Transfer of Cadmium and Mercury to Sheep Tissues

Toxic heavy metals such as cadmium and mercury can enter the diet of farm animals by a variety of environmental exposure routes and, hence, contaminate food products derived from those animals. Therefore, there is a need to be able to predict the likely levels of contamination in animal tissues if exposed to a contaminated diet and also to estimate how rapidly an animal will decontaminate once the source of contamination is removed from the diet. Data on the transfer and excretion rates of Cd and Hg from tissues have previously been inadequate to allow the development of dynamic models to predict changes in the degree of contamination of different tissues of ruminants. A study is described during which a group of sheep were given a single oral administration of 109 Cd and 203 Hg. Measurements of the concentrations of the radioisotopes in tissue samples were subsequently made over a period of 1 year. The resultant data were used to develop compartment models to describe the behavior of the two metals in sheep tissues. To our knowledge the models developed are the first to allow the time-dependent prediction of the potential Cd and Hg contamination of animal-derived food products. Previously only advised transfer coefficients were available; we demonstrate that these are of little value for cadmium and mercury due to their slow rates of accumulation and excretion.