Miljøprojekt Nr. 1183 – Resistens hos brune rotter

Resistens hos brune rotter

Summary

This report describes results of a project for monitoring of the distribution of resistance in Danish brown rats (Rattus norvegicus) in 2006. 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 on the islands of Funen and Zealand.

From 1962 to 1994 rats from locations with control problems were tested for resistance making it possible to draw a map of Denmark indicating municipalities with resistant rats. According to the Environmental Protection Act, the municipalities are solely responsible for carrying out efficient rat control. Therefore, a municipality is the registration unit for occurrence of resistance in rats. Regardless of where in a municipality a resistant rat has been caught, the whole municipality is given the same signature.

In 1994 it was decided to focus 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 is 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 have been 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.

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

During 2006 regions covering 1) Ringkjøbing Amt in western Jutland and 2) Frederiksborg Amt in North Zealand, have been in focus. Before the start of the programme these regions were characterized by many municipalities, from where rats had not been examined or resistance had not been detected since the beginning of 1994.

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

In 2006 a total of 401 rats were received at the laboratory. They had been trapped in 38 municipalities.

Resistance to difenacoum (red colour on the maps) was detected as the highest level in 4 municipalities; resistance to bromadiolone (orange) in 11 municipalities; and resistance to coumatetralyl (yellow) in 5 municipalities. These three levels of resistance are of importance to rat control in Denmark. Municipalities with resistance only to 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 17 municipalities. Rats found resistant to difenacoum have been tested for possible resistance to the more potent anticoagulant rodenticides flocoumafen or difethialone but no rats were found resistant.

The resistance monitoring programme for 2006 has revealed resistance in 15 municipalities not known previously to have resistant rats. The majority of these municipalities are situated in the western part of Jutland.