Fipronil Tissue Residues and Withdrawal Period Feasibility in Laying Hens After Extra-Label Use

Fipronil, a broad-spectrum insecticide, is often used off-label in laying hens to control red mites. This study investigated fipronil residue levels in hen tissues after simulating common extra-label administrations: in-feed (1 mg kg^-1 twice) and transdermal (1 mg kg^-1 once). Fipronil residues were quantified by HPLC-MS/MS. Both administrations resulted in quantifiable residues in edible tissues (muscle, liver, kidney, skin, fat) and feathers for up to 60 days. Fipronil-sulfone, the primary metabolite, was the dominant residue in tissues. Fat accumulated the highest concentrations. In-feed administration led to higher overall residues in edible tissues, while transdermal application resulted in higher concentrations in feathers. Calculated withdrawal periods, based on established MRLs, ranged from 36 days (muscle, transdermal) to 131 days (fat, transdermal). These long withdrawal periods render both administration routes impractical for poultry production. The study highlights the risk of fipronil residues in poultry products and suggests feather/fecal sampling for monitoring illicit use.