The so-called “superrats” are already in Madrid. The term refers not so much to size, but to what a new genetic study has confirmed: that part of the capital’s rat population has become resistant to the most commonly used rodenticides. The discovery, which includes the first-ever detection of rats with a double genetic mutation in the city, necessitates a reevaluation of rat control strategies and is causing concern among both pest control experts and health authorities.
“Superrats” are not a new species, but rather brown sewer rats (Rattus norvegicus) that have developed mutations in the Vkorc1 gene, which is key to the effectiveness of anticoagulant rodenticides. These poisons work by causing internal bleeding; the mutations alter the enzyme that regulates vitamin K, allowing the animals to metabolize the toxin without dying. The result is a rodent population that survives doses that were previously lethal and passes this advantage on to its offspring, accelerating the spread of the problem.
In Madrid , two main mutations have been identified in exon 3 of Vkorc1, designated S149I and E155K, as well as individuals carrying both simultaneously. These variants had already been described in other European populations, but the double combination in Spanish brown rats is documented for the first time in the capital.
The study that has set off alarm bells
The research that has sounded the alarm was conducted using 120 fecal samples collected from 21 one-square-kilometer quadrants , distributed across 17 neighborhoods in the urban center of Madrid. Scientists used these samples to extract DNA and search for mutations in the Vkorc1 gene, thereby mapping genetic resistance within the city. The results show that the presence of resistant rats is not uniform, but rather concentrated in certain areas depending on the level of poison exposure and environmental conditions.
At the neighborhood level, the study highlights that in El Viso, the frequency of the E155K mutation reaches 50%, while in the Sol area, up to 40% of the specimens analyzed exhibit the double S149I+E155K mutation. When broken down by district, Centro leads in total mutation prevalence (around 17.7%), followed by Chamartín (15%) and Arganzuela (12.6%). The authors interpret these data as reflecting different “selective pressure scenarios”: where more pesticide has been used and more residues accumulate, it is precisely the most resistant individuals that are more likely to survive.
An accelerated evolution right under our feet
The case in Madrid is part of a broader phenomenon: studies by the INIA and other research centers have detected rats resistant to anticoagulants in at least twelve autonomous communities. In some areas, around 20% of the sewer rats analyzed already exhibited the S149I mutation, while in black rat (Rattus rattus) populations, up to four different mutations have been found, with frequencies exceeding 30% in certain areas. In practice, this means that a growing proportion of these animals no longer respond as well to compounds such as warfarin, bromadiolone, or difenacoum—rodenticides commonly used in cities and agricultural operations.
Experts point out that rodents have very short life cycles and high reproduction rates, which accelerates the selection of advantageous mutations. If a product ceases to kill a portion of the population and continues to be used, that same pressure favors the survival and reproduction of resistant individuals, leading within a few years to a generation of “superrats” that are practically immune to conventional formulas.
What this means for public health and the city
Beyond the shocking image, the rise in resistant rats is a public health problem. These animals can transmit diseases such as leptospirosis, toxoplasmosis, or hantavirus, as well as act as vectors for other pathogens through fleas and mites. If their population grows and they become harder to control, the likelihood of contact with humans and domestic animals increases, especially in areas with high population density and poor waste management.
In Madrid, associations of environmental health companies warned years ago of the spread of the black rat, which went from about a dozen controlled outbreaks in 2019 to more than 30 detected sites in 2021. This species lives in tree canopies, rooftops, and false ceilings, complementing the traditional sewer rat that inhabits basements, drainage channels, and sewers. The coexistence of more rodents, with greater species diversity and, now, resistant mutations, further complicates the picture of Madrid’s underground ecosystem.
What experts are calling for: less poison, more prevention
Given this situation, pest control specialists insist that the only effective response involves combining science, urban hygiene, and regulatory changes. First, they call for reducing reliance on anticoagulants and adopting integrated programs that include mechanical traps, physical barriers, sealing entry points, and , above all, much stricter waste management. Wherever there are trash bags on the street, overflowing dumpsters, or food left behind in parks and squares, there is an all-you-can-eat buffet that drives up the rodent population and reinforces selective pressure in favor of “superrats.”
They are also calling for greater coordination between city councils, autonomous communities, and the central government to update rodent control protocols and harmonize the use of rodenticides, limiting their indiscriminate application.
