A rat forages for food in the Lower Ninth Ward of New Orleans. AP Photo/Alex Brandon

A team of ecologists looks at how disasters affect rat populations and human exposure to disease.

When natural disasters strike, the drastic environmental changes often bring public health concerns to the forefront of recovery efforts. Following the 2010 earthquake in Haiti, officials and international agencies rushed to curb the cholera outbreak caused by contaminated drinking water. And in the wake of Hurricane Katrina, officials in Louisiana cleared more than 38 million tons of debris in part to reduce the spread of bacterial diseases like leptospirosis, transmitted to humans via infected animals—namely, rats.

It’s not only the disasters themselves that bring humans and disease-carrying agents closer, says Michael Blum, a molecular ecologist at Tulane University. Human response to disasters also plays a key role. Since 2013, New Orleans has become a lab for scientists like Blum to study disaster ecology, or the environmental impacts of disaster and recovery. While researchers in this field commonly study mosquitoes—carriers of the deadliest vector-borne diseases—Blum focuses on rats.

Once a rat establishes a place as its home, it tends to stay within that area rather than traveling far away. That means Blum can study how the environment affects a population of rats within one block.

Throughout New Orleans, landscapes differ widely. Some areas have been rebuilt, while others have been left vacant and poorly managed. “You can see the shift in feedback between decision-making and environmental conditions at a much finer resolution [with rats] than you can with mosquitoes,” says Blum.

Yet there haven’t been many quantitative studies on rat density in urban environments. What we know about rats—their colony size, sex ratio, or movement, for example—is mostly conjecture or based on studies done in the 1940s and ‘50s. “There’s been a movement recently to use complaint calls as an indicator for estimating population density of rats, in particular New York City, [where] there’s a general rule of thumb that there’s a rat for every person,” says Blum. “None of that is validated.”

The imprecise estimates can lead to inefficient policies, especially if the perceived health risks or exposure to rats are inaccurate. So Blum and his team of student volunteers are conducting the largest study so far to look solely at rats in cities with the hope of creating a mathematical tool to predict how natural and man-made environmental changes will affect the overlap between humans and rats—and the pathogens they carry.

For the past two years, with funding from the National Science Foundation, the team has been in New Orleans identifying and mapping popular spots for rats—a process that involves going door to door to talk to community members and setting up baits and traps to catch the vermin.

A member of the “Rodent Trapping Team” sets up a trap for rats. (Center for Bioenvironmental Research, Tulane University)

So far, Blum has already noticed some patterns that go against our preconceived notions of rats. For one thing, his research doesn’t show that increase in human population density leads to increase in the rat population. “What we see is the reverse,” he says. “The city, where [human] population density has declined significantly as a consequence of the storm, is by far harboring the highest rodent density,” Blum explains. “There's sort of a sweet spot where there’s enough habitat open for rats to effectively live and just enough of a resource base from humans that provide them with sufficient food and water.”

The study also found that brown rats (also called Norway rats) aren’t the only ones cities have to worry about. “Most people think of city of being occupied by brown rats, and if you look historically, [brown] rats are thought to have pushed out black rats through competition,” says Blum. But in areas with lots of trees and vegetation—especially ones that flooded—it’s the black rats, which are arboreal creatures, that thrive.

That distinction matters because black rats and brown rats, which live underground, carry different pathogen loads. Blum and his team are working to get a better understanding of the prevalence and distribution of diseases carried by rats—such as leptospirosis, which can lead to death if left untreated. It’s often thought of as a disease that’s prevalent in Hawaii and developing countries, but not most of the continental U.S. “It’s not usually of broad concern [in America], but from what we’re finding, it should be,” Blum says. The researchers have found that the leptospira bacteria exists in almost a third of the rats in their sample.

Though the research is only in New Orleans, Blum says their results have policy applications for other cities hit by disasters. “There are new and compelling roles for environmental scientists and ecologists to play in the context of disaster response,” he tells CityLab. “Disaster ecology is going to be more and more important as human population and human footprints increase.”

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