The scarcity of information on the effects of pesticides on native Australian vertebrates constrains the development of biologically relevant risk assessments in Australia for the registration of pesticides. The concern that endemically old and unique Australian vertebrate fauna might display high sensitivity to pesticides used for locust control provoked examination of the acute oral toxicity of the organophosphorus pesticide fenitrothion for the fat-tailed dunnart, Sminthopsis crassicaudata (Gould 1844), and the stripe-faced dunnart, S. macroura (Gould 1845). By using the up-and-down method for determining acute oral toxicity, S. crassicaudata and S. macroura were found to have estimated median lethal doses (LD50s) of 129 mg/kg (95% confidence interval [CI] = 74.2–159.0) and 97 mg/kg (95% CI = 88.3–120.0), respectively. These values are 10 to 14 times lower than the reported LD50 values for a similar-sized eutherian mammal, Mus musculus (L. 1758; LD50 = 1,100–1,400 mg/kg) and lower than all other reported mammalian LD50 values. Such wide interspecific variation in sensitivity to fenitrothion may be a consequence of underlying differences in the metabolic pathway for fenitrothion detoxification in mammals and a possible explanation for the increased toxicity of fenitrothion to dunnarts, compared with other mammals, is proposed. The unexpectedly high sensitivity of these Australian marsupials to fenitrothion emphasises the importance of adequately evaluating the risks of pesticides to endemic Australian fauna. Environ. Toxicol. Chem. 2011; 30:1163–1169. © 2011 SETAC
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