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Background Imidacloprid is an active ingredient included in plant protection, biocidal and veterinary medicinal products (VMPs). VMPs containing Imidacloprid are formulated as spot-on products or collars and designed to protect pets, predominantly dogs and cats, from parasite infestation. Monitoring data collected under the Water Framework Directive between 2016 and 2018 showed detectable and varying levels of Imidacloprid in the UK surface water bodies. The aim of the work was to investigate the potential contribution of VMPs by developing a model for predicting the emissions from sewage treatment plants from the use of dog and cat spot-on and collar VMPs. Due to the absence of appropriate exposure models for VMPs, the model was built based on the principles of environmental exposure assessment for biocidal products. Results Three emission paths were considered to be the most likely routes for repeated emissions to waterways from the use of spot-on and collar VMPs, i.e., transfer to pet bedding followed by washing, washing/bathing of dogs, and walking dogs in the rain. The developed model was used to calculate the Imidacloprid concentrations in surface water after discharge from wastewater treatment plants. Realistic worst-case input parameters were deduced from sales and survey data and experimental studies. Modelled total concentrations in surface water for each pathway ranged from 0.84 to 4.8 ng/L. The calculated concentrations did not exceed the ecological thresholds for the most sensitive aquatic invertebrate organisms and were found to be much lower than the UK monitoring data for river water. For example, the calculated concentration from the bathing/washing of dogs was < 3% of the highest levels of Imidacloprid measured in surface waters. Conclusion In conclusion, a model has been successfully built and applied. The modelled data indicate that these VMPs make only a very small contribution to the levels of Imidacloprid observed in the UK water monitoring programme. Further, calculated concentrations do not exceed ecotoxicological threshold values indicating acceptable chronic safety to aquatic organisms.

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In 2020, Anthe et al. published a newly developed model to predict imidacloprid surface water concentrations stemming from sewage treatment plant (STP) effluent as a consequence of the use of veterinary medicinal products containing imidacloprid in the UK (Anthe in Environ Sci Eur (2020) 32:147, https://doi.org/10.1186/s12302-020–00424-4). The modelled data indicate that these veterinary medicinal products make only a very small contribution to the levels of Imidacloprid observed in the UK water monitoring programme.The commentary by Perkins et al. (Perkins in Environ Sci Eur (2021) 33:88, https://doi.org/10.1186/s12302-021-00533-8) questioned the validity and conclusions of the modelling approach. We believe the modelling approach, which considered what we anticipated to be, the major exposure pathways, gives a realistic picture of the chronic emission via STPs to UK rivers.

An amendment to this paper has been published and can be accessed via the original article.