This is what happens to all the rats when cities flood

By Amanda Schupak, CNN

In the wake of Hurricane Ida, the pummeling rain that hit cities up and down the East Coast at the start of September overwhelmed storm drains, poured into subway stations and filled basements like bathtubs. The devastating human toll is well known. Less clear is what happened to the denizens of those cities’ subterranean depths: the rats.

It’s impossible to know how many rats are in a city — probably on the order of millions — or how many were lost during a major storm. Experts agree that where Ida dropped record-setting rainfall, many rats living in storm sewers would surely have been killed by the sudden inundation. In New York City, 3.2 inches (8 centimeters) of rain fell in a single hour on September 1 — about an inch shy of the normal monthly total. Perhaps hundreds of thousands of rats were crushed or drowned in the deluge, Bobby Corrigan, a foremost rat expert and former rodentologist for the New York City Department of Health and Mental Hygiene, told Gothamist. Dead rats have been spotted washed up on city beaches.

The New York City health department knows some rats drown when there is severe flooding, but as the city doesn’t take rat censuses, there is no data on how many, spokesperson Michael Lanza said. The department uses complaints of rat sightings and inspection reports to track rodent activity. So far, reports have not increased since Ida passed through. The same is true in Philadelphia, which was also ravaged by rain, according to health department officials there.

But rising waters alone are not enough to take down these gritty members of a city’s Rodentia. Rats are excellent swimmers, points out Michael Parsons, an environmental biologist and visiting research scholar at Fordham University in New York City. They can swim a half a mile (0.8 kilometer) or more and tread water for three days straight. (They can even swim up your toilet.)

And rats are wily, apt to move to higher ground if they have the chance.

“To put it scientifically, rats ain’t stupid,” said entomologist Michael Waldvogel, associate extension professor emeritus at North Carolina State University and an expert in “anything people find yucky and disgusting.”

“They’re going to get to where they’re out of harm’s way,” Waldvogel said. “And if they have to, they’ll keep moving further up.”

The Norway rat, the species abundant in New York City, makes its home in sewers, sidewalks and underground burrows. But this critter can climb vertically. And once it gets into a building, it can chew into the walls and scale them. The more diminutive black rat, which is arboreal, meaning it lives in trees, naturally heads upward. This urban dweller is common in New Orleans, where it’s known, fittingly, as a roof rat.

Even if catastrophic flooding were to trap and kill many rats underground, many more would likely find their way to safety.

After the flood

Given how these animals are known to respond to crises, Parsons predicts that rats would not only survive Ida, but thrive. During the pandemic, his early research found, rat populations in New York City adapted to changes in their normal food resources that resulted from restaurant shutdowns during the height of social distancing. “The weaker or unlucky rats died off, while the more fortunate or resilient individuals found ways to survive,” he said.

The survivors reproduce — quickly and often. Twenty rats could easily become a few hundred within six months, Waldvogel said.

“It’s sort of counterintuitive,” said Michael Blum, a professor in the department of ecology and evolutionary biology at The University of Tennessee, Knoxville. “You think, in these flood-affected areas, these things should be wiped out. But really, things get wiped out, but they come back very quickly. They can become much more abundant than they were prior to flooding.”

Blum studied the impacts of Hurricane Katrina on rats in New Orleans. His research, published in August, found that 12 years after the historic 2005 storm, rats flourished in areas that had been heavily damaged by flooding, where many buildings were left vacant. Rodent populations were even larger in underserved, often predominantly Black neighborhoods, such as the Lower Ninth Ward, where vacant lots weren’t well maintained.

Indeed, what happens to a city’s rodent population after a major flood is largely determined by the human response once the waters recede.

“In the case of Hurricane Katrina, the infrastructure was so badly damaged that it took time for trash and everything that was put out curbside to be picked up,” said Claudia Riegel, director of the New Orleans Mosquito, Termite and Rodent Control Board. Emptied refrigerators and debris from damaged homes lingered on streets, providing food and resources for rats and requiring the board to mount a major control effort, including putting rodenticide in storm drains where rats were congregating. “We were trying to arrest the population from actually increasing exponentially,” she said.

Public health measures are needed

That has important public health implications, since rats carry dozens of pathogens, including salmonella and the bacteria Leptospira, which causes leptospirosis. Infection can cause fever, chills and vomiting within a few days of exposure, and can lead to kidney or liver failure.

“If you see a rat, you should assume it has some sort of pathogen,” Riegel said.

Floodwaters can become contaminated with urine from rats, which can increase the risk of leptospirosis. (Lanza noted that the disease is rare in New York City and there are no known cases associated with this or past floods.)

To keep down rat populations and prevent the transmission of disease, it’s vital for storm cleanup to happen as swiftly as possible. Damaged properties also must be maintained in the months and years that follow. As Blum’s research showed, simply mowing vacant lots can go a long way toward controlling rats.

The same principles apply in absence of a weather event, Riegel stressed. Putting lids on garbage cans, not feeding birds and picking up your dog’s poop (rats eat it) all help keep rat numbers in check. Because if there is somewhere to burrow and something to eat, the rodents are likely to take advantage.

“The bottom line,” Waldvogel said: “Rats will survive.”

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Amanda Schupak is a science and health journalist in New York City.