According to a new study, the bubonic plague could be transmitted via rats in New York City. The rats host a species of flea capable of passing the plague, also known as The Black Death, to humans. However, plague itself has not been found in the rats or fleas in the study.

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Researchers analyzed more than 6,500 specimens of fleas, lice and mites taken from 133 New York-dwelling rats.

Last month, we looked at research published in the journal Cell Traces that found a wide variety of microorganisms, including DNA traces of anthrax and bubonic plague, alongside unidentified pathogens in a citywide map of microbes in the New York subway system.

However, senior investigator Dr. Christopher E. Mason reassured that these pathogens were unlikely to be harmful to human health:

“Despite finding traces of pathogenic microbes, their presence isn’t substantial enough to pose a threat to human health. The presence of these microbes and the lack of reported medical cases is truly a testament to our body’s immune system, and our innate ability to continuously adapt to our environment.”

Now, in the first study of its kind since the 1920s, researchers from Cornell and Columbia Universities analyzed more than 6,500 specimens of fleas, lice and mites taken from 133 New York-dwelling rats.

Among the collected creatures were more than 500 Oriental fleas, described as being “notorious” for their role in transmitting The Black Death.

“If these rats carry fleas that could transmit the plague to people, then the pathogen itself is the only piece missing from the transmission cycle,” says lead author Matthew Frye.

Oriental rat fleas are also known to transmit the pathogenic bacteria Rickettsia and several species of Bartonella, which Frye says “can cause a wide range of clinical syndromes, some severe.”

Frye’s colleagues at Columbia University’s Center for Infection and Immunity used molecular screening methods to search for these pathogens in the collected samples. Although the researchers did find Bartonella, they report in the Journal of Medical Entomology that no Rickettsia or plague were found.

Plague can be found in some regions of the US, such as the American Southwest, where it infects an average of 10 people each year. The pathogen is transmitted in these areas by the fleas of ground squirrels and prairie dogs.

However, New Yorkers should not be too worried about an imminent outbreak of Black Death in the Big Apple. Commenting on the study, the city’s Health Department emphasized that plague-carrying rats have never been found in New York.

“Plague requires extreme circumstances besides fleas to pose a threat to human health, and those circumstances do not exist here,” a Health Department spokesperson told New York Daily News.

Previous research conducted by Frye on the same 133 rats, however, found that the rodents carried a “disturbing number” of viral and bacterial diseases. These included previously undocumented pathogens that could infect humans.

Frye recommends that New Yorkers remove food and water and prevent access to shelter in an effort to hold back rodent infestations. Also, when evicting rats from homes and workplaces, Frye says it is crucial that careful sanitation is followed to remove the fleas, lice and mites left behind by the rodents.

“It’s not that these parasites can infest our bodies,” Frye says, “but they can feed on us while seeking other rats to infest.”