Liz Szabo

USA TODAY

Florida confirmed Monday 10 more homegrown cases of Zika in people infected by local mosquitoes, leading federal health officials to advise women who are pregnant to avoid the area just north of downtown Miami where Zika is spreading.

Women who have visited the Wynwood neighborhood in Miami since June 15 should avoid getting pregnant for at least eight weeks, according to the Centers for Disease Control and Prevention. Zika, which can cause devastating birth defects, appears to have begun spreading among mosquitoes in this area around June 15.

CDC officials believe this is the first time that the federal health agency has warned people to avoid a community in the continental U.S., said agency spokesman Tom Skinner.

At Florida's request, the CDC is sending an emergency response team to the state to help control the outbreak. Two CDC staff are already working in Florida, with six more planning to join them.

The new Zika cases in Miami bring the number of Zika infections spread by local mosquitoes — as opposed to foreign travel — to 14.

Controlling the cluster of Zika cases is proving difficult, CDC director Tom Frieden said, noting that more diagnoses related to the neighborhood are possible.

"We don't have ideal ways to control the mosquitoes that control Zika," Frieden said. "In Miami, aggressive mosquito control measures don't seem to be working as well as we would have liked."

All pregnant women should take steps to prevent mosquito bites, Frieden said. The mosquito species that spreads Zika, the Aedes aegypti, lives in 30 states, in addition to territories such as Puerto Rico and the U.S. Virgin Islands.

CDC: All pregnant women should be assessed for Zika exposure

According to the CDC, 1,658 people in the continental U.S. and Hawaii have been diagnosed with Zika. Nearly 20% of those cases have been in Florida.

The true number of Zika cases in the U.S. is likely "many times" higher than these totals, Frieden said, because four of five people with Zika have no symptoms. Those who do develop symptoms tend to have fevers, a rash, pink eye, joint pain and other symptoms.

The local cases in Miami are a major development, because — with the exception of one Zika case related to a lab accident — all of the Zika infections in the U.S. until now have been been diagnosed in people who traveled abroad or in people who had sex with a traveler. Although Zika is largely spread by mosquitoes, both men and women can transmit the virus sexually.

The Florida Zika outbreak so far is limited to a 1-square-mile area just north of downtown Miami, Gov. Rick Scott said Monday. Six of the 10 people infected with Zika had no symptoms and were identified through door-to-door community outreach, according to Florida officials.

Mosquito control staff in Miami have used pyrethroid insecticides, which don't appear to be working, Frieden said. It's possible that the mosquitoes are resistant to the insecticides or that the mosquitoes are breeding in hidden areas that works aren't able to find.

Controlling mosquitoes in an area as diverse as Wynwood, the Miami area where the outbreak is centered, is also challenging, Frieden said.

In the past decade, Wynwood has been transformed from a warehouse district to a bustling artistic enclave, with studios, bars and outdoor restaurants all around. With so many tourists and locals teeming throughout the colorful neighborhood, Wynwood has become one of the most pedestrian-heavy sections of a city where people rarely walk due to the overwhelming heat.

The Design District, just north of Wynwood, has grown into a high-end retail destination with wine tastings luring wealthy clients from South Florida, Latin America and beyond. The streets are lined with restaurants and stores including Bulgari, Fendi, Giorgio Armani, Hermes, Louis Vuitton, Prada and Tom Ford.

Tucked in between, Midtown has seen a construction boom with a mega-mall and several high-rise condos to respond to the growing demand of people wanting to move into the revitalized pocket of Miami.

Zika Q&A: What you need to know about the virus

The CDC warned pregnant women to avoid places with ongoing Zika outbreaks, including Puerto Rico, in January. But public health experts say they can't remember the last time that a federal agency told people to avoid a community in the U.S. because of an outbreak.

The last outbreak that led people to avoid an American city may have been polio in the 1940s, said infectious disease expert William Schaffner, a professor of preventive medicine at Vanderbilt University School of Medicine in Nashville. Although health officials back then told people to avoid New York City to reduce their risk of polio, that was before the formation of the CDC.

Gov. Scott advised women with questions about Zika to talk to their doctors.

“While we continue to learn more about this virus each day, we know that it is most harmful to pregnant women and their babies," Scott said. "For women who live or work in the impacted area and are either pregnant or thinking of becoming pregnant, I urge you to contact your OB/GYN for guidance and to receive a Zika prevention kit."

Frieden said scientists are learning more about Zika every day.

"What we know about Zika is scary," Frieden said. "What we don’t know about Zika is even more unsettling."

Advising travelers to avoid any part of Florida is a sensitive decision, given the importance of tourism for the state's economy. Scott noted that a record 30 million tourists have visited Florida this year.

Zika has been diagnosed in at least 855 pregnant women, with about half of those cases in Puerto Rico, according to the CDC. Thirteen babies in the U.S. have been born with Zika-related birth defects, and seven pregnancies have been lost to miscarriage or abortion due to Zika.

The U.S. has been able to contain past clusters of mosquito-borne diseases with aggressive mosquito control. Those outbreaks have included dengue outbreaks in Texas in 2013 and the Florida Keys in 2009. Federal health officials have predicted that the continental U.S. is likely to experience clusters of Zika cases, but probably won't see the sort of large-scale epidemic that has hit Brazil, where Zika cases have been concentrated in crowded urban slums where residents have no screens or air conditioning.

In Miami, health officials have gone door-to-door to talk to residents about Zika. They've taken samples from more than 200 people who live or work near those diagnosed with Zika, all in Miami-Dade or Broward counties, according to the Florida Department of Health.

Health workers tested close contacts and community members within a 150-meter radius of the area where Zika patients were infected, because that's the maximum distance that the Aedes aegypti mosquito has been known to travel.

In all, Florida health officials have tested more than 2,300 people across the state for Zika since the outbreak began.

The health department has conducted testing for the Zika virus for more than 2,300 people statewide, including travelers who were tested after returning from an area where the virus was spreading widely.

Contributing: Alan Gomez.