Scientists have found that many urban mice and rats in major U.S. cities are developing genetic mutations that make them increasingly resistant to common poisons used for extermination.

A Rutgers University study found that most tested rodents in cities like New York, Philadelphia, and Washington, D.C. carried mutations in the Vkorc1 gene linked to rodenticide resistance.

House mice showed particularly high levels of these mutations, likely due to frequent exposure to poisoned bait, while brown rats showed fewer resistance traits.

Researchers say this is making traditional pest control methods less effective and allowing rodent populations to persist in urban areas, the Daily Mail has reported.

The study also highlights ongoing public health concerns, as these rodents can spread diseases such as hantavirus, salmonella, and leptospirosis.