Widespread distribution of rodenticide resistance-conferring mutations in the Vkorc1 gene among house mouse populations in Portuguese Macaronesian islands and Iberian Atlantic areas

Growing evidence of widespread resistance to anticoagulant rodenticides (ARs) in house mice pose significant challenges to pest control efforts. First-generation ARs were introduced in the early 1950s but resistance to these emerged later that decade. Second-generation rodenticides were then developed, with resistance being reported in the late 1970s. Research has linked resistance to ARs with mutations in the Vkorc1 gene, leading to the use of more toxic and environmentally harmful compounds. In this study, 243 tail tips of house mice from mainland Portugal and Southern Spain, the Azores and Madeira archipelagos were analysed for all 3 exons of the Vkorc1 gene. Mutations L128S, Y139C, along with the so-called spretus genotype Vkorc1spr are considered responsible for reduced susceptibility of house mice to ARs. All these sequence variants were broadly detected throughout the sampling regions. Vkorc1spr was the most often recorded among mainland populations, whereas Y139C was nearly ubiquitous among the insular populations. In contrast, L128S was only detected in mainland Portugal and four islands of the Azores archipelago. All first generation ARs such as warfarin and coumatetralyl are deemed ineffective against all Vkorc1 variants identified in this study. Second-generation bromadiolone and difenacoum should also be discarded to control populations carrying Vkorc1spr, Y139C or L128S mutations. Inadequate use of ARs in regions where resistant animals have been found in large proportions will result in the spreading of rodenticide resistance among rodent populations through the positive selection of non-susceptible individuals. Consequently, ineffectiveness of rodent control will increase and potentiate environmental contamination, hazarding non-target wildlife through secondary poisoning. We highlight the need for Vkorc1 screening as a crucial tool in rodent management, aiding in the selection of the most appropriate control/eradication method in order to prevent misuse of these toxic biocides and the spread of rodenticide resistance among house mouse populations.