More U.S. counties are finding Zika-carrying mosquitoes

With the summer mosquito season in full swing in many U.S. states, a new report shows a significant increase in counties across the South that have reported mosquitoes capable of spreading Zika and related viruses.

Two types of mosquitoes are the primary transmitters of Zika, dengue, yellow fever and chikungunya viruses. Based on updated data collected through 2016, research from the Centers for Disease Control and Prevention found that 38 additional counties -- primarily in Texas but as far north as Illinois -- documented the presence of Aedes aegypti mosquitoes, Zika's main vector. That's an increase of 21 percent compared to an earlier 2016 survey.

Although Zika has faded from the headlines, researchers say the latest findings highlight the need for continued and improved mosquito surveillance. Accurate and up-to-date information on the insects' distribution has been difficult to obtain because of the patchwork system of vector control at local levels. In some places, one employee may be responsible for snow removal as well as mosquito control in what one expert described as "Chuck in a truck."

Zika is transmitted primarily by the bite of an infected mosquito or through sex. It's especially important to monitor for Aedes aegypti in states with established populations, scientists say. In addition to Texas, those include California, Arizona, New Mexico, Florida and the other Gulf Coast and Mid-Atlantic states.

But other areas that now merit close surveillance are the Midwest and Eastern United States, where states may have only one to two years of collection records to determine whether introduced mosquito populations have become established.

Zika infection poses the greatest risk for pregnant women. Earlier this summer, a CDC report laid out for the first time the risk for Zika-related birth defects in each trimester of pregnancy.

Among women with confirmed Zika infection during the first trimester, 8 percent -- nearly 1 in 12 -- had a baby or fetus with Zika-related birth defects. For infections in the second trimester, 5 percent of babies or fetuses were affected, and in the third trimester, 4 percent, according to the CDC. The report was based on the largest number of completed pregnancies with laboratory confirmation of Zika infection so far.

With the updated data, researchers say the current distribution of Aedes aegypti extends to 220 counties in 28 states and the District of Columbia, with the heaviest concentration in Southern California, Arizona, Texas, Louisiana and Florida.

The new report was published in the Journal of Medical Entomology.

The other mosquito tracked, Aedes albopictus, has now been reported in 127 new counties, a 10 percent increase from the earlier survey. The new locations are primarily in Kansas, Texas, Arkansas and Mississippi.

This mosquito, also known as the Asian tiger mosquito, has long been widespread in the Southeast, South Central and Mid-Atlantic states. Although the natural conditions in much of the arid Southwest may be harmful for Aedes albopictus, man-made water sources such as artificial containers and irrigation ditches have allowed it to persist in Southern California, Arizona, New Mexico, and West Texas, the researchers noted.

Its survival in isolated counties in Utah, Colorado and Minnesota is the result of "repeated introductions of the mosquito in used tire facilities," the report said.

Based on the latest data, researchers said 177 counties located mostly in Southern California, Arizona, Texas, Florida and Maryland reported both types of mosquitoes between January 1995 and December 2016.

The areas are based on collection records that represent the presence of the mosquito in a county, not its abundance. The report noted that the lack of a collection record from a county "should not be interpreted as absence of these mosquitoes," especially if that species has been collected from a nearby jurisdiction.