THURSDAY, Oct. 20, 2016 (HealthDay News) — U.S. health officials are now recommending that all pregnant women who have recently spent time in any part of Miami-Dade County in Florida be tested for Zika infection.

Previously, testing had only been urged for pregnant women who had been in areas of the county where Zika had been spreading locally. This latest advisory extends that recommendation to the entire county and covers the period going back to Aug. 1, the U.S. Centers for Disease Control and Prevention said in a statement Wednesday afternoon.

Reports of local spread of the mosquito-borne virus continue to come in from Miami-Dade County, the CDC said. Florida is the only state to report local transmission of the virus that can cause devastating birth defects.

"Zika continues to pose a threat to pregnant women living in or traveling to Miami-Dade County," Dr. Lyle Petersen, director of the CDC's division of vector-borne diseases, said in the statement. "Our guidance today strengthens our travel advice and testing recommendations for pregnant women, to further prevent the spread of the infection among those most vulnerable."

There have been 165 Zika cases in the state spread by local mosquitoes, according to the Florida Department of Health.

Zika is typically spread by the bites of mosquitoes, but it can be spread through sex. In most people, the symptoms are mild. But, infection during pregnancy can cause babies to be born with severe birth defects that include microcephaly, where the brain and head are abnormally small.

The new testing advice also extends to pregnant women who weren't in Miami-Dade themselves, but who had unprotected sex with someone who had been in the county recently.

Pregnant women should still postpone travel to the county if possible and stay out of the two remaining areas where Zika is spreading locally, Miami Beach and an area just north of the Little Haiti neighborhood, the CDC said.

At one point, Florida had three areas where Zika was spreading locally, but the Wynwood neighborhood is no longer considered an active transmission zone.

In clearing the Wynwood neighborhood of Zika, a combination of insecticides was needed to beat back the Aedes aegypti mosquito that carries the virus, state and health officials said.

While ground spraying was ineffective, aerial spraying with the insecticides naled and Bti (bacillus Thuringensis) dramatically reduced the mosquito population and local transmission of Zika, CDC Director Dr. Tom Frieden said when Wynwood was declared free of Zika in September.

"This really heralds in a new era of [mosquito] control," Frieden said. "It appears that the aerial application of the one-two punch has the ability to rapidly interrupt transmission. It doesn't mean that the area is immune from future spread, but the findings are quite striking," he added.

These pesticides kill both the insect and its larvae, preventing the birth of new mosquitoes, Frieden said.

Aerial spraying is part of a comprehensive mosquito-control program that also includes encouraging people to get rid of standing water on their property and protect themselves against mosquito bites as well as using ground spraying in hard-to-reach areas, Frieden said.

This breed of mosquito has been particularly hard to control, Frieden said. And it's impossible to know if spraying will work in the long run, he added. But after spraying, the mosquito population in the Miami area dropped significantly, as evidenced by the low numbers of insects found in mosquito traps, he said.

The Zika epidemic has been centered in Latin America and the Caribbean, with Brazil reporting the largest numbers of infections and microcephaly.

To reduce the risk of local Zika transmission within the United States, the CDC recommends that people returning from countries with ongoing infections should use mosquito repellent every day for three weeks and follow the CDC's guidelines to prevent sexual transmission of Zika.

The virus can be spread by infected men and women to their sex partners. There is no vaccine or treatment for Zika.

The CDC advises that partners of pregnant women use a condom to guard against sexual transmission during pregnancy.

More information

The U.S. Centers for Disease Control and Prevention provides more information on mosquito-borne diseases.

This Q & A will tell you what you need to know about Zika.

To see the CDC list of sites where Zika virus is active and may pose a threat to pregnant women, click here.