Rats, residues and rodenticide resistance: Hepatic concentration of anticoagulant rodenticides in genetically susceptible wild brown and black rats

Invasive rats are major pests worldwide, posing economic and public health risks. Since the 1950s, efforts have been made to control or eradicate these animals by using anticoagulant rodenticides (ARs). Initially effective, ARs quickly lost efficacy due to the emergence of resistance-associated mutations in the Vkorc1 gene. When consumed in (sub-)lethal doses, these biocides bioaccumulate in the liver, becoming a risk for non-target predators. First, this study aims to assess genetic resistance to ARs in two synanthropic rat species, Rattus rattus and Rattus norvegicus, from port urban areas in Lisbon (mainland Portugal) and Ponta Delgada (São Miguel Island, Azores, Portugal). Secondly, we aim to detect and quantify the hepatic concentration of first- (FGARs) and second-generation anticoagulants (SGARs). We analysed 203 sequences of exon 3 of the Vkorc1 gene and found no known resistance-conferring mutations in either species or locations. A subsample of 177 liver tissues from genotyped rats was examined for AR residues via Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS). No FGARs were detected, but five SGARs were identified with 80.4% of individuals accumulating at least one of them. The absence of genetically mediated resistance via exon 3 of Vkorc1 mutations does not necessarily mean that the risk of AR exposure to rat predators is low, as 16.0% of the live-trapped rats bioaccumulated high levels of ARs, above the 100 ng/g toxicosis threshold. This finding underscores significant ecological risks, particularly for non-target wildlife apex-predators, that are more susceptible to AR exposure and may bioaccumulate lethal concentrations through repeated consumption of contaminated prey.