreting urine with sufficient quantities of urea results in significant alkalinity associated with increased quantities
of ammonia and ammonium ion, phosphate ion and carbonate ion. These changes increase the risk of rapid formation of uroliths
containing primarily struvite, with smaller quantities of calcium phosphate (so-called calcium apatite) and calcium carbonate
phosphate (so-called) carbonate apatite.
9. The proportions and locations of calcium apatite and carbonate apatite minerals within struvite uroliths vary depending
on the urine concentrations of these minerals, urine pH, and probably other factors. If layers of calcium phosphate precipitate
to form a layer around struvite, they can impede medical dissolution of struvite.
10. The solubility of struvite, calcium apatite and carbonate apatite decreases in alkaline urine. In contrast, struvite crystals
typically dissolve if the urine pH is less than 6.3.
11. In addition to contributing to the formation of MAP, high concentrations of ammonium cations bind to negatively charged
sulfate groups contained in the protective glycosaminoglycan (GAGS) layer coating the mucosal surface of the urinary tract.
As a consequence, struvite crystals may adhere to the mucosa, facilitating their retention and growth within the urinary tract.
Damage to the protective GAGS layer by high concentrations of ammonia may also result in increased adherence of bacteria to
the mucosal surface of the urinary tract.
12. Bacteria that become trapped within the matrix of struvite uroliths may remain viable for long periods. In this location,
they are often protected from the effects of antimicrobial drugs. Therefore, when dissolving struvite uroliths by medical
protocols, antimicrobial drugs should be administered as long as the uroliths can be identified by survey radiography. Although
the urine and surface of uroliths may be sterilized following appropriate antimicrobial therapy, viable microbes may remain
below the surface of the urolith. This observation is of clinical importance because premature discontinuation of antimicrobial
therapy may result in relapse of bacteriuria and reduced efficacy of dietary therapy.
13. Infection-induced uroliths can form within a few days to a few weeks following infection of the urinary tract with urease-
producing microbes. Struvite uroliths associated with UTIs caused by staphylococci or Proteus spp have been detected in puppies
as young as 5 weeks of age.