A recent study has found a significant increase in rat infestations in various cities worldwide, notably in Washington. The study attributes this rise to rising temperatures, urban development, and human activities.

A first-of-its-kind examination of trends and reasons in hard-to-count rat populations uses rat sighting reports in 16 cities around the world. In 11 of those cities, rat complaints have increased, according to a study in Friday’s journal Science Advances.

Based on individual trends within cities, Washington was by far the leader in rat increases, followed by San Francisco, Toronto, New York City and Amsterdam. Washington’s rising rat reporting trend was three times greater than Boston’s and 50% more than New York’s, the study said. Washington city officials did not respond to requests for comment.

Only three cities saw significant decreasing trends — New Orleans, Louisville and Tokyo — with the home of Mardi Gras showing the biggest drop in rat reporting. Experts said the Louisiana city can teach others how to combat the rat problem.

Researchers did a statistical analysis of the rising rat reporting in those cities and concluded that slightly more than 40% of the trend seen is due to warming temperatures from the burning of coal, oil and natural gas. And that comes down to sex and food, said study lead author Jonathan Richardson, a biologist at the University of Richmond.

“We’re seeing these increasing trends in rats in cities that are warming the fastest, probably because this is a small mammal that has physiological challenges in the cold weather months,” Richardson said. “If we’re warming the climate and winter starts a week or two later and spring arrives a week or two earlier, that’s one, two, maybe even three or four weeks across the entire year where those rats can be above ground foraging, acquiring more food and maybe squeezing out one or two more reproductive cycles. ”

An extra month may not sound like much, but female rats can have a litter every month. Each litter is eight to 16 baby rats, Richardson said: “That is a recipe for accelerated population growth.”

Researchers pointed to two other big statistical links — that fit with known biological issues — behind more rat reports: the increase in urbanization and more densely populated cities.

Rats like the built-up environment and being near people and their waste, the study and outside scientists said. They essentially eat at the same table as humans, multiple experts said.

“The rat is the third most successful mammal behind humans and house mice. So it evolved and engineered to live alongside us,” New York City rat czar Kathleen Corradi said during a break at a New Orleans conference on improving pest management. “They followed humans, Homo sapiens, across the continents and are in every single continent except Antarctica. So it’s considered a wicked problem.”

Even though rats are intelligent and highly adaptive, Richardson and other experts said they are a serious problem for people.

“When rodent populations are high, people get sick, motor vehicles become disabled, mental health declines, fires are started and foods fouled,” said Houston rat expert Michael Parsons, who wasn’t part of the study. People are rightly bothered by rats because of “an innate fear caused by an organism that can make us sick.”

Researchers have not had good figures on rats. Because of the way they live and hide, they are not as easily counted as other critters, so this is one of the first attempts to quantify them. The study is not really a count of rats, but of complaints by people.

Because the statistics go back years and only cities that haven’t changed their reporting methods were looked at, Richardson said the trends they spotted have scientific merit. Even though Washington has the highest increasing trend that doesn’t mean it has more rats or even rat sightings, it is just that the numbers within the city are increasing fastest, he said.

Several outside experts said the study is legitimate, sensible and was sorely needed.

“This paper is by far the largest data-driven effort to understand changes in urban rat populations ever attempted,” said Drexel University ecology professor Jason Munshi-South, who wasn’t part of the research.

Looking at the few cities where rat reports are down may help in the fight against rats, Richardson and Corradi said. The answer is not more poison or traps, but prevention, the study said.

“In New Orleans they make a big effort to get out into neighborhoods and do educational workshops and campaigns to talk to residents about what makes a property less likely to have rats,” Richardson said.

New York City’s recently stepped up rat-fighting, which includes replacing garbage in bags on the street with rat-resistant containers, hasn’t quite shown up in Richardson’s data yet, but rat czar Corradi said early results are encouraging. The city has signed people up for what it calls an elite squad of rat fighters, named ” NYC rat pack.”

It’s an uphill battle.

“As our cities warm, urbanize and (increase in) density, we create more resources for rats which could result in further increases in numbers,” Simon Fraser University health and rat scientist Kaylee Byers said in an email.

We can fight them better, but in the end people “need to coexist with wildlife in urban environments, even with rats,” University of Michigan conservation scientist Neil Carter said.

“Zero rats is impossible,” Richardson said. “But I think an expectation that we need to live with the number of rats that we’re seeing in many of these cities is also an unhealthy perspective on this problem.”