Transfer and risk assessment of fipronil in laying hen tissues and eggs

Fipronil is a persistent insecticide known to transfer into hen eggs from exposure from animal drinking water and feed, but some questions remain regarding its transfer behavior and distribution characteristics. Therefore, the dynamic metabolism, residue distribution and transfer factor (TF) of fipronil were investigated in 11 edible tissues of laying hens and eggs over 21 days. After a continuous low-dose drinking water exposure scenario, the sum of fipronil and all its metabolites (defined as fipronilT) quickly transferred to each edible tissue and gradually increased with exposure time. FipronilT residue in eggs first appeared at 3 days and then gradually increased. After a single high-dose feed exposure scenario, fipronilT residue in edible tissues first appeared after 2 h, quickly peaked at 1 day, and then gradually decreased. In eggs, fipronilT residue first appeared at 2 days, peaked 6–7 days and then gradually decreased. The TF values followed the order of the skin (0.30–0.73) > egg yolk (0.30–0.71) > bottom (0.21–0.59) after drinking water exposure, and the order of the skin (1.01–1.59) > bottom (0.75–1.1) > egg yolk (0.58–1.10) for feed exposure. Fipronil sulfone, a more toxic compound, was the predominant metabolite with higher levels distributed in the skin and bottom for both exposure pathways. FipronilT was distributed in egg yolks rather than in albumen owing to its lipophilicity, and the ratio of egg yolk to albumen may potentially reflect the time of exposure. The distinction is that the residues after feed exposure were much higher than that after drinking water exposure in edible tissues and eggs. The study highlights the residual characteristics of two exposure pathways, which would contribute to the tracing of contamination sources and risk assessment.