This map shows Zika risk for 50 U.S. cities (click to enlarge). A city's circle size represents its average monthly travelers from Zika-affected countries; color indicates observed abundance of Aedes aegypti mosquitoes in July. (Image: NCAR/NASA)
Mosquitoes are often the worst part of summer, even when they aren't carrying terrible diseases. And for many American cities, the annual boom of bloodsuckers is looming larger than usual this summer due to the Zika virus.
To help people in the U.S. visualize the danger posed by Zika-carrying mosquitoes, researchers from NASA, the National Center for Atmospheric Research (NCAR) and other institutions made a map illustrating which cities are most vulnerable. Published this week as part of a study in the journal PLOS Currents Outbreaks, the map fuses several variables into a detailed glimpse of Zika's potential risk for 50 U.S. cities.
In hopes of revealing when and where a Zika outbreak might occur, the researchers looked at key factors — including temperature, rainfall, socioeconomic status and travel patterns — that contribute to the virus's spread. Their final product, pictured above, is meant to help government agencies and health organizations prepare for possible disease outbreaks related to the expansion of Zika.
"This information can help public health officials effectively target resources to fight the disease and control its spread," says Dale Quattrochi, senior research scientist at NASA's Marshall Space Flight Center, in a statement.
The map assigns a circle to each city, with diameter indicating average incoming travelers from Zika-affected countries. It also depicts the known range of Aedes aegypti mosquitoes, which can carry Zika, and colors each city's circle to show potential abundance of Ae. aegypti in July. (Another version of the map adds more detail, with Ae. albopictus range and January mosquito abundance.)
The researchers found Ae. aegypti will likely expand across much of the Southern and Eastern U.S. as summer sets in. Winter conditions only suit the mosquitoes in South Florida and South Texas, they write, which can sustain low to moderate mosquito levels even in January. Conditions become suitable in all 50 cities during peak months of July to September, although Ae. aegypti hasn't been documented yet in all 50. Summer weather favors the mosquitoes along the East Coast as far north as New York City, and across the country's southern tier as far west as Los Angeles.
On top of climate, travel and mosquito abundance, the study also considered socioeconomic factors. That's because Ae. aegypti frequents densely populated places, and low-income areas often lack mosquito precautions like screened windows and air conditioning. In fact, given the difficulty in preventing Ae. aegypti from breeding, those are some of our best defenses against Zika.
This could be an issue in much of the U.S. South, as the map above illustrates, especially since that's also where the highest abundance of Ae. aegypti occurs. But higher poverty rates in cities along the U.S.-Mexico border also "may correlate with factors that increase human exposure to Aedes aegypti," the study's authors write.
Travel from Zika-affected countries, as well as across the U.S.-Mexico border, tends to spike at the worst possible time for Zika prevention, they add.
"Importantly, July and August are the two months with the highest estimated number of passengers arriving by air from countries on the CDC Zika travel advisory," the researchers write. "These months fall within the season of highest meteorological suitability for Ae. aegypti across the fifty cities. These months also have comparatively high traffic across the U.S.-Mexico border."
Zika seemed relatively mild for decades after its 1947 discovery in Uganda, but the current New World outbreak has revealed a darker side. Since Zika appeared in Brazil last year, we have learned it can cause a birth defect known as microcephaly, in which babies are born with unusually small heads and other neurological damage. It's also associated with serious health problems in adults, including Guillain-Barré syndrome and acute disseminated encephalomyelitis (ADEM).
No locally transmitted cases have been reported in the U.S. yet, but the country has seen cases in travelers returning from areas where mosquitoes are spreading the virus. And as the number of Zika cases continues spreading in the tropical Americas, the number of infected U.S.-bound travelers will likely grow as well, NASA warns. Eventually, some of those people may be bitten by Ae. aegypti or Ae. albopictus mosquitoes, which could then transmit the disease to more people.
"Knowledge is one of the most effective barriers to disease transmission and can alleviate unnecessary concern," says Cory Morin, a NASA postdoctoral program fellow at the agency's Marshall Space Flight Center. "By identifying the key risk factors and producing forecasts of disease transmission, we can enable citizens to take effective actions that will greatly reduce their risk of disease."