Quantitative Detection of Fipronil and Fipronil-Sulfone in Sera of Black-Tailed Prairie Dogs and Rats after Oral Exposure to Fipronil by Camel Single-Domain Antibody-Based Immunoassays

The insecticide fipronil can be metabolized to its sulfone in mammalian species. Two camel single-domain antibodies (VHHs) F1 and F6, selective to fipronil and fipronil-sulfone, respectively, were generated and used to develop enzyme linked immunosorbent assays (ELISAs) for the detection of the two compounds in the sera of black-tailed prairie dogs and rats. The limits of detection of fipronil and fipronil-sulfone in the rodent sera by the corresponding ELISAs were 10 and 30 ng mL^–1, and the linear ranges were 30–1000 and 75–2200 ng mL^–1. ELISAs showed a good recovery for fipronil and fipronil-sulfone cospiked in the control sera of the black-tailed prairie dogs (90–109%) and rats (93–106%). The VHH-based ELISAs detected fipronil and fipronil-sulfone in the sera of the rodents that received a repeated oral administration of fipronil. The average concentration of fipronil-sulfone was approximately 3.2-fold higher than fipronil in the prairie dog sera (1.15 vs 0.36 μg mL^–1) and rat sera (1.77 vs 0.53 μg mL^–1). ELISAs agreed well with a liquid chromatography–mass spectrometry method for the quantification of both fipronil and fipronil-sulfone in real serum samples. Fipronil-sulfone was identified as the predominant metabolite of fipronil in the black-tailed prairie dog and rat sera.