Distinct Toxicological Profiles of Pesticide Metabolites Compared to Parent Compounds: Evidence from a Zebrafish Model

Background:
South Africa's extensive pesticide use presents significant health risks via direct exposure and environmental residues in food and water. Limited data on human metabolism and toxicity of pesticide metabolites hinder accurate risk assessments. This study investigated the toxicity of glyphosate, imidacloprid, chlorpyrifos, mancozeb, and their metabolites using a larval zebrafish model to provide insights into potential human health risks associated with low-level pesticide exposure.

Methods:
A dose range for the pesticides was determined by translating internationally acknowledged human acceptable operator exposure levels to zebrafish-relevant doses and estimating metabolite doses to account for unknown pesticide metabolism. Zebrafish larvae at 94 hours post-fertilization (hpf) were exposed to pesticides or metabolites until 118 hpf. Acute toxicity endpoints, including survival and morphological abnormalities (e.g., pericardial edema, spinal curvature), were assessed. A 5-day exposure protocol also evaluated neurobehavioral effects via larval locomotion using the light-dark transition test (LDTT), with activity tracked via automated systems (Daniovision and Ethovision, Noldus).

Results:
Metabolites frequently exhibited greater toxicity than parent compounds. Desnitro-imidacloprid and 6-chloronicotinic acid, both imidacloprid metabolites, were more toxic than the parent compound at slightly elevated doses, inducing reduced locomotion, developmental delays, and severe malformations. Chlorpyrifos oxon caused 100% mortality at all tested levels, unlike chlorpyrifos and chlorpyrifos-methyl, which were tolerated at AOEL levels. However, chlorpyrifos oxon and chlorpyrifos-methyl caused significant motor deficits and deformities, likely due to acetylcholinesterase inhibition. Glyphosate showed low acute toxicity, but its metabolite methylphosphonic acid induced developmental defects and yolk sac edema, while increased basal activity suggested neurotoxic stress. Mancozeb resulted in complete mortality and severe malformations, with its metabolite ethylenethiourea also reducing locomotion and inducing deformities.

Conclusion:
These findings reveal significant toxic effects of both pesticides and their metabolites at or near accepted exposure levels. Including metabolites in toxicity assessments is crucial to accurately evaluate potential human health risks.