Arbejdsrapport fra Miljøstyrelsen 43/2006 – Resistens hos brune rotter - Monitering af resistens hos den brune rotte i Danmark 2003 - 2004

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 2003 and 2004. 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.

Each spring and autumn a region has been brought into focus during a period of about 12 weeks. 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 - in most cases from locations with control problems.

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

During the two-year period, regions covering 1) the southern part of Zealand and the islands south and southwest of Zealand (Storstrøms Amt plus Langeland), 2) the northwestern part of Jutland (Viborg Amt), 3) the eastern part of Zealand (Roskilde Amt), and 4) the northern part of Jutland (Nordjyllands Amt) have been in focus. Before the start of the programme these regions were characterized by many municipalities, from where rats had not been examined since the beginning of 1994.

Rats from other areas outside the regions in Jutland and on Zealand have been included in the monitoring programme.

In 2003 and 2004 a total of 737 rats were received at the laboratory. They had been trapped in 75 municipalities.

Resistance to difenacoum (red colour on the maps) was detected in 17 municipalities and resistance to bromadiolone (orange colour on the maps) in two municipalities. These two levels of resistance are of importance to rat control. Resistance to warfarin (green colour on the maps) was found as the highest level in nine municipalities. These municipalities are to be regarded as having no resistance problems because warfarin is not allowed in Denmark. No resistance was found in 47 municipalities.

Greater areas in the northern and northwestern part of Jutland and the islands south and southwest of Zealand are thus concluded to be without resistance problems. Resistance levels that have to be considered by the pest control operators were found in municipalities in the southern and eastern parts of Zealand and in two areas in central Jutland.