Pharmacokinetics of fenbendazole and its bioactive metabolite oxfendazole in northern bobwhite administered an anthelmintic feed

The northern bobwhite (Colinus virginianus) has experienced significant population declines across its native range. Parasitic nematodes are increasingly being recognized as contributing factors. QuailGuard®, an anthelmintic feed that uses fenbendazole (FBZ) as the active ingredient, has shown promise in field studies for reducing parasite loads in wild quail. However, little is known about the pharmacokinetics of FBZ in bobwhite, limiting accurate assessments of therapeutic exposure. This study investigated the pharmacokinetic profile of FBZ and its bioactive metabolite, oxfendazole (OFZ), in bobwhite fed QuailGuard®. Plasma concentrations were measured at multiple time points during (1, 2, 4, 8, and 24 h) and after (2, 4, 8, 24, and 32 h) feed administration using LC/MS-MS. Key pharmacokinetic parameters, including estimated area under the curve (AUC), apparent elimination rate, half-life, and mean residence time (MRT) were calculated using composite sampling. Peak average plasma concentrations of FBZ (2.3701 μg/mL) and OFZ (0.5526 μg/mL) occurred at 8 h post-ingestion. Estimated AUCs were 47.782 μg h/mL for FBZ and 12.783 μg h/mL for OFZ, with respective half-lives of 3.83 and 4.03 h. Combined drug exposure approached levels associated with the lethality of Oxyspirura petrowi in prior studies, indicating that intake over multiple weeks may achieve therapeutic efficacy. While sex did not affect analyte accumulation during treatment, females exhibited faster OFZ apparent elimination than males. FBZ and OFZ were rapidly absorbed and eliminated, which supports the safety of QuailGuard® for use in wild populations. These findings establish foundational pharmacokinetic data necessary for optimizing field treatment regimens and improving parasite control strategies in declining bobwhite populations.