Comment to article: Ruiz-López, María José, Laura Barahona, Josué Martínez-de la Puente, Marta Pepió, Andrea Valsecchi, Victor Peracho, Jordi Figuerola, and Tomas Montalvo. “Widespread resistance to anticoagulant rodenticides in Mus musculus domesticus in the city of Barcelona.” Science of the Total Environment 845 (2022): 157192. – bb0095


What is known about this subject?

House mice are considered to be one of the 100 invasive species of greater ecological and economic impact. House mice are also the most common species of mouse found in urban areas and mainly inhabit indoor spaces, thus, causing more incidences in buildings, business premises and dwellings. Due to their great impact on the environment, public health and economy, different biological, physical and chemical methods have been used over the centuries as an attempt to control rodent populations. At present, anticoagulant rodenticides are the most commonly used pesticides worldwide in the control of this type of murids. These compounds act by inhibiting an enzyme that makes vitamin K available for use in, for example, blood clotting processes. Blockage of this enzyme compromises the coagulation process and the animals die of hemorrhage. Different compounds carry out this function. First anticoagulants came into use in the 1940s, and are known as first-generation anticoagulants. Eventually, cases of resistance to these compounds in mice were reported and second-generation anticoagulant rodenticides were developed. However, despite the effectiveness of these new rodenticides, rodent populations have also developed resistance to them. Studies in different countries have reported resistance to first-and second-generation rodenticides associated with genetic mutations in mice. However, in Spain there was virtually no information on the subject. This study aimed to determine whether resistance does occur in common mouse populations in Barcelona city, in an effort to improve management in the use and effectiveness of biocides.


What does this study add to the existing literature?

Results show that 100% of the studied mice carried mutations associated with resistance to anticoagulant rodenticides. In addition, in more than 90% of mice, mutations were associated with past hybridization events with the Algerian mouse (Mus spretus), widely distributed throughout the Iberian Peninsula. These mutations confer resistance to both first-generation and some second-generation rodenticides, especially bromadiolone. This is particularly relevant since three out of four products marketed in Spain use bromadiolone as active substance.


What are the implications of the findings?

Considering the widespread use of bromadiolone, the main conclusion of this study is that users of this type of rodenticide (citizens, pest control companies and public health authorities) need to be aware that these products are ineffective against mice. In addition, the use of the latter may have important adverse effects on the environment due to the high risk of secondary poisoning for predators of these resistant mice, such as kestrels or owls who inhabit the urban areas of Barcelona. Second-generation anticoagulant rodenticides are very effective in general, though they can easily accumulate, especially in resistant mice who do not die after eating the poison. Predators of resistant mice that have taken rodenticides get affected and suffer from secondary poisoning, with the consequent impact on the health and preservation of these species.

Although the study was conducted in Barcelona, the high frequency of mutations, especially those resulting from past hybridization events with the Algerian mouse, highlights the need for conducting this type of studies in other Spanish locations. This includes the region of Andalusia, where such studies have never been carried out.

In summary, the findings of this study have important implications for public and environmental health, and highlight the need to identify resistances to anticoagulants in rodent populations, especially in home mice, to achieve a rational use of biocides, raise awareness amongst manufacturers and distributors, and stimulate research aimed at exploring effective alternatives in the control of these species.



María Jesús Ruíz López

Doñana Biological Station – CSIC (Spanish National Research Council) and CIBER (Consortium for Biomedical Research) in Epidemiology and Public Health.