Resistance tests and field trials with bromadiolone for the control of Norway rats (Rattus norvegicus) on farms in Westphalia, Germany.

The aim of this study was to determine the incidence and the level of resistance to bromadiolone among rats on farms suspected of being foci of resistance by using the international normalised ratio (INR)-based blood clotting response (BCR) test. Whether the level of reduced susceptibility constitutes 'practical resistance' was subsequently determined in field trials. The 2.5 multiple of the ED(50) baseline was used to test for the incidence of resistance, and higher multiples in the range of the suspected resistance factor were used to investigate the degree of resistance. The ED(50) values of bromadiolone in resistant rats were confirmed in the range 4.70-7.05 mg kg(-1) for males and 4.62-6.61 mg kg(-1) for females. Variations within these ranges appeared between farms. According to the BCR resistance tests, 50-100% of rats were classified as resistant prior to the field trials; 29-100% of rats survived the treatments. BCR tests based on the use of the INR and baselines are suitable for determining the incidence and for assessing the level of resistance in populations of Norway rats. The majority of rats of the Westphalian resistant strain, characterised by the Y139C marker in VKOR, are resistant to bromadiolone under practical control conditions.