Rodent resistance to the anticoagulant rodenticides, with particular reference to Denmark.

Inherited resistance to anticoagulant rodenticides was discovered in populations of Rattus norvegicus about 14 years ago. Similar resistance has now been reported from several countries in north-western Europe and from the USA. In order to detect resistance and to control it effectively, basic data on the susceptibility of rat populations are required for each country, and trapping surveys should be made in any area where resistance is suspected. Acute poisons are needed to control resistant rats although the shift from anticoagulants to acute poisons is a retrograde step as far as efficiency is concerned, and increases the hazard of control operations to man and other animals. Resistance to anticoagulants in Mus musculus has been reported from England, and resistant mice are probably to be found in other countries also in view of the great individual variation in susceptibility of this species to these rodenticides.