Distribution of anticoagulant resistance in the brown rat in Belgium

Anticoagulant resistance is known as one of the major factors interfering with rodent control. Within this context we investigated the distribution of anticoagulant resistance in Flanders, northern Belgium. From 2003 to 2005, we tested 691 rats from different locations with blood clotting response tests for their susceptibility to the anticoagulant compounds warfarin, bromadiolone and difenacoum. Of these, 119 were also screened for a mutation in the VKORC1 gene that is suspected to be responsible for anticoagulant resistance. Warfarin resistant rats were found in the western and eastern parts of Flanders. The same distribution pattern was found for bromadiolone with the exception of the south-eastern area, where this form of resistance was largely absent. We detected difenacoum resistance in only six rats and did not observe any resistant rats in the central part of Flanders. Susceptible rats were found all over Flanders. Genetic analyses showed that anticoagulant resistance in Belgium was related to two different mutations in VKORC1, namely Y139F and L120Q. Our results indicate that rodent control should be regionally tailored to be most effective.