Resistance and Ecotoxicology

There is published scientific evidence that anticoagulant rodenticides have the potential to cause harm to the environment; mostly by the primary and secondary poisoning of non-target wildlife. Primary poisoning happens when a non-target animal accidentally consumes bait put out for target rodents. Secondary poisoning occurs when, having consumed bait, either target or non-target animals are themselves taken as food by scavenging and predatory animals. It is generally considered that the use of the first-generation anticoagulants entails less risk to the environment than the use of the second-generation compounds. This is because the former compounds are both less acutely toxic and less persistent. But that is not to say that the first-generation anticoagulants are without risk.

As first-generation anticoagulants carry less risk to non-targets, it is sensible to use these compounds, instead of second-generation anticoagulants, where they are known to be fully effective. When resistance to any active substance occurs, either a first- or second-generation active substance, their use should be replaced either by the use of alternative methods of rodent control or by the use of anticoagulants that are fully effective. The continued use of ineffective anticoagulants in areas of resistance poses unacceptable risks to non-targets. There is evidence that target rodents in areas of anticoagulant resistance carry a higher residue burden of anticoagulant active substances than they do in areas where there is no resistance.

It is not, however, recommended that practitioners should repeatedly use the same active substances, even in areas where they are fully effective. This is particularly the case for the use of the first-generation compounds because their frequent and repeated use may lead to the development of resistance. Therefore, it is sensible occasionally to use more potent products containing brodifacoum, difethialone and flocoumafen in areas where first-generation anticoagulants, and the less potent second-generation anticoagulants bromadiolone and difenacoum, are effective in spite of increased risk to the environment. This action will serve to prevent resistance development and preserve the effectiveness of the first-generation and less potent second-generation active substances.

We need more accurate information on the geographical distribution of anticoagulant resistance in all countries to permit science-based decisions to be made so that anticoagulants can be used that are both fully effective and pose the least risk to non-target animals.