A group of wet rats emerging from a drainage pipe in a dark, abandoned setting, showcasing their fur and distinct features.

A group of wet rats emerging from a drainage pipe in a dark, abandoned setting, showcasing their fur and distinct features.
Credit: Grok AI

Rats and mice in major American cities are developing genetic mutations that make them harder to kill with common poisons, according to new research from Rutgers University.

Scientists examined nearly 300 rodents from New York, New Jersey, Pennsylvania and Washington, D.C., and found widespread signs of resistance to widely used exterminator chemicals.

Around five out of every six rodents tested carried at least one mutation linked to reduced sensitivity to poison.

More than two-thirds also had additional genetic changes previously tied to resistance against common rodenticides.

The mutations were especially common in house mice, which researchers said appear to be adapting faster than larger brown rats, also known as sewer rats.

“Genetic mutation is not that special in these creatures,” lead researcher Jin-Jia Yu said. “But we found that the house mouse shows a lot of genetic mutations related to rodenticide resistance.”

Researchers began investigating after pest control companies reported that standard anticoagulant rodenticides were becoming less effective in urban infestations.

Those poisons work by stopping blood from clotting, causing rodents to die from internal bleeding.

Brown rats showed fewer resistance mutations than house mice, possibly because they are more cautious around unfamiliar food and traps.

The findings come as several major U.S. cities continue to struggle with rodent problems.

According to Census Bureau data cited by researchers, a staggering 29 percent of households in Philadelphia have reported seeing rats in their homes.

In Manhattan, 15 percent of residents have also reported rat problems, while one in five Washington, D.C., residents said the same.

Rodents are a persistent public health threat because they can spread diseases, including hantavirus, salmonella, and leptospirosis.

New York City recently recorded 24 cases of leptospirosis linked to rodent exposure.

Researchers warned that rising poison resistance could make urban rodent infestations harder to control without new pest-management strategies.

The study was published in Pest Management Science.

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