Characteristics of the local distribution of the Y139C resistance gene in Norway rats (Rattus norvegicus) in a focus of resistance in Westphalia, Germany

The Westphalian resistant Norway rat strain is characterised by the possession of the Y139C variant of the vkorc1 gene, and practical resistance occurs in rat infestations at different frequencies to the anticoagulants warfarin, coumatetralyl, bromadiolone and difenacoum. Within the present study we investigated whether there was an obvious pattern in the distribution of resistance in relation to the distance to an identified hot spot of resistance from the site of sampling and whether the frequency of the resistance gene was connected with local conditions, such as rodent control history. Rats were trapped at a single infested site in each of 12 1km x 1km squares in a line including a resistance hot spot. Tissue samples were taken from all trapped rats, and genotyped for the Y139C variant of the vkorc1 gene. The frequency of the resistance gene was determined for each site sampled. Data were also collected about rodent control measures applied in the past and other relevant local conditions. The frequency of the resistance gene varied considerably between < 20% and > 80%. There was no obvious correlation of the frequency of the resistance gene and the distance to the hot spot, and there was no increase or decrease of the gene frequency in west-east direction. Permanent baiting and poor rodent control practice seemed to increase the incidence of resistance in the respective site. The implementation of good rodent control practice is recommended to prevent an increase in the frequency of resistance. The study was funded by the Rodenticide Resistance Action Committee (RRAC) of CropLife International.