title: Individual sensitivity distribution evaluation from survival data using a mechanistic model: implications for ecotoxicological risk assessment.
abstract: Two main alternatives are typically used to model mechanistically dose-survival relationship in ecotoxicity tests. Effects are related to a concentration of concern, for instance body concentration, and, to account for their differences relative to time-to-death, individuals have either different concentration thresholds for death ("individual tolerance approach"), or equal probability to die, with death occurring randomly ("stochastic death approach"). A general framework to unify both approaches has recently been proposed. We derived a model from this framework to analyse five datasets (daphnids exposed to selenium, guppies exposed to dieldrin and second, third and fourth instars chironomids exposed to copper), by extending the standard stochastic death approach. We showed the possibility to estimate properly the toxicity parameters together with inter-organisms differences of sensitivity for at least one of these parameters (here the threshold for effect). For the daphnids, there was no improvement of using the extended model, which confirms the expected low variability among genetically identical individuals. For all the other datasets, our model outperformed the standard approach without accounting for differences of sensitivity. We estimated coefficients of variations in the distribution of the logarithm of the threshold from 44% to 4% and showed, for chironomids, a decrease of inter-individual differences of sensitivity with the age of the larvae. All standard threshold estimates were close but above the medium value of the distribution in the new approach, which means that a concentration equal to the standard threshold would ultimately result in the death of more than half of the exposed organisms. A more relevant parameter, such as the concentration protecting 95% of the population, would be 2-4 times inferior to the standard threshold.
authors.affiliation: INERIS, Unit «Models for Ecotoxicology and Toxicology» (METO), F-60550 Verneuil-en-Halatte, France.
authors.forename: Rémy
authors.initials: R
authors.lastname: Beaudouin
authors.forename: Florence A
authors.initials: FA
authors.lastname: Zeman
authors.forename: Alexandre R R
authors.initials: AR
authors.lastname: Péry
mesh_terms: D000818:Animals; D002683:Chironomidae / Q000187:drug effects / Q000254:growth & development; D007814:Larva / Q000187:drug effects; D008962:Models, Theoretical; D018570:Risk Assessment; D012984:Software; D018675:Toxicity Tests