Rats! The flea that carried the bubonic plague is in New York City, according to a rodent parasite survey. Fortunately these rats don’t currently carry the disease — but the bacteria that they do carry hints that we should probably be paying more attention to Big Apple rodents.

Given that the plague only infects about 7 people each year in the US and that those infections tend to occur in the South, the plague probably won't become established in NYC. But some of the rats in the study carried Bartonella, bacteria that cause a flu-like disease. So, when a flea feeds on the blood of a rat infected with Bartonella, that flea becomes a vector. If it happens to bite a human, it regurgitates its gut contents — and infects them as well.

The study highlights a need for more research on rats as potential disease reservoirs, Frye says. "We've been able to do these survey since the early 1900s," but New York City hasn't surveyed its rats for parasites since the 1920s. People who work in the pest management industry are particularly at risk but if we don't know about them, diagnosing them becomes very difficult. Studies like this one let us know that rats and the fleas that live in their fur are a potential transmission pathway.

To figure out exactly which parasites reside on New York City rats, Frye and his team trapped 133 specimens in five different locations, three of which were residential buildings. They collected the parasites by euthanizing the rats and placing them in containers filled with a vapor that kills insects. The dead insects then dropped out of the fur — and any that didn't were combed out by researchers.

The oriental rat flea, the tropical rat mite, the spine rat louse, and the spiny rat mite were all unevenly distributed among the 133 rats. Because scientists don't really know of human diseases transmitted by the mites or the louse, the researchers focused mainly on the oriental rat fleas, which infected 30 percent of rats. "That’s the one that’s been documented to transmit plague and Bartonella," Frye says — in addition to some tapeworms. Scientists checked the fleas and found Bartonella. This wasn't entirely surprising; medical professionals already know Bartonella is present in the city, Frye says. But knowing that rats are participating in the transmission cycle may help with diagnosis.

A total of 6,500 parasites were found on the 133 rats — a number that Frye calls impressive. One rat had 83 fleas on it. "That was a lot," Frye says. "Fleas were just pouring off its body, and that was rather shocking." Rats groom themselves like cats, but this particular rat just couldn’t keep up. "Given its global distribution, I am not surprised that the rat flea (Xenopsylla cheopis) was collected, though it was interesting that it was the only species of flea found," says Kaylee Byers, an entomologist at the University of British Columbia who didn't participate in the NYC rat study.

The study is a follow-up to an October report that looked into the pathogens that NYC rats carry. Among them were food-borne illnesses and diseases that were entirely new to science. But New Yorkers shouldn’t worry too much about the parasites identified in Frye’s study. The majority of the pests found don't harm humans, though those that carry diseases that can infect people are obviously problematic. Still, the parasites' preferred host is the rat, so when they bite humans, it tends to be an accident, Frye says. "The likelihood of them establishing a colony on a human is low to impossible."

As for the plague, if it was ever introduced in New York City, it probably wouldn’t be too hard to control. "A lot of these are easily controlled with modern medicine," such as antibiotics, Frye says. But knowing where disease reservoirs reside makes that process a lot easier. "We wouldn’t see the Black Death — or the hysteria that comes with it."