The large accumulation of lead in livestock organs and bone 

 represents a potentially significant source of lead in the human 

 diet . 



No documentation has been found relating chronic exposure of 

 livestock to lead and the subsequent development of cancer. 

 Studies of rats and mice subjected to rather high doses of lead 

 compounds via oral or parenteral administrations exhibited 

 malignant and benign renal neoplasms (Environmental Protection 

 Agency 1977) . 



The synergistic effects of lead and cadmium have been 

 documented for ponies and calves (Burrows and Borchard 1982, Lynch 

 et al . 1976b). Zinc appears to be antagonistic to lead and 

 inhibits symptoms of lead toxicity in young horses (Willoughby et 

 al. 1972b). These authors found that, in the presence of toxic 

 amounts of lead and zinc, the symptoms and tissue lead accumula- 

 tion normally associated with lead toxicity were suppressed and 

 that the clinical symptoms were those associated with zinc 

 toxicity. Willoughby et al . (1972b) found that dietary doses of 

 lead and zinc necessary to experimentally produce clinical 

 toxicity in foals were considerably higher than lead and zinc 

 levels in diets associated with natural toxicosis, thus suggesting 

 interaction with unknown additional elements occurred in the 

 natural poisoning cases. Lead has been shown to also, disrupt 

 tissue levels of iron, copper and manganese in cattle (Doyle and 

 Younger 1984). There is conflicting data concerning the effect of 

 calcium on the absorption and excretion of lead (Pearl et al . 

 1983, Willoughby et al. 1972). 



6.1.4 Zinc toxicology 



Animals have high tolerances for zinc, and only under large, 

 excessive exposures have toxic effects been documented. Diets 

 with 3,000 ppm have been required to induce zinc toxicosis experi- 

 mentally, and 1,000 ppm zinc has not produced adverse effects if 

 there has been an adequate amount of copper and iron in the diet. 

 Ott et al. (1966a) has shown that 1000 to 2000 ppm zinc is 

 necessary to adversely affect the performance of lambs. Zinc is 



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