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The study published by Science Advances on Friday, Jan. 31 analyzed public records from 16 cities worldwide to determine why some urban environments are more infested with rats than others. The researchers found that 11 cities—including Washington, Boston, and New York City—had seen major rat population growth over an average of 12 years.
Washington's increase was the most extreme, tripling compared to Boston and growing 1.5 times faster than New York.
"There’s no silver lining," said lead researcher Jonathan Richardson, an urban ecologist at the University of Richmond. "Cities who are truly committed are going to have to dedicate more resources and larger staff."
The study said rising temperatures, growing human populations, and shrinking green space have created ideal conditions for rodents. Warmer weather extends rats' active seasons, allowing them to breed more frequently.
According to the researchers, cities where temperatures are increasing faster reported more rat activity and sightings.
"Given the projections of continued warming for the foreseeable future, cities need to be prepared for the potential for this warming to exacerbate current rodent pest infestation levels," the study's discussion section said. "More financial and personnel resources will need to be dedicated to municipal rodent control efforts to limit this expected increase in rat populations and activity."
Rapid urbanization provides a steady food supply from dumpsters, restaurants, and waste piles. Shrinking green space, which researchers once thought would limit rat growth, is actually pushing them further into urban environments where they thrive.
Large parks, which tend to have less garbage, don’t sustain rat populations the way dense commercial areas and housing developments do.
"Not all green spaces are equally beneficial to rats," said Drexel University urban ecologist Jason Munshi-South.
Even as US cities spend an estimated $500 million annually on rat control, infestations in some cities are getting worse. New York has appointed a "rat czar" to collaborate with city agencies, community groups, and private businesses to prevent infestations.
Mayor Eric Adams said the city's "war on rats" has focused on stopping rodents from being attracted to the roughly 14 billion pounds of trash the city annually produces.
"Thanks to our citywide integrated pest management strategy and our 'Trash Revolution,' we are giving rats the boot, driving down rat sightings, and improving quality of life across the five boroughs," Mayor Adams said before he hosted the first National Urban Rat Summit in October 2024.
Washington DC's health department also has a "Rodent and Vector Control Division" helping people take steps to prevent rat infestations.
"Rodent and Vector Control Division staff alerts the Division of Food and Division of Community Hygiene of specific areas where waste food sources contribute to rodent activity in residential neighborhoods or commercial neighborhoods," the health department's website said. "DC Health Food Protection Program will initiate enforcement at food establishments if needed."
In July 2024, Boston created a rodent action plan to prevent rats from damaging property and spreading disease.
"Boston, being one of the oldest cities in the Northeast, has infrastructure that greatly influences the rodent population," the city's website said. "The densely populated neighborhoods with intertwining alleyways and old sewer systems, along with century-old cobble or brick streets and sidewalks, provide an environment for rodents to thrive."
A few cities, including Louisville, Kentucky, and New Orleans saw declines in rat populations. New Orleans attributes its success to aggressive public education campaigns and rapid extermination efforts.
In Tokyo, strict building codes and social media-driven business accountability have also helped control infestations.
"Like the proverbial 'canary in the mine,' our 'rats in the city' provide an indication of human welfare," said Inbal Ben-Ami Bartal, a neuroscientist at Tel Aviv University who wasn't involved in the study.
The study also estimates rats cost the US $27 billion per year in damage to infrastructure, crops, and food supplies.
You can click here to see the full study from Science Advances.
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