Page 885 - Small Animal Clinical Nutrition 5th Edition

916 Small Animal Clinical Nutrition

higher quantities of silica. For example, grasses contain between eter. Some protrusions are long and slender, whereas others
VetBooks.ir 1 and 4% silica by dry weight. Plants notable for their high sil- are blunt, imparting a mammillary appearance to the urolith.
Protrusions from individual uroliths are usually but not
ica content include rice and scouring rushes (genus Equisetum),
invariably similar in length and diameter. These features of
which contain up to 16% silica (Salisbury and Ross, 1985).
silica uroliths often impart a distinctive appearance, which can
EPIDEMIOLOGY AND MINERAL often be identified by radiography. Cross sections of canine
COMPOSITION silica jackstones reveal that they are distinctly laminated;
however, these laminations cannot be detected by radiogra-
Prevalence phy. In most dogs, silica jackstones occurred in multiples with
Silica accounted for 0.4% of all canine uroliths submitted to the some patients having more than 30. However, a few dogs had
Minnesota Urolith Center from 1981 to 2007, (1,414 of solitary uroliths. Silica uroliths ranged in diameter from less
350,803 [Table 38-8]) and 0.33% of (134 of 40,612) uroliths than 1 mm to more than 3 cm. Not all uroliths composed pri-
submitted in 2007. Silica accounted for 0.34% of all upper tract marily of silica had a jackstone configuration. However, all sil-
uroliths analyzed at the Minnesota Urolith Center from 1981 ica uroliths formed by dogs had some form of surface protru-
to 2007 (19 of 5,591). Silica uroliths were more commonly re- sions at more or less regular intervals, imparting a regularly
moved from the lower urinary tract (99%) than the upper uri- uneven surface contour to the uroliths. Some silica jackstones
nary tract (1%) (Table 38-9). were coated with layers of struvite, which altered their char-
acteristic shape. In some instances, struvite completely sur-
Age and Gender rounded silica jackstones.
In approximately 310,000 cases of urolithiasis evaluated at our Calcium oxalate monohydrate, calcium oxalate dihydrate and
center, only one case of silica urolithiasis was documented in ammonium urate uroliths may also have a jackstone-like ap-
dogs less than 12 months of age. The mean age of dogs at the pearance as detected by survey and contrast radiography.
time of urolith retrieval was 7.7 years (range eight months to 19 However, the macroscopic appearance of non-silica jackstones
years). Males were affected (95%) more commonly than fe- is typically different than that of silica jackstones.
males (5%).However,female dogs may void small silica uroliths
before they induce clinical signs, reducing the detection rate. ETIOPATHOGENESIS AND RISK
FACTORS
Breeds
From 2000 to 2006, 82 different breeds were affected including Overview
mixed breeds (19%), German shepherd dogs (13%), golden Naturally occurring silica jackstones were first reported in dogs
retrievers (6%), Shih Tzus (6%), black Labrador retrievers (6%), living in the United States in the mid-1970s (Osborne et al,
rottweilers (2%), miniature schnauzers (2%) and cocker spaniels 1986). An extensive review of the literature revealed a conspic-
(1%). A high prevalence of silica uroliths in German shepherd uous absence of this type of canine urolith before that time. In
dogs, Yorkshire terriers, Shih Tzus, Lhasa apsos, golden retriev- the mid-1970s, silica uroliths were reported to occur only in
ers, miniature schnauzers and old English sheepdogs has been dogs from the United States and Canada. However, in 1985,
recognized (Aldrich et al, 1997, Ling, 1995). Several of these canine silica jackstones were recognized in Japan and shortly
breeds also appear to be at risk for calcium oxalate uroliths.This thereafter in Europe. Calcium magnesium aluminum silicate
association, and the observation that silica uroliths often con- uroliths without a jackstone configuration were identified in
tain calcium oxalate, prompt questions about the possibility of dogs native to Kenya in 1977 (Brodey et al, 1977).
epitaxy associated with silica and calcium oxalate urolithiasis.
Relationship of Silica Uroliths to Food
MINERAL COMPOSITION AND Hypothesis
ARCHITECTURE Several observations prompt the hypothesis that development
of canine silica uroliths may be related to hyperexcretion of sil-
Of 1,414 canine silica uroliths, 982 were composed entirely ica in urine following consumption of an absorbable form of sil-
(100%) of amorphous silica, and 432 were composed of at least ica in various foods. It is noteworthy that silicic acid is readily
70% of this mineral. Silica uroliths may contain varying quan- absorbed across the intestinal wall (Ammerman et al, 1980;
tities of other minerals, especially calcium oxalate. Ammonium Sutor et al,1970;White and Porter,1969).The fact that ingest-
urate and calcium phosphate are encountered less frequently in ed silica is rapidly cleared by the kidneys from plasma of dogs
association with silica. and other animals following absorption into the body also sup-
Most canine silica uroliths have a jackstone configuration ports the “dietary risk factor” hypothesis (Benke and Osborn,
(Osborne et al, 1981). The name jackstone was selected be- 1979; King et al, 1933).
cause their shape is similar to the small, six-pronged metal
pieces used in the children’s games of “jacks.” Protrusions Possible Dietary Sources of Silica or Silicate
from different uroliths vary in number (usually from 15 to Silicate minerals occur in very low concentrations in most ani-
30), length (from a few mm to more than one cm) and diam- mals. Therefore, ingredients in pet foods derived from animal