VKORC1 mutations in rodent populations of a tropical city-state as an indicator of anticoagulant rodenticide resistance

A survey of VKORC1 missense mutations in eleven Italian islands reveals widespread rodenticide resistance in house mice

To protect native wildlife, more than one hundred rodent eradications have been attempted in the Mediterranean islands by using anticoagulant rodenticides (ARs). Despite their high efficiency, resistance to ARs has been observed in many countries and it is mostly related to missense mutations (SNPs) in the VKORC1 gene. The presence of resistant individuals reduces the efficiency of rodent management, leading to an excessive use of ARs. Thus, the risk of poisoning in non-target species increases. In this study, the first survey of ARs resistance in the house mouse Mus domesticus covering multiple islands in the Mediterranean was performed. Tissue samples of eighty-two mice from eleven islands in Italy were analysed and eight missense SNPs were found. In addition to some well-known missense mutations, such as Tyr139Cys, six new missense SNPs for the house mouse were discovered, four of which were new even for any rodent species. Furthermore, the frequency of Tyr139Cys significantly increased in Ventotene Island after a four-year long rat eradication. This could be due to the selective pressure of ARs that allowed the mice carrying the mutation to survive. This study demonstrates once again the importance of assessing resistance to ARs before undertaking rodent eradications. Indeed, this would allow an informed decision of the most effective AR to use, maximizing the success rate of the eradications and minimizing secondary poisoning and other deleterious effects for non-target species and the environment.

Exposure and resistance to anticoagulant rodenticides in invasive and endemic Chadian urban rodent species to develop a rational management strategy

Rodent management involves the use of anticoagulant rodenticides (ARs). This use has resulted in the selection of numerous resistance alleles in the Vkorc1 gene, encoding the target enzyme of ARs. In Africa, although rodents are a major problem as a consequence of their transport and transmission of zoonotic pathogens, and damage to crops, the use of ARs and the spread of resistance alleles are poorly documented. We attempted to address both issues in Chad which is one of the largest countries in Africa. Owing to its location at the crossroads of central and northern Africa, Chad is representative of many African countries.

METHODS

Using a sampling of nearly 300 rodents composed of invasive and endemic rodents collected in six of Chad's largest cities, exposure to ARs was analyzed by their quantification in the liver; the spread of AR resistance alleles was analyzed by Vkorc1 sequencing.

RESULTS

We demonstrate the use of both ARs generations in Chadian cities and report the total sequencing of the Vkorc1 for 44 Mastomys natalensis with detection of two different haplotypes, the sequencing of the Vkorc1 for two other endemic rodent species, M. kollmannspergeri and Arvicanthis niloticus, and finally the detection of three new missense mutations - V29E, V69E and D127V - in R. rattus, potentially associated with resistance to ARs.

DISCUSSION

These results should argue for the implementation of a reasoned management of rodent populations in Africa to avoid the spread of ARs resistance alleles. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

No evidence of mutations associated with anticoagulant resistance in gene Vkorc1 in brown and black rats from Barcelona

Synanthropic rodents such as the brown rat (Rattus norvegicus) and black rat (Rattus rattus) are a source of disturbance in urban areas and the focus of control programs. Control measures often rely on anticoagulant rodenticides, but their broad use is compromised by the emergence of resistance. Here we studied the prevalence of anticoagulant resistance genotypes in the Vkorc1 gene in rats in the metropolitan area of Barcelona. In this area, part of the management practices to control brown rats include anticoagulant rodenticide use, but rodenticides with different active ingredients are used in rotation. Brown rats were sampled from the sewage system during two periods: from December 2016 to November 2017 when difenacoum and brodifacoum were used, and from August 2021 to July 2022 when bromadiolone was used. Because black rats have just recently been detected in Barcelona, we only studied them during the latter sampling period, with samples obtained from a control action carried out in a green urban area. Exon 3 of the Vkorc1 gene was characterized in both species, while exon 1 was additionally analyzed in black rats. Synonymous mutations, not resulting in amino-acid changes, were found in both exons, indicating no evidence of anticoagulant resistance in the rats of Barcelona. This finding indicates that the current rodent management plan in Barcelona, which involves anticoagulant rotation for brown rats and the use of life capture traps in specific situations for black rats, has helped to prevent the emergence of resistance to anticoagulant rodenticides in rats in Barcelona. Future actions should aim to diversify the control measures included in the rodent management plan.