Anticipating Hurricane Harvey's Effect on West Nile and Zika

Following several days of heavy rain and flooding in the Houston metro area from Hurricane Harvey, parts of Texas and Louisiana along the Gulf Coast will face an increased risk of mosquito-borne illnesses, such as the West Nile and Zika viruses.

So far this season, the Centers for Disease Control and Prevention (CDC) reports that 45 states and the District of Columbia have seen either human, mosquito, or avian cases of West Nile virus, including 190 cases of neuroinvasive disease and 124 cases of non-neuroinvasive disease. The virus has been present in Louisiana since 2001 and in Texas since 2002, and together, the two states have reported more than 6,500 human cases of West Nile. As of August 22, 2017, Texas has reported 33 human cases of West Nile this year and Louisiana has reported 20 cases.

In November of 2016, Texas reported its first case of Zika virus believed to have been transmitted by a mosquito in the state, and since then, has reported 23 cases thus far in 2017. On August 24, 2017, the Texas Department of State Health Services (DSHS) issued a health alert, expanding the area considered at risk for increased likelihood of local Zika transmission. The alert noted the addition of three more counties in the southern part of the state to the DSHS list of counties receiving enhanced Zika surveillance and screening, including testing recommendations for symptomatic and asymptomatic pregnant women.

On the night of August 25, 2017, Hurricane Harvey made landfall as a Category 4 storm near Corpus Christi, Texas, bringing heavy rains, winds, and surging coastal waters to create devastating flooding along its path. Setting new calendar-day record rainfalls exceeding 51 inches in some parts of the Houston metro area, and lingering in the region for several days, the storm made landfall again on August 30, near the border of Texas and Louisiana. While the focus of local and federal officials has been to rescue individuals trapped in flooded areas, news reports note that health departments in the region could see an increase in mosquito-borne diseases such as West Nile and Zika virus. Floodwater mosquito species, such as the Aedes vexans, rarely carry the viruses, so current floodwaters are not expected to be breeding grounds for vectors of West Nile or Zika. However, in the coming weeks, as floodwaters recede and form pools of stagnant standing water, the new pools may become breeding areas for the Aedes aegypti mosquitos that carry and spread the viruses.

Offering hope that Texas and Louisiana may not see a spike in West Nile and Zika cases in the aftermath of Harvey, a 2007 study in the CDC journal Emerging Infectious Diseases found that Louisiana and Mississippi did not experience an increased number of West Nile neuroinvasive disease cases or later peaks following Hurricane Katrina. However, another study published the following year in the same journal also examined cases of neuroinvasive West Nile virus in the areas affected by Katrina, finding that no cases were reported in Louisiana in the 3 weeks before Katrina made landfall, but 11 cases were reported 3 weeks after. A similar rise in cases during that period was not observed in 2002, 2003, 2004, or 2006. Similarly, the affected area of Mississippi saw cases rise from 0 to 10 in the 3 weeks following Katrina, while unaffected parts of the state saw only a minor increase in cases. The study authors noted a correlation with the 3- to 14-day incubation period for West Nile virus, and that the increase in the number of cases occurred despite the population loss from the extensive migration of local residents relocating away from flooded and damaged areas.

While officials don’t yet know how Harvey will impact the spread of West Nile or Zika viruses, for now, the Texas DSHS is dealing with the various public health effects of the storm, and has issued precautions for the public to avoid contact with floodwater, which can contain bacteria, hazardous chemicals, and dangerous debris.