Parkinson's disease and pesticides: a toxicological perspective

Abstract

Environmental factors have been shown to contribute to the incidence of Parkinson's disease (PD). Pesticides, which represent one of the primary classes of environmental agents associated with PD, share the common feature of being intentionally released into the environment to control or eliminate pests. Pesticides consist of multiple classes and subclasses of insecticides, herbicides, rodenticides, fungicides, fumigants and others and exhibit a vast array of chemically diverse structures. In this review we examine the evidence regarding the ability of each of the major pesticide subclasses to increase the incidence of PD. We propose that, from a toxicological perspective, it would be beneficial to identify specific subclasses, common structural features and the propensity for widespread human exposure when considering the potential role in PD, rather than using the overly broad term of 'pesticides' to describe this diverse group of chemicals. Furthermore, these chemicals and their environmentally relevant combinations should be evaluated for their ability to promote or accelerate PD and not merely for being singular causative agents.

Figure 1

Classification of Pesticides. Pesticides are classified by their target species and can be further subdivided based upon chemical structure. Of those compounds associated with Parkinson’s disease, all are classified as insecticides, herbicides or fungicides. Compounds with asterisks are used as example compounds in Figures 2, 3 and 4.

Figure 2

Structures of representative pesticides. Log organic carbon–water partition coefficient (Koc) and aqueous solubility (Sol) of selected pesticides at 25 °C. *Estimated Koc value because paraquat sorption is primarily due to cation exchange rather than partitioning into soil organic matter.

Figure 3

Introduction and use of various pesticides in the U.S. The commercial introduction of several widely used pesticides are shown for the past century. Whereas many of these compounds have had their use restricted, some continue to persist in the environment. Aldrin and dieldrin, as well as DDT and DDE, are grouped together given their similarities in structure. Similar trends of use and restriction have been observed in Europe. Naturally occurring rotenoids have been used since the 1800s. Other organophosphates were developed and utilized starting in the 1940s.

Figure 4

Environmental fate of various pesticides. The vast majority of pesticides are found in the solid (soil) phase and exhibit a wide range of environmental half-lives. Equilibrium phase distributions of selected pesticides are shown for a system containing equal volumes of water, air and soil (1% organic carbon) at 25 °C, based on recommended or average Henry’s law constant, aqueous solubility, sorption coefficient and soil half-life data [75,76]. Although paraquat has a high water solubility, its charged nature results in strong surface adsorption, whereas the other compounds are lipophilic and primarily partition into soil organic matter.