Summary

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

This report describes the results of a project for monitoring of the distribution of resistance in Danish brown rats (Rattus norvegicus) in 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 are requested and encouraged to cooperate in trapping rats.

Two different regions are brought into focus during a period of about 12 weeks; one region in the spring and another in the autumn. Rats are trapped by the local rat control operators, preferably at locations where the rat control operator has experienced unsuccessful control.

Other rats are received from locations outside the selected region, especially from locations with control problems.

The rats are brought to the laboratory while alive to be tested for resistance against anticoagulant rodenticides. Two methods, i.e. a blood clotting response test (BCR) and a feeding test, are used for testing of the level of resistance in a specific rat.

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.

During the spring season of 2008 regions covering 1) Københavns Amt in Zealand and 2) the island of Bornholm have been in focus.

Rats from areas outside the regions mentioned above have been included in the monitoring programme.

In 2008 a total of 262 rats were received at the laboratory. They had been trapped in 18 (former) municipalities.

Resistance to difenacoum (red colour on the maps) was detected as the highest level in 3 municipalities; resistance to bromadiolone (orange) in 1 municipality; and resistance to coumatetralyl (yellow) in 1 municipality.

No resistance was found in rats received from 13 (former) municipalities. Rats found resistant to difenacoum have been tested for possible resistance to the more potent anticoagulant rodenticides brodifacoum, flocoumafen or difethialone but no rats were found resistant.

The resistance monitoring programme for 2008 has revealed resistance in 2 municipalities not previously known for having resistant rats.