Exposure of nontarget white-footed mice (Peromyscus leucopus) to second-generation anticoagulant rodenticides in an urban context.

The pathways of nontarget exposure to anticoagulant rodenticides (ARs) are poorly understood and have yet to be examined in Ontario, Canada. We sought to determine if second-generation anticoagulant rodenticide (SGAR) baiting practices used by pest management professionals targeting commensal rodents may be causing rodenticide spillover into nontarget rodents in urban wildland areas, which could act as a vector of ARs to predators. We used snap traps to capture white-footed mice (Peromyscus leucopus) in urban wildland areas of Toronto and Vaughan, Ontario, near ongoing rodenticide baiting programs utilizing bromadiolone and difethialone products. We detected bromadiolone in only nine individuals and difethialone in two individuals trapped out of a total of 111. These exposed white-footed mice were caught at only five out of seven study sites. AR concentrations were between 0.008 and 0.03 µg/g of liver tissue for bromadiolone and 0.008 and 0.023 µg/g for difethialone, which may be sublethal for raptors. SGAR residues were detected in individuals caught at 0 m, 5 m, 20 m, 40 m, 70 m, and 100 m from active rodenticide stations. SGAR-positive mice did not differ significantly from SGAR-negative mice in terms of sex, age, body condition, distance to the AR source, capture date, or capture site. Four exposed individuals were in "poor" body condition. This indicates that the pest management industry's use of rodenticides in these urban and suburban settings is likely causing some degree of nontarget spillover in Toronto and the Greater Toronto Area. Additionally, nearby residential SGAR usage may have accounted for some of these exposures.