Anticoagulant resistance is now established in rat and mouse infestations in many countries. Indeed, in some European countries several different resistance mutations (SNPs) are present in both species and are sometimes widely distributed (see also Resistance Maps). This means that those who conduct rodent pest management in these countries frequently encounter resistant rodent infestations in some areas and are required to deal with them effectively. Only if this is done comprehensively, and by a large proportion of practitioners, will the spread of resistance be curtailed. Conversely, if it is not done resistance will continue to spread and will become more severe.
It is important to note that resistance in the Norway rat in most cases is restricted to certain resistance areas or foci of resistance, where the probability is high that rats of a certain resistant strain occur, in particular in habitats where these rats are adapted to, e.g. in relation to certain farming systems. In some countries, these resistance areas are well known (see also Resistance tests and Resistance Maps). In contrast, the appearance of resistant house mice is not linked to known areas. In most cases known so far, the distribution of a resistant strain of the house mouse is connected to the transportation of goods and to the control history in the respective premise. Therefore, options to assess the probability of the presence of resistant mice are mostly limited to the conduct of resistance tests.