Amid the devastation left by Hurricane Harvey, numerous media reports speak of widespread fears that Houston, Texas and surrounding cities may become ideal breeding grounds for various infectious diseases, including diarrhea-causing bacterial and viral infections, as a result of the record-setting 50+ inches of rain engulfing cars, houses, and highways. However, reports of major outbreaks resulting from direct exposure to flood waters in the United States are rare.

The risk of communicable diseases after natural disasters are influenced by the availability of safe water, degree of crowding, and local disease ecology. According to the CDC, unless a disease is brought into a disaster area from elsewhere (as was seen with the cholera epidemic in Haiti after the 2010 earthquake), any outbreaks that occur will more likely be from diseases that were already present in the disaster-affected area before the disaster struck.

It is true that flood water is often contaminated with sewage, chemicals, and various pathogenic agents in the wake of disasters involving wide-scale flooding, but this does not necessarily result in waterborne disease outbreaks. In developing countries, it is not uncommon for victims displaced by floods to acquire waterborne disease such as typhoid and hepatitis A. But in the US, which doesn’t have the same disease ecology as in the developing world, outbreaks of such illnesses due to exposure to flood waters are uncommon. Of greatest concern is one report of 22 cases of Vibrio illnesses (V. vulnificus, V. parahaemolyticus, and nontoxigenic V. cholerae) among those affected by Hurricane Katrina. Most of these cases were associated with infections of open wounds, likely due to exposure to flood waters, rather than from direct consumption or contaminated food or water or from person-to-person transmission. This report highlights the potential for infection following injury among those affected by hurricanes and the flooding that follows.

While large scale outbreaks due to general exposure to floodwaters in Texas is unlikely, it is still a good idea to limit one’s exposure, to the extent possible. The CDC advises those affected by flooding to keep out of floodwaters as much as possible and to be cautious of their drinking water, which may potentially be contaminated. Health officials recommend that residents in Houston boil their water if possible to kill off any bacteria or to rely on bottled water. Children should not be allowed to play with toys that have been contaminated by flood waters, unless they have been properly disinfected. Those with opens wounds should take additional precautions to keep the wound as clean as possible and away from flood water.

Most likely there will be a rise in relatively common diarrheal diseases due to either increased crowding in evacuation centers or due to consumption of spoiled or improperly cooked foods. After Tropical Storm Allison hit southeast Texas in 2001, there was an uptick in diarrheal illness, and following Hurricane Katrina, norovirus, Salmonella, and toxigenic and nontoxigenic V. cholerae infections were confirmed among evacuees—the majority of these cases occurred among individuals in evacuation centers. FEMA estimates that more than 30,000 residents will ultimately be displaced from their homes, and some shelters have already exceeded their capacity. As an example, a convention center in Houston is currently home to 9,000 people, nearly doubling its design capacity of 5,000.

Normally, person-to-person transmission of diarrheal illnesses can be easily prevented through proper hand hygiene, including the use of alcohol-based hand sanitizer, after using the restroom and before eating [NOTE: alcohol-based hand sanitizer is not effective against norovirus, Clostridium difficile, and some parasites]. After a disaster of this magnitude, however, basic hygiene measures maybe difficult to maintain, especially in overcrowded shelters.

There is also a reasonable concern that the flooding could increase risk of vector-borne illnesses. Although the flood has washed away existing mosquito breeding sites, as the floodwaters recede and pools of stagnant water increase, there could be a boom in the mosquito population and potentially an increase in mosquito-borne diseases in the future. Recovery efforts will likely focus on larger tasks such as debris removal and home or building repairs, potentially ignoring small pools of standing water that can serve as ideal breeding grounds for various mosquitoes including the Culex and Aedes mosquitoes—both common vectors for a number of diseases, including West Nile and Zika viruses. The West Nile virus has been endemic in Texas since 2002, with more than 350 cases in 2016 and 36 confirmed cases so far this year. In a previous Outbreak Thursday post, we discussed the first report of Zika in Hidalgo County, Texas. Hidalgo County is south of the affected area, but the fact that Zika is present nearby could mean additional risk in Houston as the area recovers from the hurricane. As of August 29, the Texas Department of State Health Services reported 23 total cases of Zika identified across the state in 2017, including 6 in counties hit hardest by Hurricane Harvey.

Both Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Texas, and Robert Wright, environmental health supervisor for Austin’s public health department, believe that you would expect an increase in the mosquito populations in the weeks after the flood waters recede, as was seen after Hurricane Katrina struck New Orleans and Mississippi in 2005. Initially after the storm, there was a decrease in both West Nile disease and St. Louis encephalitis; however, the following year there was more than a twofold increase in the incidence of these diseases in the areas affected by Hurricane Katrina. Unless measures are taken to reduce the pools of standing water in areas affected by Hurricane Harvey, a similar rise in West Nile, and potentially Zika, would be expected.

Mosquito surveillance programs will also play a key role in controlling any potential outbreak of vectorborne diseases in Houston and the surrounding areas. According to Chris Deusen from the Texas Department of State Health Services, Harris County, Texas has a “very robust” mosquito control program.

But with reports that approximately 30% of Harris County is under water, the region is facing a public health crisis of almost unprecedented scale. Under these circumstances, it is hard to imagine how local government officials will be able to maintain routine programs, such as mosquito surveillance, without a swift and comprehensive infusion of emergency funding and other resources. Reports indicate that Congress is considering issuing emergency funds to Texas in stages rather than in one lump sum. If this phased allocation scheme results in delays or minimizes the total allotment of emergency funds, this will greatly compromise response and recovery efforts. Lack of timely and sufficient resources will only exacerbate Hurricane Harvey’s toll on public health.


Photo by Sgt. 1st Class Malcolm McClendon/US Department of Defense. The appearance of US Department of Defense visual information does not imply or constitute DoD endorsement.


Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly in near-real time to allow others to learn from these experiences in order to improve global outbreak response capabilities.