The World Health Organization reports that floods can increase the transmission of multiple infectious diseases, including water-borne infections like cholera and hepatitis A, and vector-borne diseases like dengue, West Nile, and malaria. The specific illnesses that will emerge during a flood vary based on a region’s ecology and epidemiology, but from Haiti to Houston, New Orleans to South Asia, significant flooding should increase public vigilance of infectious diseases. According to the WHO, though, the risk of infection is usually low “unless there is significant population displacement and/or water sources are compromised.” If water sources are compromised, prompt detection and action can still continue to mitigate risk.
During extreme flooding, sewage systems may spill out into roadways. Drinking water reservoirs can rupture and levees burst, causing clean water to mix with rainwater and sewage. The result? Contaminated floodwater that infiltrates everything, from property to people. Water-soaked buildings are a mold’s paradise. Hurricane season also coincides with mosquito season, and pools of standing water – clean or dirty – are breeding grounds for these disease vectors.
Besides ingestion, water-borne diseases can be contracted by skin contact with contaminated water; for instance, someone with an open wound is susceptible to developing an infection. Leptospirosis, for example, is a zoonotic bacterial disease of public health concern in some parts of the world. It is transmitted through the urine of infected animals, which can seep into water or soil and persist for months. Human contact either directly with urine or with contaminated water enables bacteria to enter the body through skin abrasions or mucous membranes (eyes, nose, and mouth). Leptospirosis causes a range of symptoms including fevers, chills, aches, jaundice, abdominal pain, and diarrhea. Severe, untreated infections can lead to kidney or liver failure, or meningitis.
In developing countries, flooding can lead to outbreaks of dangerous illnesses like cholera, typhoid, and yellow fever. In developed nations with robust public health infrastructure, flooding is more often accompanied by enteric diseases like E. coli, respiratory infections, and conjunctivitis. The WHO has stated that none of these pathogens – from cholera to conjunctivitis – are of extremely high epidemic risk after a natural disaster. More often than not, outbreaks keep to a small cluster of cases.
Of course, exceptions occur which later serve to inform future public health efforts. For example, after Hurricane Katrina, the CDC reported a greater than two-fold increase in West Nile Virus, which is transmitted by mosquitoes. This was not until after Katrina itself had passed, when affected regions in Louisiana and Mississippi contained large pools of standing floodwater (in contrast, during the actual hurricane, turbulent water would have the opposite effect on mosquito breeding grounds). Now, in the wake of Hurricane Harvey, people are speculating about the same public health issues; namely, whether certain mosquito-borne illnesses like West Nile Virus (which is endemic in Texas) or even Zika might increase in transmission in waterlogged areas. This, again, would likely be a delayed consequence rather than a direct one, since shortly after a hurricane water is still flowing and interrupting any existing insect breeding sites.
Altogether, the risk of major outbreaks is low in the wake of flooding. Public health-focused regulations such as boiling orders for water consumption help further push existing risks down. There are, of course, exceptions – such as the devastating cholera epidemic that struck Haiti shortly after a massive earthquake decimated much of the country. The source for this outbreak, however, was not the natural disaster itself. Instead, cholera was imported by UN aid workers from Nepal, who excreted the bacteria into poorly-contained latrines.
As one study outlines, one of the largest risk factors for outbreak after disasters is associated with population displacement. The availability of safe water, the underlying health status of a population, the presence of proper sanitation facilities, the degree of crowding, and the availability of healthcare services are further all interconnected in a complex ecological web that dictates the risk of an epidemic anywhere. Nonetheless, floodwater, especially in the event of sewage overflow and levee rupturing, is considered unsafe to consume. Individuals with skin abrasions such as cuts or dermatitis are advised to take proper precautions against coming into direct contact with potentially contaminated water, as is everybody else when it comes to drinking tap water in the wake of a natural disaster.