These cities have big rat problems, and there’s one thing to blame

There’s a saying that in a big city you are never more than six feet away from a rat. It’s an urban myth, but scientists are warning cities across the globe are becoming far rattier, and the boom is primarily driven by one factor: climate change.

Jonathan Richardson, a biology professor at the University of Richmond, decided to research urban rat trends after seeing media reports of rats taking over cities. These tended to focus on single locations and “usually without a lot of hard data,” he told CNN.

He and his team decided to change that. They requested rat stats from the 200 biggest US cities by population, but found only 13 had the quality long-term data they needed. To give more geographical range, the researchers also included three international cities: Toronto, Tokyo and Amsterdam.

The data collected spanned an average of 12 years and comprised rat sightings, trappings and inspection reports.

It revealed “significant increasing trends” in rat numbers in 11 of the 16 cities, according to their study, published Friday in the journal Science Advances. Washington DC, San Francisco, Toronto, New York City and Amsterdam experienced the biggest growth. Just three saw declines: New Orleans, Louisville and Tokyo.

The study linked rat increases to several factors, including high population densities and low amounts of urban vegetation, but the predominant influence was warmer average temperatures.

Rats are small mammals and limited by the cold, Richardson said. Warmer temperatures, especially in the winter, give them longer to be outside foraging and, crucially, longer to reproduce through the year.

A warmer climate can also extend growing seasons, providing rats more food as well as vegetation to hide in, said Michael Parsons, an urban field ecologist and wild rat expert who was not involved in the research. “Even scents of food and rubbish can travel farther in warmer weather,” he told CNN.

Burgeoning rat populations are a big problem for cities. Rats damage infrastructure, contaminate food, and can start fires by gnawing through wires. They cause an estimated $27 billion of damage each year in the US, according to the report.

They are a health hazard, too. “Rats are associated with more than 50 pathogens that affect people,” which they transfer through their urine, feces, saliva nest, materials and parasites, said Matt Frye, a pest expert at Cornell University, who was not involved in the research.

Some of these can be severe, such as leptospirosis, also called Weil’s disease, which can cause kidney and liver damage and even death without treatment.

There is also increasing evidence rats have “huge mental health impacts” on the people living around them, Richardson said.

Even among the rattiest cities identified in the study, Washington, DC, stood out. It had a 1.5 times greater growth in rat populations than New York City.

The tell-tale sign of a rat problem in DC is a hole chewed through a hard plastic trash can. “The only way you can rodent proof a trash can is not to put food in it,” said Gerard Brown, who runs the city’s rodent control program.

Last year was DC’s hottest on record — bad news for attempts to control rats. Brown is hoping the cold snap in December and January will help cull the population. “Cold acts as a natural exterminator,” he said.

Brown and other city officials attempted a rat birth control pilot project several years ago but abandoned it after inconclusive results. The rats had to consume a liquid birth control daily, an impossible task to guarantee.

Brown said DC’s numbers could be so high because the city encourages residents to call in each rat sighting.

Public reports of rats are very useful but can be flawed, said field ecologist Parsons. People normally only make a call when they see something “unusual,” he said, and not when rats are expected in any given area.

It is incredibly hard to pin down accurate urban rat numbers, Parsons added. “Rats are small, cryptic and usually nocturnal.”

Richardson said the high number of rats in some cities is no indictment of authorities’ commitment to tackling the problem, but rat-reduction efforts are often underfunded.

Lessons can be learned from the three cities in the study that reduced rat populations, he said. He chalks their success up to campaigns informing residents how to avoid attracting rats and making city resources available to help.

Richardson also encouraged authorities to move away from lethal control, “because it’s just responsive to infestations that are already there,” and think more about how to take away access to what rats rely on, such as food waste, garbage access and debris piles.

The findings are a wake up call about the challenge rats may pose in a warmer world, Richardson said. “If you don’t have a handle on this, it’s only going to get worse. You don’t want to be like Sisyphus pushing that boulder up a hill.”

In DC, Brown said he is optimistic about the city’s battle to keep its rats under control. “Nobody in the world thinks we are totally going to get rid of rats, but we can reduce them to a manageable level,” he said. “The goal is to control and reduce.”

For more CNN news and newsletters create an account at CNN.com