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ESSENTIALITY, SYNERGISM, AND ANTAGONISM <br />Limited data are available on the beneficial, protective, and essential <br />properties of arsenic, and on its interactions with other chemicals. <br />Arsenic apparently behaves more like an environmental contaminant than as <br />a nutritionally essential mineral (NAS 1977). Nevertheless, low doses (<2 <br />ug/daily) of 'arsenic stimulated growth and metamorphosis in tadpoles, and <br />increased viability and cocoon yield in silkworm caterpillars (NAS 1977). <br />Arsenic deficiency has been observed in rats: signs include rough haircoat, <br />low growth rate, decreased hematocrit, increased fragility of red cells, and <br />enlarged spleen (NAS 1977). Similar results have been documented in goats and <br />pigs fed diets containing less than 0.05 mg As/kg (NAS 1977). In these <br />animals, reproductive performance was impaired, neonatal mortality was <br />increased, birth weight was lower, and weight gains in second-generation <br />animals were decreased; these effects were not evident in animals fed diets <br />containing 0.35 mg As/kg (NAS 1977). <br />The use of phenylarsonic feed additives to promote growth in poultry and <br />swine and to treat specific diseases does not seem to constitute a hazard to <br />the animal or to its consumers. Animal deaths and elevated tissue arsenic <br />residues occur only when the arsenicals are fed at excessive dosages for long <br />periods (NAS 1977). Arsenic can be detected at low levels in tissues of <br />animals fed organoarsenicals, but it is rapidly eliminated when the arsenicals <br />are removed from the feed for the required 5-day period before marketing <br />(Woolson 1975). <br />Selenium and arsenic are antagonists in several animal species. In rats, <br />dogs, swine, cattle, and poultry, the arsenic protects against selenium <br />poisoning if arsenic is administered in the drinking water and selenium <br />through the diet (NAS 1977; NRCC 1978; Pershagen and Vahter 1979). Inorganic <br />arsenic compounds decrease the toxicity of inorganic selenium compounds by <br />increasing biliary excretion (NRCC 1978). However, in contrast to antagonism <br />shown by inorganic arsenic-inorganic selenium mixtures, the toxic effects of <br />naturally methylated selenium compounds (trimethylselenomium chloride and <br />dimethyl selenide) are markedly enhanced by inorganic arsenicals (NRCC 1978). <br />Toxic effects of arsenic can be counteracted with saline purgatives, with <br />various demulcents that coat irritated gastrointestinal mucous membranes, and <br />11