Proceedings of the Vertebrate Pest Conference

In the European Union (EU), anticoagulant rodenticides (AR) represent more than 90% of the commercially available products for use against commensal rodents. The only other active ingredients (CO2, chloralose, corn cob) represent minor alternatives. A major issue in the EU is the resistance level of rat and mice populations, as well as potential non-target species exposure. This study presents results of surveys of anticoagulant resistance in Norway rats based on the sequencing of the VKORC1 gene, the major gene involved in AR and an investigation of the presence of AR residues detected in rodents trapped alive in urban and rural areas in order to investigate the potential risk of secondary poisoning of predators and scavengers. For resistance monitoring, rats were either trapped alive in the city of Lyon or its surroundings, or alternatively rat tails were obtained from pest control operators from France. Specific DNA primers were used for DNA sequencing and mutation identifications. AR residues were monitored by LC-MS-MS (for the 8 ARs marketed in Europe), with a limit of quantification of 1.0 µg/kg in liver samples. AR resistance appears to be extremely common (45-70% of all rats tested, depending on the part of France), with the notable exception of downtown Lyon where all rats are susceptible to AR. AR residues are detected in almost 100% of the rats trapped and tested (>200 individuals in/around Lyon). These results show that resistance is common in France, and evidence from neighboring countries suggests that this is an EU-wide problem. More surprising is the fact that all rodents tested contain detectable residues of AR, which could potentially result in secondary poisoning.