The new map shows counties in which scientists, over the past two decades, have collected Aedes aegypti mosquitoes — the type of insect thought to be spreading Zika in Latin American and the Caribbean.
"The new map is more accurate than the initial one," says Thomas Scott, an entomologist at the University of California, Davis. "The distribution of the A. aegypti mosquito is much more restricted than the initial map showed."
In the map, counties colored yellow reported A. aegypti mosquitoes during one year between 1995 to 2016. Orange counties had the mosquitoes in two years. And red counties are the hotspots: Scientists there found A. aegypti mosquitoes during three or more years in the past two decades.
This map represents "the best knowledge of the current distribution of this mosquito based on collection records," entomologist John-Paul Mutebi and his colleagues at the CDC wrote in the Journal of Medical Entomology.
Many of the hot spots for this mosquito aren't surprising. They're places that we already knew are vulnerable to Zika, including counties in southern Florida, along the Gulf Coast and southern Texas. These places have had problems with a virus closely related to Zika, called dengue. They're already on high alert for Zika.
But several hot spots are bit more unexpected — and concerning. "Perhaps the most concerning development for A. aegypti is its establishment in the Southwest, most recently in California in 2013," Mutebi and his co-authors write.
Other surprises include parts of the Bay Area, greater Washington, D.C., and the Dallas-Fort Worth region, which all have established populations of A. aegypti, the map shows.
"The country is really a patchwork," Scott says. "When you drill down into one particular state, you find that the mosquito isn't found across the whole state. And when you drill down into a county, you find the same thing. The mosquito is found in just a small part."
So why did the first map from the CDC make it look like such an extensive part of the country was at risk for Zika?
"The two maps show different things," Mutebi tells Shots. "The first map showed where the climate is able to sustain populations of A. aegypti. This new map shows reports from counties where these mosquitoes were found in the last 20 years."
And the new map, Mutebi says, is not complete. "Not all counties have mosquito surveillance programs looking for mosquitoes," he says. In places that do, they are often targeting the mosquito that causes West Nile virus, not A. aegypti.
"So just because a county hasn't reported having any A. aegypti mosquitoes, doesn't mean they're not there," Mutebi says.
A. aegypti mosquitoes are nasty critters. They chase down people so they can feed on their blood, says virologist Scott Weaver at the University of Texas Medical Branch in Galveston.