The Synergistic Toxicity of Pesticide Mixtures: Implications for Risk Assessment and the Conservation of Endangered Pacific Salmon

The Synergistic Toxicity of Pesticide Mixtures: Implications for Risk Assessment and the Conservation of Endangered Pacific Salmon

http://www.ehponline.org/docs/2008/0800096/abstract.html

Cathy A. Laetz, David H. Baldwin, Tracy K. Collier, Vincent Hebert, John D. Stark, and Nathaniel L. Scholz

 

Abstract

Background: Mixtures of organophosphate and carbamate pesticides are commonly detected in

freshwater habitats that support threatened and endangered species of Pacific salmon

(Oncorhynchus sp.). These pesticides inhibit the activity of acetylcholinesterase (AChE), and

thus have potential to interfere with behaviors that may be essential for salmon survival. While

the effects of individual anticholinesterase insecticides on aquatic species have been studied for

decades, the neurotoxicity of mixtures is still poorly understood.

Objectives: We assessed whether chemicals in a mixture act in isolation (resulting in additive

AChE inhibition) or whether components interact to produce either antagonistic or synergistic

toxicity.

Methods: We measured brain AChE inhibition in juvenile coho salmon (O. kisutch) exposed to

sublethal concentrations of the organophosphates diazinon, malathion, and chlorpyrifos as well

as the carbamates carbaryl and carbofuran. Concentrations of individual chemicals were

normalized to their respective EC50 concentrations and collectively fit to a non-linear regression.

This curve was used to determine whether toxicological responses to binary mixtures were

additive, antagonistic, or synergistic.

Results: Addition and synergism were both observed, with a greater degree of synergism at

higher exposure concentrations. Several combinations of organophosphates were lethal at

concentrations that were sublethal in single chemical trials.

Conclusion: Single chemical risk assessments are likely to underestimate the impacts of these

insecticides on salmon in river systems where mixtures occur. Moreover, mixtures of pesticides

that have been commonly reported in salmon habitats may pose a more important challenge for

species recovery than previously anticipated.