Scientists have uncovered a disturbing change in both mice and rats infesting America's biggest cities, making the rodents harder to kill than ever before.
Researchers at Rutgers University in New Jersey found the vast majority of smaller house mice and larger brown rats in New York, New Jersey, Pennsylvania and Washington DC have genetically mutated to become more resilient to poison.
Specifically, five out of every six rodents tested displayed at least one mutation in the Vkorc1 gene, which has been found to affect how well the poisons work.
This study of nearly 300 house mice and brown rats also confirmed that more than 69 percent of the rodents carried additional genetic mutations, which previous studies have proven make rodents immune to common exterminator poison.
The changes were significantly greater in the smaller house mouse, which typically measures between two and four inches in length and can squeeze through the tightest space to get inside homes.
Meanwhile, the brown rat, also known as the sewer rat or Norway rat, which can grow up to 10 inches long, was found to be much more susceptible to common pesticides as they displayed fewer of the mutations.
Rutgers postdoctoral researcher and lead study author Jin-Jia Yu said in a statement: 'Genetic mutation is not that special in these creatures. But we found that the house mouse shows a lot of genetic mutations related to rodenticide resistance.'
In large urban areas, rodent infestations have been a continuous public health threat for decades, as scientists consider them a major source of disease transmission from animals to humans, passing on illnesses such as hantavirus, salmonella and plague.
Researchers study mouse and rat DNA have found that rodents in multiple US cities have mutated to resist common poisons (Stock Image)
Yu noted that the smaller mice were likely mutating faster than the brown sewer rats Americans are used to seeing in the streets because this species tends to be more curious than their larger peers.
The researchers told the Philadelphia Inquirer that house mice were more likely to eat unfamiliar foods they come across, which leads to them consuming rodent bait more often.
The more exposure these tiny rodents have to fatal poisons, the more their DNA has a chance to develop a mutation, which protects them from falling ill and dying - making current exterminator tactics less successful.
Meanwhile, sewer rats are still playing catch-up because Yu called them 'pretty smart' creatures who know to avoid live traps and new food sources that may be poisoned.
According to Census Bureau data cited by the researchers, 29 percent of all households in Philadelphia have spotted a rat in their home.
Fifteen percent of New Yorkers living in the borough of Manhattan have experienced rat problems in their apartments or homes, and one in five residents in Washington DC said the same.
The Rutgers team started looking for DNA mutations in city rodents in 2021 after local pest control companies began noticing that common poisons called anticoagulant rodenticides were becoming less effective at stopping rodent infestations.
These poisons work by stopping blood from clotting, causing internal bleeding in the pests.
Larger brown rats were still less protected against commons poisons but researchers noted they still displayed several genetic changes (Stock Image)
While the plague may be an extremely rare illness humans can contract from contact with mice and rats, new fears have recently emerged over the return of hantavirus.
Hantaviruses are a deadly strain of respiratory illnesses that typically spread by inhaling particles released by rodent droppings, urine or saliva. The animals' bites and scratches can also cause infection.
Early symptoms include fever, fatigue and muscle aches, which can later progress to headaches, chills, and nausea, vomiting and diarrhea. Health experts have warned that a resilient hantavirus strain could be a candidate for a future pandemic.
Researchers wrote in the journal Pest Management Science: 'The house mouse and the Norway rat are globally distributed commensal rodent species. They cause substantial economic losses by damaging furniture and buildings and pose serious public health risks through the transmission of zoonotic diseases.'
In early 2026, the World Health Organization (WHO) confirmed at least nine cases of hantavirus on the MV Hondius, which was sailing from Argentina to Cape Verde.
Three of the passengers died, including a Dutch couple who health officials believed were the first exposed to the virus while visiting South America.
In New York City, 24 cases of the potentially deadly bacterial infection leptospirosis were traced back to contact with rodents and their urine.
It often starts like a bad case of the flu, with symptoms such as high fever, chills, headache, muscle aches, vomiting, diarrhea and red eyes, according to the CDC.
In more serious cases, however, it can lead to jaundice - yellowing of the skin and eyes - kidney or liver problems, bleeding and death if not treated immediately.