Environmental Risks and Toxicity of Fipronil and Imidacloprid Used in Pets Ectoparasiticides

Fipronil (FIP) and imidacloprid (IMID) are two of the most commonly used ectoparasiticides to control parasites in pets. Compared with those of farm animals, their environmental risks have generally been considered low because of their limited use; however, the growing pet population and evolving treatment practices make this assumption challenging. To assess these risks, water samples were collected at an animal shelter in Italy to monitor the abundance of ectoparasiticides in aquatic environments. Additionally, laboratory-based ecotoxicological assays were carried out on a range of marine non-target species across different trophic levels (algae, copepods, and mussels). In vitro toxicity tests on human epithelial cell cultures were also implemented to examine potential cytotoxic effects at the levels of human exposure detectable in a domestic setting after pet treatment. Wastewater samples from the shelter contained 0.18 µg L⁻¹ of IMID, 0.50 µg L⁻¹ of FIP, and 0.20 µg L⁻¹ of FIP-sulfone, with these concentrations remaining stable for 60 days. Chronic exposure to FIP and IMID at 30.0 µg L⁻¹ impaired the mobility of the copepods. The EC₁₀ and EC₂₀ values were determined to be 1.7 (0.06–6.59) µg L⁻¹ and 2.8 (0.436–8.51) µg L⁻¹ for FIP and 2.6 (0.80–6.33) µg L⁻¹ and 7.6 (3.12–15.8) µg L⁻¹ for IMID, respectively. FIP and IMID exposure led to lipid peroxidation in the digestive glands and gills of mussels, whereas only IMID exposure increased acetylcholinesterase activity in the digestive glands at concentrations between 0.5 and 5.0 µg L⁻¹. Additionally, both fipronil and imidacloprid triggered the production of reactive oxygen species and lipid peroxidation and decreased the viability of human keratinocyte cells in a concentration-dependent manner. These findings highlight the persistence and potential risks of FIP and IMID, stressing the need for stricter regulations and further research on chronic environmental exposure to safeguard ecosystems and public health.