Pesticides and birds — mechanisms of selective toxicity

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On present evidence certain pesticides tend to be more acutely toxic to birds than to mammals. This trend is marked with carbamates, less marked with organophosphates. There is also some evidence that birds have a greater capacity for bioaccumulation of persistent pesticides such as dieldrin and DDE than do mammals of similar size. Consideration is given to biochemical and physiological contrasts between the two groups which may be connected with these differences. Small liver size, the maintenance of relatively high body temperatures, the direct movement of blood to the kidneys, the process of egg laying and the release of urine into the cloaca are all special features of birds which may affect selective toxicity. There is also evidence that birds are deficient in certain enzymes which aid the detoxication of pesticides. Birds tend to have very low levels of ‘A’ esterase in comparison to mammals, and this may explain their high susceptibility to organophosphates such as pirimiphos-methyl and diazinon. Hepatic microsomal monooxygenase activity also tends to be low in birds and this may have far-reaching consequences. In some birds the relatively high susceptibility to carbamates and the tendency to bioaccumulate persistent organochlorine compounds may be due, at least in part, to a deficiency in this enzyme system. A case can be made for more detailed investigation of comparative detoxication mechanisms in birds and other vertebrates to aid the explanation and the prediction of patterns of selective toxicity.