Brodifacoum isomer formulations with potentially lower risk to non-target wildlife

Anticoagulant rodenticides (ARs) have a long history of successful use in controlling vertebrate pest and invasive species. Despite regulatory efforts to mitigate risk, non-target wildlife may be unintentionally exposed to ARs through various trophic pathways, and depending on dose, exposure can result in adverse effects and mortality. Second-generation ARs (SGARs) are mixtures of cis- and trans-diastereoisomers (each including two stereoisomers) that exhibit similar in vitro inhibitory potency for vitamin K epoxide reductase in rodent microsomal assay systems. Some diastereoisomers and hence some individual stereoisomers are preferentially metabolized in vivo, resulting in residue patterns in exposed target rodents that differ from the bait formulations. Use of less persistent but equally potent SGAR stereoisomers in baits results in lower tissue residues in target rodents, which in turn constitutes lower risk when consumed by non-target wildlife. The toxicity of two brodifacoum formulations with stereoisomers having markedly different elimination half-lives in rats (Formulation A containing the two least persistent stereoisomers, and Formulation B containing the most persistent stereoisomer) were tested in a 7-day dietary feeding trial with American kestrels. Based on previous kestrel studies using commercially available brodifacoum, Formulations A and B were each provided at three dietary concentrations (0.05, 0.1 and 0.5 µg/g diet, 4 kestrels/dose level) predicted to cause a range of toxicity. Compared to unexposed controls, all kestrels that ingested 0.5 µg/g diet of the longer-lived Formulation B exhibited extreme coagulopathy. In contrast, the 0.5 µg/g diet of the shorter-lived Formulation A yielded only a modest lengthening of clotting time in just 1 of the 4 exposed kestrels. These findings support the notion that SGAR baits enriched with less persistent stereoisomers may pose lower hazard and ultimately risk to non-target wildlife.