Active metabolite of the neurotoxic rodenticide bromethalin along with anticoagulant rodenticides detected in birds of prey in the northeastern United States.

Little is known about the ecologic fate of the neurotoxic rodenticide bromethalin, which is currently registered for use in the United States, Canada, and other countries including Australia. There is minimal research on bromethalin's potential to cause secondary toxicosis in nontarget wildlife. The aim of this study was to evaluate adipose tissue in four species of birds of prey presented to a wildlife clinic in Massachusetts, USA, for desmethylbromethalin (DMB), the active metabolite of bromethalin. Birds were also screened for anticoagulant rodenticides (ARs) in liver tissue to present a more complete picture of rodenticide exposures in this geographic area and to evaluate the impact of current mitigation measures in place during the time of sampling, 2021-2022. A total of 44 hawks and owls were included; DMB was found in 29.5% of birds and ARs were present in 95.5%. All birds with DMB detections also had residues of ARs. Among birds positive for ARs, 81% had two or more compounds. To the authors' knowledge the data presented here represent the first published monitoring study to document bromethalin/DMB bioaccumulation in obligate carnivores. As DMB is a more potent neurotoxicant than its parent compound, these results are cause for concern and an indication that further monitoring and study of the potential risk of bromethalin to wildlife species is needed. These findings have global implications as increasing concern regarding exposure to and toxicosis from ARs in nontarget wildlife worldwide leads to a search for alternatives and effective mitigation approaches.