Rapportage landelijk beeld resistentie tegen rodenticiden bij bruine ratten en huismuizen

In this report the results of the national study into rodenticide resistance in house mice and rats in the Netherlands are described. Between September and December 2021 a total of 2318 rodents were collected with the help of professional pest managers from all over the country. A total of 218 different postal code areas was sampled. This sample size provides a good and realistic overview of the current situation. There were 397 house mice (Mus musculus) collected (and 286 mice from different species), 1553 brown rats (Rattus norvegicus), and 82 black rats (Rattus rattus). For the house mouse the test on Vkorc1 mutations at amino acid 139 resulted in 246 wild-type (62%), and 151 mice showed resistant (38%). For the brown rat 1476 of the 1553 samples gave a reaction. Of those 1476 rats, 1224 tested wild-type (83%) and 252 rats showed resistant (17%). In comparison to the study in 2012/2013, the percentage of resistant brown rats is lower (25% in 2012), but lies within the same range. Although the sample size and geographic spread of the rats tested in the previous study was somewhat limited, the new numbers could indicate a positive trend which might be due to more restricted and conscious use of rodenticides. Rodenticide resistance was found in a large part of the same regions as in the previous study, but also a few ‘new’ regions. This could be due to the fact that resistant rodents have spread, or/also because there were more samples collected with a larger geographic spread than in the earlier study. However, the 17% resistant brown rats is an average for the Netherlands, with clear differences between regions. Despite the fact that the numbers of 2012 were lower with a less balanced geographic sampling, comparison of the results could indicate a positive trend of a decrease in rodenticide resistance in rats. Results of the house mouse are not comparable to previous studies as there are none. Nonetheless the number of rodenticide resistant mice in the Netherlands seems higher than for the rats. It is recommended to continue the monitoring, also for the house mouse. This way, the effect of the IPM approach with less rodenticide use and when used, with more conscious application, can be seen in a hopefully reduced development of rodenticide resistance. There was also held a survey amongst 91 professional rodent managers. Over 24% of them states to work without rodenticides. In case of rodenticide use, products with bromadiolone as effective compound are used most often. In the literature it is shown that brown rats with a heterozygous mutation prevailing in France (tyr-phe) are insensitive to coumatetralyl and bromadialone, but still sensitive to difenacoum, brodifacoum, flocoumafen, and difethialone. For the house mouse it is for several mutations known what compounds are still effective or not. However, only one of the found mutations from this study (tyr-cys) is in that list. This means that house mice with a tyr-cys mutation are no longer susceptible to bromadialone, and in some cases sensitive to difethialone. House mice with this mutation are still sensitive to brodifacoum, flocoumafen, and difethialone.