Summary

Resistens hos brune rotter - Monitering af resistens hos den brune rotte i Danmark 2001-2008

A nationwide survey of Danish brown rats (Rattus norvegicus) for resistance to anticoagulant rodenticides has been conducted from 2001 to 2008.

Only anticoagulant rodenticides are allowed for chemical rat control in Denmark. Active ingredients on the Danish market are coumatetralyl, bromadiolone, difenacoum, brodifacoum, flocoumafen and difethialone. Ever since the first occurrence of resistance to warfarin was recorded in 1962, resistance to coumatetralyl, bromadiolone and difenacoum has been found in Jutland and in the islands of Funen and Zealand.

From 1962 to 1994 rats from locations with control problems were tested for resistance, and a map of Denmark could be drawn indicating municipalities with resistant rats. According to the Environmental Protection Act, the municipalities are responsible for carrying out efficient rat control. Therefore, a municipality is the registration unit for occurrence of resistance in rats. Regardless of the location in a municipality where a resistant rat has been trapped, the whole municipality is given the same mark.

In 1994 it was decided that areas where resistance might occur because resistance was known in the surrounding municipalities should be brought into focus.

In 2001 the resistance monitoring was intensified and a mapping programme for the whole country with regard to occurrence of resistance in brown rats was launched. Since then the basic principle has been to select a region the size of a Danish county (in Danish: Amt). All municipalities within the region have been requested and encouraged to cooperate in trapping rats.

The number of municipalities was reduced by January 1^st, 2007, because many municipalities merged. The new municipality structure resulted in bigger units than previously. In order to maintain the same structure as in the foregoing years, the previous names and sizes of the municipalities and counties (Amt) have been used in this study. This means that in this report e.g. a new municipality now covering three former municipalities is treated as three separate units, namely the three original municipalities.

Two methods, i.e. a blood clotting response test (BCR) and a feeding test, have been used for testing of the level of resistance in a specific rat.

This report describes and analyses the results obtained during the years and compares the findings with the status of resistance known at the start of the study period.

Rats from 210 municipalities have been tested. The highest level of resistance is resistance to difenacoum which was found in 55 municipalities, to bromadiolone in 37 municipalities, and to coumatetralyl in 10 municipalities. Rats from 108 municipalities were found either non-resistant or with resistance to warfarin which is of no concern in Denmark because warfarin is no longer allowed.

Maps of Denmark showing the highest level of resistance found in each municipality and at each location are shown in the report.

In the majority of municipalities registered for resistance in rats before 2001 resistance has been confirmed at the same or a higher level. Because only anticoagulant rodenticides are used for rat control, further development and spread of resistance is predictable.