Sources and mitigation of nationwide parasiticide water pollution
File(s)
Working paper
Author(s)
Type
Working Paper
Abstract
Despite being banned for outdoor agricultural use since 2018, the neonicotinoid insecticide imidacloprid continues to be detected in English freshwaters at potentially harmful concentrations. This study investigates the contribution of domestic pet ectoparasiticides, e.g., treatments for flea, tick, and mite infestations, as putative sources of imidacloprid. For the first time, high frequency daily and year-round measurement of untreated influent into a major wastewater treatment plant (WWTP) in the UK is used to characterise imidacloprid sourced from the highly urbanised catchment it serves. In addition, all available samples from WWTP effluent and Environment
Agency (EA) monitoring sites from running waters (e.g., rivers and streams) across England in the years leading up to and after the ban are used to quantify concentrations in receiving waters at a national scale. These data quantify the year-round presence of imidacloprid in untreated and treated wastewater and in waterways across England since the ban for the first time. More than a third of EA samples in rivers and streams are shown to have exceeded lowest predicted no-effect concentration (PNEC) for adverse biological effects. Imidacloprid in untreated wastewater, and at
many sites downstream of WWTPs, exceeded chronic and acute toxicity thresholds year-round. We demonstrate that if a monitoring site has a WWTP upstream, it is > 3 times more likely to be contaminated with imidacloprid. Bayesian regression modelling shows that concentrations were likely to be higher if a WWTP had a higher pet population in its catchment. These results indicate that parasiticide use has been the primary source of pollution through down-the-drain emissions since the ban. We explored hypothesised mitigation strategies and suggest several options to
substantially reduce contamination without need for extensive changes to wastewater treatment practices. These findings highlight the need to revise consumer advice, product marketing, authorisation and monitoring procedures for ectoparasiticidal products to ensure that their environmental risks are adequately assessed and managed.
Agency (EA) monitoring sites from running waters (e.g., rivers and streams) across England in the years leading up to and after the ban are used to quantify concentrations in receiving waters at a national scale. These data quantify the year-round presence of imidacloprid in untreated and treated wastewater and in waterways across England since the ban for the first time. More than a third of EA samples in rivers and streams are shown to have exceeded lowest predicted no-effect concentration (PNEC) for adverse biological effects. Imidacloprid in untreated wastewater, and at
many sites downstream of WWTPs, exceeded chronic and acute toxicity thresholds year-round. We demonstrate that if a monitoring site has a WWTP upstream, it is > 3 times more likely to be contaminated with imidacloprid. Bayesian regression modelling shows that concentrations were likely to be higher if a WWTP had a higher pet population in its catchment. These results indicate that parasiticide use has been the primary source of pollution through down-the-drain emissions since the ban. We explored hypothesised mitigation strategies and suggest several options to
substantially reduce contamination without need for extensive changes to wastewater treatment practices. These findings highlight the need to revise consumer advice, product marketing, authorisation and monitoring procedures for ectoparasiticidal products to ensure that their environmental risks are adequately assessed and managed.
Date Issued
2026-06-23
Citation
2026
Copyright Statement
© 2026 The Author(s).
Publication Status
Submitted