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Zika Mosquitoes Are in More Places Than You Thought, CDC Says

States especially affected include California, Arizona, New Mexico, Texas, Florida and other Gulf states, and the mid-Atlantic states.
A mosquito on a man's leg.Ye Aung Thu / AFP - Getty Images

The mosquitoes that carry Zika virus, dengue, chikungunya and yellow fever are more common across the United States than previously believed, federal experts reported Tuesday.

Updated maps for 2016 show the Aedes aegypti mosquito in 38 counties where it wasn’t found before — a 21 percent increase, the Centers for Disease Control and Prevention reported.

States especially affected include California, Arizona, New Mexico, Texas, Florida and other Gulf states, the mid-Atlantic states, as well as big cities such as Chicago, where the mosquitoes keep getting brought back, the CDC said.

The newly discovered populations don’t necessarily mean that Aedes aegypti is newly arrived. It more likely is being found because of better surveillance, the CDC said.

Related: Meet the Mosquito Hunters

“Most of the states have intensified their surveillance efforts in the past year,” CDC research biologist Rebecca Eisen told NBC News.

Zika arrived in Brazil in 2013 or 2014 and has rapidly spread across the Americas, carried by infected travelers who transmit it to local mosquito populations. The main carrier is Aedes aegypti, a mostly tropical mosquito. A cousin, the Aedes albopictus, also known as the Asian tiger mosquito, can also carry Zika but it’s not been shown to be a major source of transmitting Zika to people.

The CDC also showed that Aedes albopictus is more common than previously believed, however. It’s been found in 127 new counties, a 10 percent increase over 2015, the team reported in the Journal of Medical Entomology.

Mosquitoes spread all sorts of viruses, including West Nile virus and St. Louis encephalitis. Different species spread different viruses. CDC’s worried about Zika because it causes profound birth defects in babies born to women infected while they are pregnant.

So far, 1,883 pregnant women in the 50 U.S. states have been reported infected with Zika, and 80 babies have been born with birth defects or have miscarried or been aborted because of severe, Zika-related birth defects in the U.S.

Related: Birth Defects Seen in 5 Percent of Affected US Pregnancies

Just because a county has reported finding mosquitoes doesn’t necessarily mean they are common there or that they are carrying any viruses, Eisen stressed. Sometimes a single mosquito has been found in a trap filled with other species of mosquito that don’t spread Zika, dengue or yellow fever.

And states with no evidence of Aedes species are not necessarily completely in the clear, either. Just because a mosquito has not been trapped doesn’t mean it’s not there.

For instance, Missouri, Georgia and parts of Florida should have more evidence of Aedes mosquitoes than have been found, the CDC team said.

Related: U.S. Not Prepared for the Next Biothreat

Mosquitoes only travel a few hundred yards in their lifetimes but they can hitch rides in tropical plant containers and in tires.

“One method that we think is important is the transport of new tires,” Eisen said. “Tire recycling facilities are also associated with these species.”

Mosquitoes lay their eggs right above the water line in containers and in tires. The eggs stay stuck there until the tire or container fills up with water again.

If the mosquito larvae hatch and if the temperature is warmer than 50 degrees, they’ll survive. Aedes albopictus can survive even cooler temperatures, and mosquitoes can overwinter in sewers, heated garages, unused basements, bathrooms and elsewhere — even in places as far north as Chicago.

The CDC's advice for preventing mosquito bites:

  • Empty anything that can hold water where mosquitoes might breed, and that includes pet dishes, roof gutters and even bottle caps
  • Wear protective clothing including long sleeves and trousers
  • Use a DEET-based insect repellent when outside (CDC has a list of good ones)
  • Stay inside behind screens during mosquito weather