Proceedings of the Vertebrate Pest Conference

Warfarin resistance in the Norway rat (Rattus norvegicus) has been studied over the past 30 years. To determine the status of this resistance phenomenon wild Norway rats were collected from Colorado and Chicago, Illinois. As reported previously, warfarin resistance in the Chicago area exceeds 50%, while rats from Colorado remain very susceptible to warfarin. The theory that true genetic resistance may not exist was examined, implying that geographic variation in intestinal flora contribute to the rapid degradation of warfarin after ingestion, along with production of sufficient Vitamin K in the bacteria to reverse the effect of warfarin. Antibiotics in combination were tested with warfarin and demonstrated that efficacy in the laboratory can be increased by using the combination in a bait form.