The influence of anticoagulant resistance on effective rodent control in the UK¹

Widespread use of warfarin, and other multiple‐dose anticoagulants, selected populations of warfarin‐resistant rodents in the UK and in other countries. The effectiveness of rodent control was dramatically reduced when the resistant animals formed a large proportion of a population. The second‐generation anticoagulants were developed to overcome these practical control problems with warfarin and were considered to be effective rodenticides against warfarin‐resistant populations. In the UK, however, difenacoum showed reduced efficacy for controlling an established warfarin‐resistant Rattus norvegicus population covering a substantial area of farmland in southern England. Continued use of difenacoum selected for difenacoum‐resistant animals, so that the rodenticide is now ineffective for controlling rats in three counties. Brodifacoum was subsequently used to control some infestations and there is evidence that these treatments selected for animals with an increased level of resistance to brodifacoum. Anticoagulant resistance in Mus domesticus is less of a practical problem because of the availability of non‐anticoagulant rodenticides for the control of this species. There is evidence, however, of bromadiolone resistance in M. domesticus trapped on farms in the UK. Recent investigations of the mechanism of anticoagulant resistance in R. norvegicus have indicated that vitamin K3 (menadione) is an antidote to anticoagulants in resistant animals. This form of the vitamin is included as a dietary supplement in poultry and pig food and would aid the survival, and therefore increase the selection, of resistant animals when anticoagulants are used exclusively.