Counting New York’s Rats

The rodents carry diseases, but maybe there are fewer of them than we fear.

WildRat

In October last year, scientists at Columbia University looked at New York City’s rats and identified pathogens that the rats carry, some of which can cause serious and life-threatening illnesses for people. The study showed that rats—as you might expect—indeed carry such germs, for example, as E. coli and salmonella. Some of the New York rats even carry viruses never documented in the city before, such as Seoul hanta­virus, which can cause fever and kidney failure in humans.

The study was based on research on more than 130 Norway rats from five New York City sites, mainly residential areas. According to one of the authors, Cadhla Firth, “New Yorkers are constantly exposed to rats and the pathogens they carry, perhaps more than any other animal,” she said in the Mailman School of Public Health release.

“Rats are sentinels for human disease,” said W. Ian Lipkin, director of Columbia’s Center for Infection and Immunity, in the release. “They’re all over the city; uptown, downtown, underground. Everywhere they go, they collect microbes and amplify them. And because these animals live close to people, there is ample opportunity for exchange.”

But how big is the problem? How many rats are there?

Urban legend says that there are approximately eight million rats in New York City, one rat per person. The myth is based on claims by W.R. Boelter, an author of a book called “Rat Problem.” Based on surveys with the English countryside, Boetler estimated that England had one rat per acre of cultivated land. Since England had 40 million acres of cultivated land at the time, and also had a population of 40 million people, the one rat per person claim prevailed, and the idea spread to other world cities.

But Jonathan Auerbach, a Ph.D. student of statistics at Columbia—and who used to work for New York City Council on data analysis projects—says the assumption is misleading. He says the city and many organizations for years have been using the same wrong assumption because there was no actual research that would prove or disprove the claim. Auerbach thinks that many agencies have also used the wrong statistics to get resources and funds for actions related to rat prevention, without knowing how big the problem actually is. Last year, Auerbach decided to “count” the rats himself.

In October 2014, Auerbach published his rat paper in Significance magazine and won a statistical competition sponsored by the 180-year-old Royal Statistical Society of London. Aurebach began collecting data by looking at the rat complaints to the city, and defined areas of the city and neighborhoods that rats either inhabit or not. Once he had target areas, “then you can use capture-recapture model, which is used in estimating population sizes,” said Auerbach. “So we tried to estimate the number of inhabited building … and in order to get a number of rats, you just multiply the estimated number of buildings that have the rats by the average colony size.” In short, his research determined that there are approximately 2 million rats in New York City.

“The fact that there are fever rats means something for taxpayers,” says Auerbach. From a policy standpoint of view, he explained, a better metric for determining the scope of a problem allows the city to distribute resources better. “From an individual standpoint, it doesn’t matter if there is eight million or two million rats, if you have rats in your building, you’re gonna be pretty upset,” Auerbach said.

So if Auerbach is right, the bad news is some two million rats still carry harmful diseases. The good news? At least there are not eight million of them.

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