The sight of a rat in a crowded city is nothing extraordinary. New York City vermin, like the yellow cab, have come to be an identifiable symbol of the city. Although we may instinctually know that these rats are not to be touched, a recent study from Cornell University investigated just how dangerous the rodents actually were, and what they found may be surprising.

The study, published in the Journal of Medical Entomology, was led by Matthew Frye, an urban entomologist with Cornell University's New York State Integrated Pest Management (IPM) Program. As reported in the press release, Frye and his team collected and analyzed more than 6,500 specimens of five well-known species of fleas, lice, and mites from 133 rats. Although the study concluded that the parasites on the collected rodents did not currently pose a threat to human health, the results could not confirm that they would always remain so innocuous.

Modern Day Plagues

Fleas are behind the spread of some of the most dangerous illnesses in human history, with the oriental rat flea being responsible for the spread of the Bubonic Plague. However, a recent study has suggested that the rat was not responsible for plague transmission. Instead, it seems that the gerbil and other Asian rodents hold the most blame for spreading this terrible disease in the 14th century.

While the rat isn’t completely innocent in the transmission of the plague, scientists have long been confused as to why the plague has all but disappeared from Europe while the common rat has not. The new theory proposes that this is because the majority of the fleas responsible for the plague were carried on the bodies of gerbils and other Asian rodents who only occasionally travelled to Europe in large numbers.

“We show that wherever there were good conditions for gerbils and fleas in central Asia, some years later the bacteria shows up in harbor cities in Europe and then spreads across the continent,” Nils Christian Stenseth, one of the study’s authors, told BBC News.

Today, small occurrences of the plague continue to pop up, with the United States experiencing an average of 10 cases a year. In countries such as Madagascar, plague outbreaks have become a yearly occurrence.

What worried the Cornell scientists most is that the rat populations they found in New York City had full potential to spread serious diseases if the circumstances were right.

"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," Frye said in the press release.

In order to stop this from ever occurring, the researchers emphasized the need to practice safe pest control in order to keep rodent numbers down.

"Removing food and water and preventing access to shelter are key to knocking back rodent infestations," Frye said.