Grappling with the Health Consequences of Floods
Cover Story

Grappling with the Health Consequences of Floods

Hurricanes, monsoons, and heavy rains have researchers studying how to prevent the diseases that often follow.

Above, Figure 1: 2. Seth Pedersen, a graduate student from Rice University, wading into a flooded street for collection of water samples after Harvey flooded Houston, TX. Photo Credit: Ya He.

In early September—when the rains from Hurricane Harvey finally subsided in Houston, Texas—Seth Pedersen loaded up his pickup truck with sample collection kits, waders, rubber boots, buckets, and a small aluminum fishing boat. On that particular day, Pedersen—a second year graduate student in environmental engineering at Rice University—accompanied by another graduate student, was on a mission to test the water in homes flooded by Hurricane Harvey. He was looking for E. coli bacteria, chemicals, heavy metals, and other pathogens (Figure 2).

Figure 1: Graduate students from Rice University accessing a flooded home by boat for collection of water samples from inside the house after Harvey hit Houston, TX. Photo Credit: Qilin Li.

Figure 2: Graduate students from Rice University accessing a flooded home by boat for collection of water samples from inside the house after Harvey hit Houston, TX. Photo Credit: Qilin Li.

Pedersen was part of a team of researchers in the greater Houston region who were sampling floodwaters in areas affected by Harvey to better understand the public health risks they posed (Figures 1 [above] and 3 [below]). The test results did not surprise him: the samples revealed a host of bacteria and chemicals.

“There were definitely bacterial pathogens present—that’s why researchers often see an uptick in gastrointestinal diseases after floods in public health studies,” says Lauren Stadler, an environmental engineer at Rice University and one of the leaders of the project. “There was definitely sewage running off and being untreated, because that’s what happens in the event of a flood. Even with household chemicals that you keep under the kitchen sink—when your house gets flooded, where do you think those go?”

Figure 3. Seth Pedersen, a graduate student from Rice University, collecting a water sample from a flooded creek after Harvey hit Houston. Photo Credit: Ya He.

Figure 3. Seth Pedersen, a graduate student from Rice University, collecting a water sample from a flooded creek after Harvey hit Houston. Photo Credit: Ya He.

Just this year, Hurricane Harvey was preceded by a huge flooding event in South Asia that affected 41 million and killed more than 1,000 across Bangladesh, India, and Nepal. And after Harvey came Hurricane Maria, which swept through Puerto Rico in September 2017. In 2010, flooding in Pakistan displaced more than 20 million people; Hurricane Katrina in 2005 forced approximately 200,000 to leave their homes and killed nearly 2,000.

Worldwide, floods are the most frequently occurring natural disaster causing the most human suffering and loss, according to Jorge Castilla, an emergency officer at the World Health Organization (WHO). And it’s getting worse: Rains during monsoon season are getting stronger in Asia, India, Australia, and other tropical regions. While the immediate hazards are from injuries or even death, there are also long-term public health consequences.

In the days and weeks following floods, the threat of infectious disease becomes high. For instance, after the 2010 foods in Pakistan, more than 6 million people suffered from gastroenteritis, respiratory infections, malaria, and skin conditions. Diarrheal diseases were rampant due to a lack of safe drinking water. In 2015, floods in Chennai, India also led to a spike in cases of diarrhea, cholera, dysentery, respiratory infections, hepatitis A and E, typhoid fever, leptospirosis, and vector-borne diseases. Recent research from the Karolinska Institute in Stockholm, Sweden, confirmed the impact of floods on human health after examining 113 cases of floods that occurred worldwide. Notably, they found that skin infections, gastrointestinal infections, wounds, and poisonings from carbon monoxide, gasoline, bleach, and hydrocarbons all spiked after storm events in both developing and developed countries.

The specific public health impacts of floods depend greatly on where the flooding event occurs. For instance, since Houston is home to many oil and gas companies, the floods that accompanied Harvey sent a cocktail of chemicals into the floodwater. Sewer overflows unleash a host of microbes into water that can infect open wounds and cause disease, as Pedersen and Stadler have seen in their test results. As such, health authorities in Houston were concerned about the possibility of a surge in flesh-eating bacteria, as well as cases of antibiotic-resistant staph infections, diarrheal disease from noroviruses, along with other respiratory infections and disease. (One woman from a Houston suburb has died from coming into contact with flesh-eating bacteria in water.) And because it takes time for high floodwaters to recede, pools of standing water become a breeding ground for mosquitos, which can carry viruses such as West Nile, dengue, chikungunya, and Zika. (At the time this story went to press, there was no significant increase in respiratory illnesses in the month after Harvey—according to data collected by Houston’s Health Department, which tracks over 70 conditions in real-time—compared to the same time period in 2016 when Houston was not flooded.)

Molecular Surveillance of Disease

In sampling floodwaters after Harvey, Stadler is particularly interested in the microbial communities present. Right now, she and her colleagues are using 16s rRNA sequencing to profile the microbial communities present. However, she doesn’t have data from before the flood. “That’s actually one of the biggest concerns I have,” she says. “That, from a scientific perspective, we don’t have baseline data to compare it to.”

Elsewhere, researchers are sampling components in their environment and investigating the unique microbial communities that exist.

Sewage, for example, teems with a host of microbes and viruses that could be pathogenic. One team of researchers at the Massachusetts Institute of Technology in Cambridge, Massachusetts, called “Underworlds” is using technology to understand the microbial composition of sewage. Tapping into this reservoir of information, project manager Newsha Ghaeli says, can help cities and municipalities monitor urban health patterns. While the project is not there yet—team members are still manually sampling sewage and performing metagenomic analysis on these samples—their hope is that this type of platform can be automated and used in real time to help inform public health risks and decisions.

Chris Mason

Chris Mason

One global consortium, MetaSub, is working towards a better understanding of a molecular and genetic view of cities. “There’s a global push to understand cities, sewage, buildings—the entire built environment—at a molecular and genetic level,” says Christopher Mason (Figure 4, right), a computational biologist at Weill Cornell Medical Center in New York, and director of MetaSub. MetaSub, which spans over 70 cities and six continents, uses metagenomics to determine what microbes are present on distinct surfaces.

“So when you do have a flooding event, you already have a genetic baseline of a city. You’ll know what the state was like when things were healthy and normal, and it wasn’t disrupted,” Mason says. Following Hurricane Sandy, which hit New York City in fall 2012, floodwaters inundated the South Ferry subway station in lower Manhattan. Afterwards, Mason and his colleagues swabbed the walls of the station and found that many of the microbes they detected resembled marine and fish-related bacteria. “Things that you normally found in the ocean were still present on the surface a year later as they were repairing the station,” Mason says. “It carried what we like to call a ‘molecular echo,’ of what had occurred at the station.”

But building a baseline profile of cities can be labor intensive. Right now much of the molecular profiling is done manually: researchers go out into their communities, collect samples, and bring them back into the lab to be analyzed. Members of Mason’s lab at Weill Cornell, however, are developing automated samplers that can either be attached to someone’s arm (e.g., a swab that cones out and then retracts back), or prototypes that can be attached on bike wheels. “You’d go out, ride on your bike for a while, and then come back to the lab and take it off,” Mason explains.

Understanding the Medical Needs of the Community

Cities and municipalities prone to flooding should also develop preparedness, contingency, and emergency health response plans tailored to their specific needs and consider many of the risks associated with flooding, says WHO’s Castilla. One way for cities to do this would be to understand the medical needs of their communities that they serve.

Juanita Graham, a professor of nursing at the University of Southern Mississippi has looked back on the impacts of Hurricane Katrina to help come up with solutions. She writes that, if public health officials already have an understanding of the overall health of the populations that they serve, they can use this type of information to inform decision making while responding to and recovering from disasters. For instance, gathering data and showing that a population might have a high rate of diabetes means that during disasters, the emergency response team should be able to immediately provide insulin for the community.

Kimberly Gill

Kimberly Gill

And understanding the needs of patients who live with chronic illnesses beforehand could translate to healthcare facilities carrying their medication even in the face of disaster so these patients can take their medications without any type of disruption. “Some counties have made efforts to collect this type of health data, although it is not collected uniformly and comprehensively at the county level, nor is it always readily accessible,” says Kimberly Gill (Figure 5, right), a sociologist at University of Delaware’s Disaster Research Center. Gill is currently working with researchers at Johns Hopkins University on a community resilience project, called COPEWELL, which looks at community health during and after a disaster at the county level.

Apps can also alert users if there are public health issues lurking in their proximity. ProMED, the Program for Monitoring Emerging Diseases, is a web-based reporting system that aims to provide early warning of outbreaks of diseases so public health precautions can be taken at all levels to prevent transmission.

In 2006, a group of researchers at Boston Children’s Hospital founded HealthMap, a tool that aids in the real-time surveillance of emerging public health risks. HealthMap takes data from local health departments, travelers, eyewitness reports, and governments and organizes it so that people who utilize their app, “Outbreaks Near Me,” can be alerted of any health risks in their neighborhood. App users can also set a notice so that they can be automatically alerted when an outbreak is happening in their neighborhood. (ProMED and HealthMap collaborate in their efforts to provide real-time information on health hazards.)

Researchers at the University of Arizona Zuckerman College of Public Health have recently developed Kidenga (Figure 6, below), a crowdsourcing app aimed to detect outbreaks of Zika. The app allows public health investigators to track the location of mosquitos, and identify people with symptoms of illness. Kidenga uses data from citizen scientists to track data on the Aedes aegypti mosquito, which not only transmits Zika, but also viruses like dengue and chikungunya.

Figure 6. Kidenga is an app to track the emergence of mosquito-borne diseases. Photo credit: Kidenga.

Figure 6. Kidenga is an app to track the emergence of mosquito-borne diseases. Photo credit: Kidenga.

Rethinking Location

Sabrina McCormick

Sabrina McCormick

In order to respond to diseases that may arise after floods, cities should ensure that their main hospitals are located outside of flood zones. Sabrina McCormick (Figure 7, right), a sociologist at George Washington University’s Milken Institute, and her colleagues performed a case study of Tampa, Florida, a city vulnerable to hurricanes. “Their main hospital is out on a peninsula; people are going to have a really hard time using that critical infrastructure during a flood because it may be flooded itself,” she says.

These low-lying health care centers should be moved, if possible, says Castilla of the WHO, to an area “above normal flood levels to ensure continuous operations after a seasonal flood.”

In the longer term, floods may impact the mental health of those who have been displaced. “We see a lot of stress, anxiety, and other mental health effects following these extreme events,” McCormick says. After Katrina, for instance, there were a large number of suicides in New Orleans. “We need to be building our healthcare infrastructure to support people who have experienced these events, because they’re becoming more frequent and more severe.”


  1. Saulnier DD, Ribacke KB, Schreeb JV. “No calm after the storm: a systematic review of human health following flood and storm disasters.” Prehosp Disaster Med. doi: 10.1017/S1049023X17006574 (October 2017)