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

Here we report the results of a prevalence study of AR resistance (Vkorc1 gene polymorphism) in populations of brown rat and house mouse in Finland. The brown rat and the house mouse are pest rodent species in the country. The yellow-necked mouse (Apodemus flavicollis), although regarded as a common pest in Finland, was not selected for this study, as pilot sequencing was unable to identify relevant Vkorc1-polymorphism from samples of this species. Rodent control is required as a part of own control in food and feed production sectors in Finland and ARs are commonly used in farms and urban areas. No systematic screening on the prevalence of AR resistance has been conducted in the country before. We collected tissue samples from 96 animals (48 mice and 48 rats) in both farming areas of southwestern Finland (rural environment) and in the cities of Helsinki, Turku and Pori (urban environment) in years 2017-2019. We found evidence for Vkorc1 mutations occurring in both species in Finland.

For mice, 65% of the sampled individuals were found to carry Vkorc1 polymorphism. The majority of positive individuals (27 out of 31) had a type Y139C polymorphism (16 heterozygous and 8 homozygous). Three positive individuals had a type L128S polymorphism (one heterozygous and two homozygous). In addition, one individual was tested positive for both heterozygous Y139C and L128S. Type Y139C confers resistance to FGARs and of the SGARs bromadiolone and difenacoum, and L128S to FGARs and SGARS bromadiolone, difethialone and brodifacoum. The prevalence of a Vkorc1 polymorphism ranged between study sites from 25% (Loimaa) to 100% (Jokioinen, Salo, Turku). Mice harbouring a type Y139C were found in all the sites, whereas type L128S was encountered in one location only (Salo).

In rats, however, only two sampled individuals were tested positive for a Vkorc1 polymorphism and the type found was of a rare one (R33P). Type R33P probably confers resistance to warfarin.

These results suggest that the Vkorc1 polymorphism type Y139C is fairly common in Finnish house mouse populations and this knowledge should be taken into account when planning control actions targeting house mice. The low number of AR-resistant rats found, and the absence of most common Vkorc1 polymorphisms in the sampled individuals, suggests a low prevalence of resistance in rats in Finland. However, 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 (Myodes glareolus), would be greatly needed.