Anticoagulant resistance in the UK and a new guideline for the management of resistant infestations of Norway rats (Rattus norvegicus Berk.)

Anticoagulant resistance was first discovered in UK Norway rats (Rattus norvegicus Berk.) in 1958 and has been present ever since. The possible detrimental impact of resistance on effective rodent control was quickly recognised and, for almost three decades, extensive research was conducted on the geographical distribution and severity of anticoagulant resistance in UK rats. Various schemes for the eradication of resistant rats were also implemented. At first, surveys showed resistance only to the first-generation anticoagulants, such as warfarin, chlorophacinone and coumatetralyl, but later resistance to the more potent second-generation anticoagulants, such as difenacoum and bromadiolone, was also discovered. Unlike some European countries, where only one or two resistance mutations occur, virtually all known rat resistance mutations occur in the UK and five are known to have significant impacts on anticoagulant efficacy. Little is currently known of the geographical extent of anticoagulant resistance among Norway rats in the UK because no comprehensive survey has been conducted recently. At an operational level, anticoagulants generally retain their utility for Norway rat control but it is virtually impossible to control resistant rats in some areas because of restrictions on the use of the more potent resistance-breaking compounds. This paper describes the development of resistance in Norway rats in the UK, outlines the present situation for resistance management and introduces a new resistance management guideline from the UK Rodenticide Resistance Action Group (RRAG, 2010)