Finnish rats are not resistant to rodenticides

A new study led by Finnish Safety and Chemical Agency and Natural Resources Institute Finland shows anticoagulant resistance in house mice but not in rats.

In the first systematic screening on the prevalence of rodenticide resistance 48 mice and 48 rats in both farming areas of

southwestern Finland and in the cities of Helsinki, Turku and Pori were genetically tested. While 65% of the mice showed a potential for anticoagulant resistance, only one rat 

The most common method for rodent control worldwide is the use of anticoagulant rodenticides, which block the vitamin K cycle and thus cause death by haemorrhage. First-generation anticoagulant rodenticides were introduced into the pest control already in the 1940s and some of them are still in use. Second-generation anticoagulant rodenticides, which are toxic at a lower dose, were developed after rodents began to display resistance to first generation agents. As anticoagulant rodenticides are an easy and cost-effective way to control rodents, their use is widespread. Consequently, several resistant strains of rodents have emerged, especially in the brown rat  and the house mouse.

The resistant strains have evolved a modification of the VKORC1 enzyme involved in the catalytic recycling of vitamin K. Polymorphism in the Vkorc1 gene can be identified by genetic analyses from DNA extracted from the tissue samples of rodents. The majority of positive mice (27 out of 31) had a type Y139C polymorphism which confers resistance to first-generation anticoagulants and some second-generation anticoagulants, whereas three positive individuals had a type L128S polymorphism which confers resistance to first-generation antcoagulants and a number of second-generation anticoagulants. In rats, however, only two sampled individuals were tested positive and the type found was of a rare one probably conferring resistance to warfarin.

More studies are needed to gain a better picture about the prevalence of Vkorc1 types in Finnish rat populations. In addition, to fully understand the state of resistance in Finland, especially studies on effectiveness and resistance performed on yellow-necked mouse and potentially also bank voles, would be greatly needed.

Link to the full report