Federal disease detectives are investigating what they believe to be the first Zika infections transmitted by local mosquitoes in the continental United States.

“We consider this and are proceeding as though these cases are confirmed, local mosquito-borne transmissions,” Dr. Tom Frieden, director of the Centers for Disease Control and Prevention (CDC), said in a news conference Friday.

Today’s announcement represents a significant, but unsurprising, development in the spread of the virus. Public health officials have long warned that the U.S. mainland would likely see individual cases, or small clusters, of Zika transmission in areas where the Aedes aegypti mosquito species is prevalent. South Florida is among those places.

More on this...

At a Friday news conference, Florida Gov. Rick Scott (R) told reporters that one man and three women living in Miami-Dade and Broward Counties are believed to have contracted the virus through mosquito bites.

He said state health officials believe the transmissions occurred in a one square mile area just north of downtown Miami. However, tests of mosquitoes trapped in that area have yet to find any insects infected with the virus.

“Confirming mosquito-borne transmission is not as easy as confirming an infection in a person,” Frieden explained. “It’s like finding a needle in a haystack. So, if infected mosquitoes are not found, that doesn’t imply that it’s not spreading by mosquitoes.”

Epidemiologists rely on interviews with patients for much of their information on how they may have become infected.

Prior to today’s announcement, Zika cases on the U.S. mainland and Hawaii had involved travelers returning from countries and territories with active Zika transmission, a handful of people who had sex with these travelers and one case involving the accidental exposure of a laboratory worker.

Public health officials believe the four current cases under investigation in South Florida are unrelated to travel and that the people became infected by mosquitoes in a relatively small section of Miami back in early July.

Frieden said state officials followed up with aggressive mosquito control efforts in that area. He added that Aedes aegypti mosquitoes generally travel less than 150 meters (164 yards) over their entire lifespan.

At this point, the CDC is not recommending any travel restrictions to the area.

“If, however, we were to see cases in this area and people infected after the mosquito control efforts were undertaken, this would be a concern and warrant further advice and action,” Frieden said.

As a precaution, the Food and Drug Administration (FDA) has asked blood banks in the two affected counties to stop collecting donations until they implement ways to properly screen for the virus.

Gov. Scott said none of the South Florida cases under investigation required hospitalization.

According to public health officials, four in five people infected with Zika develop no symptoms at all. And symptoms are usually mild for those who do become ill.

However, public health officials are concerned about protecting pregnant women because of the virus’s link to microcephaly and other serious birth defects.

“We at CDC continue to recommend that everyone in areas where Aedes aegypti mosquitoes are present, especially pregnant women, take steps to avoid mosquito bites,” Frieden said.

He recommended the daily use of DEET repellant on exposed skin, wearing long sleeves and pants and staying indoors or in screened areas whenever possible.

In a statement, The March of Dimes expressed their concern about local transmission of Zika.

“This is the news we’ve been dreading,” Dr. Edward R.B. McCabe, PhD, senior vice president and chief medical officer of The March of Dimes, said in a news release. “It’s only a matter of time before babies are born with microcephaly, a severe brain defect, due to local transmission of Zika in the continental U.S. Our nation must accelerate education and prevention efforts to save babies from this terrible virus.”

Jonathan Serrie joined Fox News Channel (FNC) in April 1999 and currently serves as a correspondent based in the Atlanta bureau.