Anticoagulant rodenticides (ARs) are widely used to suppress rodent pests responsible for significant economic loss and threaten human health, however, their use has had unintended impacts to non-target wildlife. In response, California legislation limiting AR use has led to greater reliance on other rodenticides, i.e. bromethalin. The California Department of Fish and Wildlife’s Wildlife Health Laboratory (WHL) and the UC Davis California Animal Health and Food Safety Laboratory routinely test wildlife for rodenticide exposure as part of statewide postmortem investigations. The WHL summarizes rodenticide exposure cases annually to monitor non-target wildlife pesticide exposure in California. In 2024, we tested 166 individuals for ARs (54 raptors and 112 mammals). Of the 166 animals tested, 114 were exposed to one or more ARs and 14 cases resulted in a diagnosis of AR toxicosis. Additionally, 100 mammal carcasses were screened for the bromethalin metabolite, desmethylbromethalin (DMB). DMB was detected in 23 mammals, 22 of which also had ARs present in their liver tissue. These findings indicate use of AR alternatives and ARs is resulting in co-exposures in wildlife across California. Understanding the implications of co-exposure to rodenticides with differing modes of action will be important following changes in statewide rodenticide use.

Ryan is a wildlife ecologist working as a Senior Environmental Scientist and the Pesticide Investigations Coordinator at CDFW's Wildlife Health Lab.