Richardson, L. F. | bioRxiv Science

bioRxiv2026

Environmental Correlates for Anticoagulant Resistance in house mice Mus musculus

Genetic resistance to anticoagulant rodenticides is a world-wide evolutionary phenomenon among invasive commensal rodents, that has not been investigated in Canada. We sequenced exon 3 of the VKORC1 gene of house mouse Mus musculus samples obtained from pest management professionals in Ontario and Quebec, to search for mutations known to confer resistance. Sanger sequencing was used alongside a novel qPCR assay for codons 128 and 139. We detected high prevalence (99%) of two non-synonymous single nucleotide polymorphisms in house mice, previously found in the USA and Europe, known to cause extreme resistance to first and some second-generation anticoagulants (L128S & Y139C). Homozygous resistant mice were more common in high population density areas. L128S alleles were more common in Southwestern Ontario, and Y139C in Central Ontario, despite high linkage disequilibrium. Detection success was far greater with qPCR than with Sanger sequencing. We conclude that uncoordinated rodenticide usage has selected for extreme resistance in mice throughout Ontario. Therefore, chemical control of the house mouse may be ineffective with first-generation anticoagulants throughout Ontario. This suggests that the evolution of wild urban mice is influenced by pest management practices, which may vary by region within the province.

evolutionary biology↗

bioRxiv2026

Exposure of non-target white-footed mice (Peromyscus leucopus) to Second-generation Anticoagulant Rodenticides in an urban context

The pathways of non-target exposure to anticoagulant rodenticides (ARs) are poorly understood, and have yet to be examined in Ontario, Canada. The spillover of ARs into non-target rodents and high-risk landscapes has been investigated numerous times, but usually in agricultural regions as opposed to urban ones. 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. Our goal was to determine if second-generation anticoagulant (SGAR) baiting practices used by pest management professionals targeting commensal rodents may be causing rodenticide spillover into non-target rodents in urban wildland areas, which could act as a vector of ARs to predators. Only 11 out of 111 mature white-footed mice trapped near ongoing urban rodenticide operations tested positive for an anticoagulant, at five out of seven study sites. Concentrations were between 0.008-0.03 ppm, which may be sublethal for raptors. We did not detect brodifacoum, despite its detection in a recent study on Ontario raptors. Exposed individuals were caught at 0m, 5m, 20m, 40m, 70m and 100m from active rodenticide stations. They did not differ from unexposed individuals in terms of sex, age, body condition, distance to the AR source, capture date or capture site. This indicates that the pest management industrys use of rodenticides in urban and suburban settings is causing some degree of non-target spillover in Toronto and the Greater Toronto Area, and that SGAR usage should be avoided near naturalized landscapes.

ecology↗