A Rutgers study found known resistance mutations in nearly 70% of sampled house mice from four Northeastern urban areas.
NEW BRUNSWICK, N.J. — Many house mice living in cities across the Northeast carry genetic mutations known to help them survive commonly used rodent poisons, according to new research from Rutgers University.
The findings offer a possible explanation for reports from pest management workers who say some urban rodent infestations have become harder to control. The researchers found that 84% of the house mice they tested carried at least one change in a gene associated with resistance to anticoagulant rodenticides. Nearly 70% carried mutations already known to reduce the effectiveness of those poisons.
The study examined DNA from 147 house mice and 143 Norway rats collected in urban areas of New York, New Jersey, Pennsylvania and Washington, D.C. Researchers focused on a gene called Vkorc1, which affects how rodents respond to anticoagulant poisons widely used in homes, businesses and public spaces.
About 35% of the Norway rats also carried at least one mutation in the gene. However, researchers cautioned that most of the mutations found in the rats have not been proven to make the animals resistant. The results therefore provide stronger evidence of resistance among house mice than among rats.
“We found that resistance appears to be much more widespread in house mice than many people realized,” said Jin-Jia Yu, a postdoctoral fellow in the Department of Entomology at the Rutgers School of Environmental and Biological Sciences and the study’s first author.
Yu said scientists still do not know whether most of the genetic changes found in Norway rats affect their response to rodenticides. The team also identified several previously undocumented variants in both species. More laboratory work will be needed to determine whether those variants help rodents survive poison exposure.
Anticoagulant rodenticides interfere with blood clotting. Rodents that carry helpful mutations may be more likely to survive exposure, reproduce and pass those traits to later generations. Decades of repeated poison use could therefore favor resistant animals within heavily treated populations.
The researchers said differences in mouse and rat behavior may help explain why known resistance mutations were more common in mice. House mice are generally more willing to investigate unfamiliar food, including bait. Rats tend to be cautious around new objects and may avoid bait or traps before consuming enough poison to be affected.
The findings do not mean every mouse or rat in the Northeast is resistant. The samples came from selected urban locations, and the presence of a mutation does not always prove that a particular animal will survive treatment. Still, the results show that genetic traits linked to resistance are established in several major metropolitan areas.
Researchers said mapping those traits could help pest management professionals choose more effective control methods and avoid repeated use of products that may no longer work well in a particular location. Additional testing is needed to determine how resistance differs among neighborhoods and which rodenticides remain effective against local populations.
Author note: Last updated July 24, 2026.