Development of Resistance to Anticoagulant Rodenticides in Rodents

Control of commensal rodents worldwide relies on the use of rodenticides. In Europe, mostly anticoagulants are available. First generation anticoagulants were developed in the late 1950s’. Their use resulted in the selection of resistant strains of rats and mice. As a consequence, second generation anticoagulants were developed, more potent but also more stable and persistent. These second generation anticoagulants are active after a single feeding and can overcome the resistance of rats and mice. Resistance is a genetic trait related to a single mutation in the Vkorc1 gene. This gene encodes for Vitamin K epoxyde reductase, an enzyme involved in the catalytic cycle of vitamin K. The mutated strains produce an enzyme which is much less susceptible to inhibition by anticoagulants and resistant rats or mice can tolerate much higher amounts of rodenticides than susceptible individuals. Many different mutations have been identified in Europe or worldwide and their relative potency also described. Today, it is possible to determine the potential susceptibility of a rodent strain just by identifying the mutation. There is evidence to support the general idea that mice are less susceptible to anticoagulants than rats, especially in Spain or France where the house mouse crossbred with the Algerian mouse and integrated a mutated portion of the Vkorc1 gene of high potency. Resistance may also be conferred by increased metabolic degradation of rodenticides, but this type of resistance is much less potent and less commonly observed. It has been described in all commensal rodent species. Overall, resistance is present in most areas sampled in Europe and up to 60% of rodents trapped in large surveys can carry a mutation. It is possible to detect 100% mutated individuals in some locations. Resistance monitoring today should be implemented at the reginal/national scale by means of genetic identification of suspected strains, in order to adapt the use of rodenticides locally.