Patterns of bromethalin exposure and toxicosis in California mammals across a shifting rodenticide use landscape

Bromethalin is an acutely neurotoxic rodenticide used as an alternative to anticoagulant rodenticides (ARs) for commensal rodent control. Non-target exposure to bromethalin, and its highly toxic hepatic metabolite, desmethylbromethalin (DMB), is an emerging concern for non-target wildlife yet the ecological implications remain poorly understood. We evaluated patterns of bromethalin exposure and toxicosis in wild mammals using postmortem data collected from 2017 to 2024 in California, USA. Wildlife samples tested for bromethalin exposure during the study period were screened opportunistically, under suspicion of rodenticide toxicity, or part of species-focused mortality research projects. To contextualize these findings, we analyzed Pesticide Use Reporting (PUR) records to compare reported bromethalin and second-generation AR (SGAR) applications before (2018-2020) and after (2021-2023) implementation of statewide SGAR restrictions. Bromethalin exposure was confirmed in 24.4% (131/537) of tested carcasses across 13 species, with mesocarnivores and omnivores comprising most detections. Among exposed individuals, 25.2% (33/131) met criteria for bromethalin toxicosis (DMB detection and characteristic clinical signs and/or lesions, with alternative causes excluded), primarily raccoons, gray foxes, and striped skunks. Adipose tissue provided the highest detection frequency among tested tissues, indicating its utility for wildlife investigations and biomonitoring. Reported statewide bromethalin use increased over the study period, coinciding with reductions in reported SGAR applications following regulatory restrictions. These findings demonstrate widespread bromethalin exposure in both mammalian omnivores and carnivores, highlighting the need for continued postmortem testing to monitor non-target wildlife exposure as rodenticide use patterns shift in response to regulatory changes.