SILICA METABOLISM AND SILICA UROLITHIASIS IN RUMINANTS: A REVIEW

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Abstract
Calculi composed mainly of silica form in ruminants (cattle, sheep and buffalo) in North America, Australia, the U.S.S.R. and India. Death due to obstruction of the urethra by large calculi is a serious problem in some areas. Obstruction is more common in castrate males than in intact males or females because of differences in the anatomy of the urethra. In addition to silica, calculi contain up to 20% organic matter and, occasionally, appreciable quantities of calcium oxalate or carbonate. Although the ultrastructural characteristics of calculi vary, silica is present entirely as hydrated (isotropic) silicon dioxide. The graminous species that constitute the main diet of afflicted animals contain high levels of silica (up to 6%). Dissolution of the more soluble fractions of this silica saturates the reticulo-rumen fluid with silicic acid, the chemical species that is absorbed and excreted. In the kidney, reabsorption of water raises the urinary concentration of silicic acid more than 100-fold compared with the concentration in glomerular filtrate, and often to values well above saturation. At these concentrations, silicic acid polymerizes in vitro to form large molecular weight micelles of silica. In the process of calculus formation, similar micelles are thought to coalesce and precipitate in association with proteinaceous sustances in urine. This would account for the presence of organic matter in calculi. Under the scanning electron microscope, particles can be seen in calculi that resemble silica particles produced in vitro from solutions of silicic acid and protein. Calculus formation in steers on Western Canadian range, or given range grass hay in confinement, is largely abolished by increasing the intake of water. Water intake was increased by providing supplements containing 15–25% sodium chloride. The effect was to increase urine output, lower the urinary concentration of silicic acid, and prevent deposition of silica in the bladder. This simple procedure constituted the most effective and least costly means of preventing calculus formation under range conditions in southern Alberta.