Summary

Resistens hos brune rotter

This report describes the results of a project for monitoring of the distribution of resistance in Danish brown rats (Rattus norvegicus) in 2007. Anticoagulant rodenticides only are allowed for rat control in Denmark. Active ingredients on the Danish market are coumatetralyl, bromadiolone, difenacoum, brodifacoum, difethialone and flocoumafen. 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 sign.

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

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. The basic principle has since then 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 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 meant 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 e.g. a new municipality now covering three former municipalities is in this report treated as three separate units, i.e. the three original municipalities.

During 2007 regions covering 1) Ribe Amt in western Jutland and 2) Vejle Amt in eastern Jutland and 3) the municipalities Copenhagen and Frederiksberg in Zealand have been in focus.

Rats from areas not included in the regions mentioned above have been included in the monitoring programme.

In 2007 a total of 312 rats were received at the laboratory. They had been trapped in 33 (former) municipalities.

Resistance to difenacoum (red colour on the maps) was detected as the highest level in 16 municipalities; resistance to bromadiolone (orange) in 7 municipalities; and resistance to coumatetralyl (yellow) in 1 municipality. These three levels of resistance are of importance to rat control in Denmark. Municipalities with resistance to only warfarin (green colour on the maps) are to be regarded as having no resistance problems because warfarin is not allowed in Denmark.

No resistance was found in rats received from 9 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 2007 has revealed resistance in 5 municipalities not known previously for having resistant rats. Three of these municipalities are situated in western Jutland whereas two others are situated west of Copenhagen in Zealand.