Page 886 - Small Animal Clinical Nutrition 5th Edition

Canine Silica Urolithiasis 917

sources are an unlikely source of this mineral. In contrast, plants The high prevalence of silica urolithiasis in native Kenyan
VetBooks.ir contain larger quantities of silica. As mentioned above, grasses dogs has been hypothesized to be related to consumption of
unprocessed Kenyan corn, a common ingredient in their diet
contain between 1 and 4% silica by dry weight and rice and
garnered primarily by scavenging (Brodey et al, 1977). It is also
scouring rushes (horsetails, genus Equisetum) contain up to
16% silica (Salisbury and Ross, 1985). conceivable that Kenyan dogs could have consumed silica con-
One plausible explanation as to why canine silica uroliths tained in the soil as they scavenged for food.
began to be recognized in the mid-1970s is that at that approx- Another potential source is micro-fine silica, which is used in
imate time, the pet food industry initiated use of an increased small quantities as an anti-caking agent in the manufacture of
quantity of plant-derived ingredients in moist and especially many pet foods. Although a cause and effect relationship be-
dry dog foods. Silicon is taken up by the roots of plants and tween micro-fine silica and silica urolithiasis is unlikely, until
deposited in their cell walls as silica, soluble silicates and organ- additional information becomes available, it seems logical to
ic combinations. Although unlikely, another factor could have avoid giving foods containing this ingredient to dogs with
been the addition of fillers, which contain relatively large quan- recurrent silica urolithiasis.
tities of silica (e.g., rice or soybean hulls), to some pet foods
designed for reduction in obesity (Underwood, 1977). Ruminants
Corn gluten feed, a by-product of the wet milling and distill- The association of food and silica urolith formation in rumi-
ing process designed to separate shelled corn into various com- nants is relevant to consideration of diet-related risk factors in
ponents, was another suspected source of silica in some pet dogs. Silica uroliths are common in range cattle and sheep that
foods. Corn gluten feed remains after extraction of starch, glu- consume forage grasses with a high concentration of silica
ten and germ from shelled corn. The term gluten, meaning (Bailey, 1970; Emerick and Embry, 1960; Emerick et al, 1959;
“glue” in Latin refers to the sticky characteristic of substances Pyrah, 1979; White and Porter, 1969). In Canada, prairie grass
derived from corn, wheat and other grains. Corn gluten feed (Festuca scabrella) has been found to contain 4 to 8% silica
contains about 40% protein and is contained in some low qual- (Bailey, 1966). It is noteworthy, however, that attempts to in-
ity pet foods. We emphasize that corn gluten feed is not the duce silica uroliths in sheep with inorganic forms of silica (sodi-
same as corn gluten meal. Corn gluten meal is contained in um silicate) have been unsuccessful (Beeson et al, 1943; Em-
many higher quality manufactured foods designed for dogs erick et al, 1959).
because it is readily digestible and a relatively inexpensive form Dietary risk factors for induced silica urolithiasis in sheep
of protein (approximately 60%), vitamins, minerals and energy. include low phosphorus concentrations, a high calcium-phos-
Corn gluten meal is an unlikely source of the silica in uroliths. phorus ratio and factors contributing to alkalinization (Em-
Contamination of various types of plants with soil during erick et al, 1959).
harvesting is also conceivable. Another possibility that may
apply to some dogs is consumption of soil secondary to diet- Rats and Guinea Pigs
associated pica. Silica uroliths have been experimentally induced in rats fed
Caution: studies performed in rats indicate that the type of diets containing 2% tetraethylorthosilicate (Emerick, 1984;
silica compound ingested influences its absorption from the Emerick et al, 1963; Stewart et al, 1993). Siliceous deposits
gastrointestinal tract (Yoko and Saboro, 1979). In addition, were detected in the renal tubules of guinea pigs given large oral
other factors (e.g.,pH) may be involved (Pyrah,1979;Yoko and doses of soluble silica (Coe et al, 1991).
Saboro, 1979). Therefore, detection of a relatively large quanti-
ty of silica in food is not itself synonymous with intestinal ab- Applications of Observations to Diet Hypothesis
sorption and urinary excretion of silica. Affected dogs that formed silica uroliths submitted to the
Minnesota Urolith Center were consuming a large variety of
Dogs and People commercially manufactured moist and dry foods in addition to
Silica uroliths developed in male dogs fed experimental foods homemade foods. A widespread change in the formulation of
containing a high concentration of silicic acid and talc commercially manufactured dog food in the U.S. was probably
[Mg SiO 10 (OH) ] for several months (Ehrhart and associated with the onset of silica urolithiasis in the mid-1970s
3
2
McCullagh, 1973; McCullagh and Ehrhart, 1974). Re- and early 1980s, but this assumption has not yet been con-
placement of dietary silicic acid with purified cellulose prevent- firmed.
ed further urolith development. In a pilot study, we detected We consider foods containing large quantities of plant-
multiple tiny silica uroliths in the urinary bladder of an adult derived ingredients as risk factors for silica urolithiasis in suscep-
male beagle dog after it was given magnesium trisilicate orally tible dogs. Corn gluten feed, rice hulls and soybean hulls have
for approximately four months. also been incriminated as dietary risk factors (Osborne et al,
Silica uroliths have been reported in several people who con- 1986).
sumed large quantities of antacids containing magnesium trisil- Concentrations of lithogenic substances in urine are depend-
icate to alleviate signs of peptic ulcers (Farrer and Rajfer, 1984; ent on urine volume. Because dry foods (~10 to 20% water) are
Forman et al, 1959; Herman and Goldberg, 1960; Levison et al often associated with production of more concentrated urine
1982; Pyrah,1979). compared with canned formulated diets (~75 to 80% water),