Brodifacoum is effective against Norway rats (Rattus norvegicus) in a tyrosine139cysteine focus of anticoagulant resistance in Westphalia, Germany.

The tyrosine to cysteine amino acid substitution at location 139 of the vkorc1 protein (i.e. tyrosine139cysteine or Y139C) is the most widespread anticoagulant resistance mutation in Norway rats (Rattus norvegicus Berk.) in Europe. Field trials were conducted to determine the incidence of the Y139C mutation at two rat-infested farms in Westphalia, Germany, and to estimate the practical efficacy against them of applications, using a pulsed baiting treatment regime, of a proprietary bait (Klerat™) containing 0.005% brodifacoum. DNA analysis for the Y139C mutation showed that resistant rats were prevalent at the two farms, with an incidence of 80.0 and 78.6% respectively. Applications of brodifacoum bait achieved results of 99.2 and 100.0% control at the two farms, when measured by census baiting, although the treatment was somewhat prolonged at one site, possibly owing to the abundance of attractive alternative food. The study showed that 0.005% brodifacoum bait is fully effective against Norway rats possessing the Y139C mutation at the Münsterland focus and is likely to be so elsewhere in Europe where this mutation is found. The pulsed baiting regime reduced to relatively low levels the quantity of bait required to control these two substantial resistant Norway rat infestations. Previous studies had shown much larger quantities of bromadiolone and difenacoum baits used in largely ineffective treatments against Y139C resistant rats in the Münsterland. These results should be considered when making decisions about the use of anticoagulants against resistant Norway rats and their potential environmental impacts.