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By Jasmine Laws
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Rats and mice continue to be a stubborn cause of public-health problems in U.S. cities, spreading disease as they thrive in aging buildings, subway tunnels and sewers. However, new research suggests some urban rodents may not only be surviving despite repeated control efforts, but they might also be genetically adapting to them.
A new study by Rutgers University found that nearly 70 percent of the mice examined had mutations that made them harder to control using common rodent poisons on the market, study author Changlu Wang, who is an extension specialist in entomology at the university, told Newsweek.
Wang said that this prevalence of resistance is "a result of repeated exposure to rodent poisons over many years," and that while rat samples contained "some mutations," they are not known to have the same resistance as mice, and can still be killed with commonly used rodent poisons.
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The findings add urgency to an already difficult problem for many U.S. cities, and Wang warned that this resistance is "likely to increase further as people continue to use rodent poisons."
Not only can rodents pose harm to human health, but they can also cause significant damage to property, crops, and food supplies across the U.S., according to the U.S. Environmental Protection Agency (EPA).
What Does The Rutgers Report Say?
The Rutgers University-led study published in Pest Management Science in April this year found evidence that certain urban rodents in the Northeast carry genetic mutations associated with resistance to anticoagulant rodenticides, the chemicals most commonly used in rat and mouse control. Rodenticides are a type of pesticide.
Researchers analyzed DNA from 147 house mice and 143 Norway rats collected from urban areas in New York, New Jersey, Pennsylvania and Washington, D.C., focusing on identifying mutations in the Vkorc1 gene, which are linked to survival against anticoagulant poisons.
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The results were especially striking for house mice: 84 percent carried at least one Vkorc1 mutation, while nearly 70 percent had mutations already known to help mice survive common rodenticides.
About 35 percent of Norway rats also carried mutations in the same gene, though the researchers said they do not yet know whether those mutations directly reduce vulnerability to rodenticides.
Other studies, including a 2005 paper published in the journal Genetics, have found similar results, though with stronger results for rats.
Where Are The Most Rat-Infested Neighborhoods In New York? A Map
In 2024, Newsweek published a map revealing where New York's estimated 3 million rats were living.
At the time of reporting, several of these places have not received treatment from the NYC Health Department for almost 10 years, with one property on East 45 Street not visited since 2015.
Now, after some reforms, city officials report roughly 15 percent declines in rat sightings and complaints, though experts caution that these metrics don’t necessarily reflect the total population size.
Why Are Mice Becoming Resistant To Pesticides?
Megan Phifer-Rixey, one of the study authors and a professor at Drexel University, told Newsweek that rodenticide resistance increases as populations of mice able to "tolerate rodenticides" also rises, given that they are "much better able to survive and reproduce compared to mice that can't tolerate them."
"Over time, then, more and more of the mice will have those mutations and our rodenticides will stop working effectively," she said.
Anticoagulant rodenticides work by disrupting the vitamin K cycle, preventing blood from clotting and eventually causing fatal internal bleeding, meaning that rodenticide resistance in this case "comes from changes in genes that affect blood clotting," Phifer-Rixey said.
While there is not enough data to determine if mutations are increasing, Phifer-Rixey said, "We can say that they are common and that is concerning because it suggests that our methods for pest management need to change."
History of Rodent Pesticide
Anticoagulant rodenticides began to be used widely in the early 1950s, replacing many faster-acting poisons, though resistance emerged relatively quickly, with cases observed in 1958. The majority of reports came from Europe but were also documented in the U.S., Canada, Japan, and Australia.
In the 1970s and 1980s, as anticoagulant resistance led to significant reductions in the efficacy of pest control, more potent anticoagulant compounds were developed, but again resistance was quickly recorded, as early as 1981.
Pesticide Effects on Pets, Humans, Birds, Wildlife
Concern around the impact of these chemicals on pets, humans and other wildlife then began to amplify as well. Phifer-Rixey said that rodenticides can "impact wildlife that we are working to conserve (like birds that eat mice) and can affect pets and other non-target animals."
Research has found that first-generation anticoagulants led to secondary poisoning on non-target wildlife, while the high-potency second-generation chemicals could bioaccumulate in the liver tissues of a wide range of species, given that they have a prolonged half-life.
The consequences of this include mortality, impacts on species' reproduction and behavior, as well as broader disruption to ecosystem balance, according to a Nature Index review of five studies between 2018 and 2024.
The EPA has also worked to reduce the risks of these chemicals to humans and non-target organisms, finding ways of decreasing non-target exposure.
How To Get Rid Of Rats
The researchers concluded that the findings suggest that different types of rodenticides should be used in pest control, and that reliance on anticoagulant rodenticides should be reduced.
Wang said that the findings are "concerning," as without "effective rodenticides, it will be more challenging to control large scale infestations."
"Additionally, presence of resistance will require higher dose or more frequent use of chemicals, which causes greater environmental impact, especially to non-target wildlife and predator birds," he added.
Experts say the most effective response is not simply a stronger poison, given that second-generation anticoagulants are harmful to the wider environment, but incorporating a wider range of control measures.
Phifer-Rixey said that managing rat and mouse populations is "very challenging, and strategies are different for individual residents versus large property managers versus urban city managers."
Wang said people should be educated about pesticide resistance and their non-target effects on wildlife. He added they should focus on prevention measures like sealing buildings well and repairing defects, while removing garbage and other food sources for rodents.
The Centers for Disease Control and Prevention (CDC) also advises that "it's easier to control rodents before they can establish themselves in your home or building," and that Americans should "regularly check your home for rodent droppings and gnaw marks as these are both signs of a rodent's presence."
For pest control professionals, Wang said that they should "carefully observe the effectiveness of poisons and other control methods and adjust their strategies," and "rotate among different classes of rodenticides, incorporating non-chemical methods and rely less on chemicals will be needed."
Phifer-Rixey added that continuing to invest in scientific studies is also "critical."
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