Most people wouldn’t dream of rising at 4 a.m. on a Sunday to go catch rats. But for 10 months, while most of us snoozed in our beds, biologist Matthew Frye and his colleagues were preparing to go on the prowl. Before sunrise, they’d don white Tyvek suits, hairnets, hard hats, and headlamps, and descend into the cramped crawl spaces under residential buildings to collect rats that had scurried into their live traps. But rodents weren’t what the researchers were really after—their true quarry were the parasites feeding on those rats.
New York’s Norwegian rats—a misnomer since they originally hail from Asia—can harbor a bevy of pathogens that are harmful to humans if passed on directly through bites (their powerful chompers bear down with 6,000 pounds of pressure per square inch) or via their urine, feces, or ectoparasites, like fleas. The latter is what Frye is particularly interested in. Rats are infamous for spreading the plague in Europe in the mid-14th century (though recent evidence suggests that rats took the fall for Asian gerbils, the true villains).
Last year Frye and his intrepid team concluded the first survey since 1925 of the parasites that hitch rides on New York City rats, to get a sense of the risk of zoonotic disease that residents face. The much-maligned rodents are creatures of the night, and, as Frye, of Cornell University’s integrated pest management program, puts it, “it’s hard enough to get around the city, much less carrying animals that freak people out.” Hence the ungodly early weekend mornings.
While the city’s subway system might seem like an obvious target, project leader and zoonotic disease expert Cadhla Firth decided to focus on residential properties. “If they’re carrying pathogens we should be worried about, then we’ll be most worried when rats are inside houses,” she explains, adding that she looks back on the project with “a mix of nostalgia and post-traumatic stress.”
With guidance from New York City rat czar Bobby Corrigan, the team set out traps in five locations in Manhattan from late 2013 through mid-2014: the crawl spaces beneath three residential buildings, the garbage room of a mixed residential/commercial building, and outside in a park.
Anyone who has spent any time in New York City has crossed paths with a rat. In Rats, a book about rats in the city, author Robert Sullivan says at any given time, one is never more than seven feet away from a rat in Manhattan. They are under the streets, on roofs, and in the walls. Despite their ubiquity, Frye has found these savvy squeakers are hard to catch. You see, they are suspicious creatures. “Rats are neophobic” Frye says, “they are afraid of anything new in their environment.”
To lure the rodents to the traps, Firth and another colleague, Meera Bhat, baited the traps with chicken, apple, or cucumber every night in the week leading up to the Sunday morning harvest.
“We’d be walking from the subway station on the Lower East Side in our full gear, carrying rat traps, passing through crowds of well-dressed guys and women in heels and strappy dresses,” Firth recalls. They received a lot of curious looks and quickly learned to tell any inquisitive passersby that they were exterminators.
The full team—Fry, Bhat, Firth, and Firth’s husband Matthew—pulled a couple of all-nighters when they trapped rats in a park, where the animals were less skittish because they’re used to disturbances. They didn’t want to leave the traps unattended, so they stayed up all night, answering questions posed by homeless people and park workers. (Frye told me when the park caretakers would learn of the mission, they’d direct the team to hot spots after saying, “Oh, kill the rats, we hate them,”—a suggestion the researchers took them up on (but in the name of science, not hate).
After trapping 133 Norway rats, Frye’s team euthanized them in a lab. Then Frye literally went over each one with a fine-tooth comb to collect the ectoparasites. In the March issue of the Journal of Medical Entomology, they reported what they found: 6,500 fleas, lice, and mites of various varieties, including tropical rat mites, spiny rat mites, spiny rat lice, and Oriental rat fleas. The latter, the rat fleas, are the ones that can transmit plague and murine typhus, but thankfully, they didn’t find any fleas carrying the bacteria that causes those nasty illnesses.
They did discover, however, that the rats, on average, had 4.1 fleas per individual, compared to .22 found back in the 1925 study. When the average number of fleas on rats is greater than 1, it’s a potential red flag to public health officials: it increases the likelihood that the bacteria that caused the Black Death could become a problem if it ever makes its way across the country to New York City. (The bacterium is currently only found in the western United States; the main hotspot is around the Colorado/Arizona/New Mexico borders.)
If rat-borne diseases do appear, your risk might depend on where you live. “If I looked at the data from one particular rat, I could probably tell you where we collected it, based on what ecoparasites it’s carrying,” says Frye. “So a person’s exposure to these ectoparasites would depend on rats in your area.”
Firth also found that the rats carry an array of disease-causing bacteria, viruses, and protozoa, reported in the journal mBio in September. They’re reservoirs for E. coli, salmonella, and C. difficile. Those bacteria can cause mild to life-threatening gastroenteritis in people, Seoul virus (which leads to hemorragic fever and kidney failure), and two novel viruses that are each related to hepatitis C.
While there’s no reason to panic, Frye and Firth say that the findings underscore the importance of reducing human exposure to rats. The traditional approach, of course, has been poisoning the pests, but that route can unintentionally sicken or even kill humans, pets, and wildlife species, including red-tailed hawks and foxes. Frye is part of a new initiative, the Scientific Coalition on Pest Exclusion, that aims to find the best techniques for preventing rats and mice from moving into buildings in the first place. Ultimately, that might be the best way to take care of those dirty rats.
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