Assessment of Malathion Environmental Kinetics Using Earthworm and Salamander Models

Abstract

This research was designed to assess the toxicity, toxicokinetics, and bioaccumulation of malathion in an earthworm and amphibian laboratory model. Toxicity was assessed by measuring cholinesterase activity with the Ellman assay in the earthworms, Lumbricus terrestris and Eisenia foetida, and in the tiger salamander, Ambystoma tigrinum. All exposed animals had decreased activities of tissue cholinesterase compared to unexposed controls. Cholinesterase activities were suppressed approximately 50% in Lumbricus and 90% in Eisenia. Additionally, there were significantly different basal enzyme activities between the two species. A. tigrinum exposed to malathion by soil or by soil and food exhibited 50-90% suppression in cholinesterase depending on exposure concentration. Regardless of the degree of cholinesterase suppression, clinical signs did not correlate well with enzyme activity. There were detectable xenobiotic burdens after exposure to malathion as measured by gas chromatography, in L. terrestris, E. foetida, and A. tigrinum. However, the tissue concentrations were not high enough when compared to soil concentrations to suggest bioaccumulation. Multiple earthworm exposure methods were compared and it was determined that exposing L. terrestris to malathion by filter paper contact significantly increased body burdens compared to exposure to similar concentrations by soil contact. The toxicokinetics of malathion absorption and elimination in L. terrestris were studied and used to parameterize a mathematical model for the estimation of the bioconcentration of xenobiotics in earthworms. The model can be used to accurately predict the bioconcentration of multiple xenobiotics within 85-103% of experimentally determined values. Published literature was used to validate the model?s effectiveness and the rate of absorption was determined to be the most sensitive parameter.