House mice aren't just a nuisance but a potential source of infections, say researchers who trapped and tested more than 400 of the rodents from apartments across New York City.

City dwellers tend to fear rats more than mice because they're bigger and can be seen scurrying in subways or in alleys. The researchers previously studied how rats carry disease-causing bacteria such as E. coli, salmonella and C. difficile.

But they were concerned mice might actually pose a greater health risk because they live with us in houses and apartments.

Dr. Ian Lipkin, a professor of epidemiology at Columbia University, and his colleagues spent 13 months trapping 416 mice living in large apartment buildings across the city, particularly in their garbage collection areas.

The researchers conducted genetic analyses of the mice droppings. They found mice from all parts of the city carried bacteria associated with gastrointestinal and other diseases. And some strains had genes that help thwart the effect of antibiotics.

Scientists are learning how mice act as a reservoir of disease-causing bacteria. (Robert Corrigan/Columbia University)

"We found a lot of antibiotic resistance in some of these bacteria, which we think may have implications for understanding not only the ubiquity of antibiotic-resistant bacteria in the environment but also potential sources of human infection," said Lipkin, whose research was published Tuesday in the journal mBio.

In the vicious cycle of resistance, antibiotics kill some bacteria, but some of the fast-growing microbes evolve and find ways to protect themselves against the drugs. When that happens, people can develop infections that are much harder to treat.

Antibiotic resistance is a major challenge, and scientists are still trying to figure out how it develops and how the resistance genes might be spreading among species.

If you have mice in your house, particularly if they're in and around your living space, this is a potential health risk - Dr. Ian Lipkin

Lipkin said while they haven't proven mice are responsible for transmitting disease to humans, he recommends exercising caution.

"If you have mice in your house, particularly if they're in and around your living space, this is a potential health risk," he said.

"If you've got mouse droppings and they're anywhere near food, for example, these need to be cleaned up with sterilizing disinfectant."

And if a mouse actually touched any food, he said, even just a nibble, it's not safe to eat.

City mice with C. difficile

The researchers say their study is the first to document C. difficile DNA in house mice in an urban setting. Lipkin suspects mice in other major cities carry it, too.

His team also found Leptospira in mice. The bacteria, commonly found in rat urine, can cause an infection called leptospirosis. The risk to humans is thought to be greatest during floods — like those caused by Hurricane Maria in Puerto Rico last year — because the rodents' urine gets washed into the flood waters.

While human cases of leptospirosis are rarely reported in New York, there was a recent cluster of three cases in the Bronx, including one death, the study's authors said.

Researchers in Vancouver previously discovered Leptospira in rats in the Downtown Eastside, but they didn't observe a link to disease in humans.

"We went looking in Vancouver to see if we had evidence of excess leptospirosis in the human population and we couldn't find any," said co-author Dr. David Patrick, a medical epidemiologist with the B.C. Centre for Disease Control. "Obviously it gets wet here but people aren't wading through flood waters."

Dr. Scott Weese, an infectious disease specialist at the Ontario Veterinary College in Guelph who worked on the Vancouver study, said a bit of perspective is needed when evaluating the possible threat of mice spreading drug-resistant diseases.

"Everything that moves" could be carrying such bacteria, he said, so it depends on the likelihood of exposure. For example, there's probably a much better chance you could be exposed to disease-causing microbes in your food or from touching a pet without washing your hands than from rodents.

"If you say, 'OK cook your meat properly or get mice out of your house, which is going to be the best thing to protect you?' Well, cook your meat properly."

Slaughterhouse legacy

Lipkin and his team in New York hope to study outbreaks of human disease to see if antibacterial resistance can be linked back to rodents and to explore how that might occur.

The researchers also published a second study in the same issue that focuses on viruses found in the house mice. They found nine viruses, none of which is known to infect humans.

Interestingly, Lipkin said, one of the viruses looks like a bug that typically infects pigs. The researchers presume the virus has been circulating in an area of the city that used to have a lot of slaughterhouses, and that the virus jumped from pigs to mice.