The first thing to know about the mosquito that can carry Zika is that it is not widespread in California. Most of California’s 58 counties do not have — and have never had — any infestation of the Aedes aegypti mosquito, the most worrisome carrier when it comes to Zika.

The Centers for Disease Control and Prevention has crunched data from across the country and created a national map showing where the Aedes aegypti mosquito has been found any time over the past two decades. We asked CDC for the county-specific data in California. Click on the map above to see how your county fares.

If your county has a faint neutral color, then Aedes aegypti has not been found there since 1995. Counties colored red mean the Aedes aegypti has been found in three or more years since 1995, orange means two years and yellow means one. Hover your mouse over the county and see all the years the mosquito has been found. In some cases, the mosquito has not been found there for several years.

Even if the mosquito is present in your county, that doesn’t mean it’s widespread in your county. For example, the mosquito has been found in Alameda County — but only in the city of Hayward.

Prof. Thomas Scott, a UC Davis entomologist, says that distribution of the mosquito “tends to be patchy.” Even in places where it’s very common, like Thailand, he said, “distribution will fluctuate, even through the course of a season.”

The Aedes aegypti mosquito was introduced to the Americas from Africa, likely on slave ships, perhaps as early as the 16th century, Georgetown historian Prof. John McNeill told CNN. Along with it came yellow fever, dengue, chikungunya and now Zika virus.

While the spread of the virus has been very rapid across much of South and Central America, Scott says it is less likely to spread as swiftly in the U.S. primarily because “our lifestyle is different than places where [many] people are infected.”

In the U.S. window screens or air conditioning are common, so mosquitoes are less likely to be in people’s homes. In Central and South America, people’s homes are more open, the “mosquitoes move frequently [in homes] and bite frequently,” Scott noted.

Case in point: the Aedes aegypti mosquito has been present across southern Arizona “for a long time,” Scott says, but the risk of dengue fever in Arizona is low. Meanwhile, just across the border in Mexico, the risk of dengue to people is much higher.

Zika spreads when a mosquito bites a person who is infected. Then the mosquito becomes infected itself. But Scott says it cannot spread Zika to another person right away. The virus must replicate within the mosquito first. Then the virus must travel to the insect’s salivary glands and replicate there. Only then has the mosquito moved from being infected to being infective, Scott said, and that takes awhile.

“That period of time is about the average life span of the mosquito, 10-14 days, for them to become infective,” Scott says. In other words, it’s possible the mosquito could die before it’s able to infect another person.

Zika can also be sexually transmitted and a pregnant women can transmit the virus to her fetus during pregnancy, according to the CDC.

The California Department of Public Health lists the cities where Aedes aegypti has been found. It did not get back to us by deadline to tell us when the mosquito was found in these places.

Alameda: Hayward

Fresno: Clovis, Fowler, Fresno, Kerman, Mendota, Sanger

Imperial: Andrade, Brawley, Calexico, El Centro, Heber, Holtville, Imperial, Seeley

Kern Arvin

Los Angeles: Commerce, East Los Angeles, Florence, La Mirada,Los Angeles, Maywood, Montebello, Pico Rivera, South Gate

Madera: Madera, Madera Ranchos, Parkwood

Orange Anaheim, Costa Mesa, Garden Grove, Lake Forest, Mission Viejo, Orange, Santa Ana

Riverside Coachella, Riverside, San Jacinto

San Bernardino: Colton, Montclair

San Diego: Bonita, Chula Vista, El Cajon, Escondido, Imperial Beach, Oceanside, San Diego, Spring Valley, Tecate, Vista

San Mateo: Atherton, Menlo Park

Tulare: Exeter

MAP: Here’s Where Zika Mosquitoes Are Likely Found in California 5 July,2016Lisa Aliferis

  • John Wallace

    Funny how the East Bay has them, the Peninsula riddled with them, but not a one in SF.

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Lisa Aliferis

Lisa Aliferis is the founding editor of KQED's State of Health blog. Since 2011, she's been writing and editing stories for the site. Before taking up blogging, she toiled for many years (more than we can count) producing health stories for television, including Dateline NBC and San Francisco's CBS affiliate, KPIX-TV. She also wrote up a handy guide to the Affordable Care Act, especially for Californians. Her work has been honored for many awards. Most recently she was a finalist for "Best Topical Reporting" from the Online News Association. You can follow her on Twitter: @laliferis

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