Florida and federal officials on Friday confirmed the first local spread of the Zika virus through infected mosquitoes in the continental United States.

Florida Gov. Rick Scott (R) made the announcement following a state health department investigation into four suspected cases in Miami-Dade and Broward counties. Scott said transmission was confined to a small neighborhood just north of downtown Miami and involved one woman and three men.

“We learned today that four people in our state likely have the Zika virus as a result of a mosquito bite," he said during a news conference. "All four of these people live in Miami-Dade and Broward counties, and the Florida Department of Health believes that active transmissions of this virus could be occurring in one small area in Miami."

The Centers for Disease Control and Prevention, which has been closely coordinating with Florida and sent a medical epidemiologist at the state's request, made a similar announcement a short time later that left no doubt about the genesis of the cases.

"These are the first cases of locally transmitted Zika virus in the continental United States," CDC Director Tom Frieden said in a news briefing Friday. "As we have anticipated, Zika is now here."

Frieden praised state officials for responding rapidly with aggressive mosquito-control measures and a community-wide search for additional Zika cases. Because the virus can have devastating consequences for a fetus, the CDC recommends that pregnant women or women thinking about becoming pregnant postpone travel to Zika-affected regions. However, Frieden said no travel limitations in this country are currently warranted.

"We don’t expect widespread transmission in the continental United States," he said. "That is not the situation we're in today."

But if more people become infected despite ongoing mosquito-control measures, "this would be concerning and warrant further advice and action." The CDC will reassess daily and revise its recommendations accordingly, he said.

The infections occurred in early July in a several-block radius of Miami's Wynwood neighborhood, a popular restaurant and entertainment area. The individuals became sick a week later and were diagnosed a few days after that. Strong evidence suggests that at least two of the people were bitten at work sites near each other in that area, Frieden said.

Lyle Petersen, who is managing the CDC's Zika response, said the agency expects "there may be additional cases of ‘homegrown’ Zika in the coming weeks."

Scott said state officials continue to "put every resource available into fighting the spread of Zika in our state," adding that Florida has been preparing for this type of situation for months, much in the way it prepares for hurricanes.

"We know this virus is most detrimental to expecting mothers," he said. "If you are pregnant or thinking of becoming pregnant and live in the impacted area, I urge you to contact your OB/GYN for guidance and to receive a Zika prevention kit. I also ask every Floridian to take proper precautions by eliminating any standing water and wearing insect repellent."

The announcement had been expected since officials revealed a week ago that they were investigating a case of non-travel-related infection. They subsequently acknowledged an additional case in Miami-Dade County and two more suspected cases in neighboring Broward County. Frieden said Friday that all four individuals were infected in Wynwood but that two live in Broward County.

No mosquitoes have yet tested positive for the virus, though. Frieden explained that confirming infections in mosquitoes is much harder than confirming them in people -- which is why there can be local transmission even in the absence of positive insect tests.

Celeste Philip, the state's surgeon general and health department secretary, said officials have been successful in limiting transmission of similar viruses carried by the same mosquito species in the past and expect they will do so for Zika.

"We believe at this time, the likelihood of ongoing transmission is low," she said.

Yet the risk of a local Zika outbreak prompted the Food and Drug Administration on Wednesday to direct all blood collection centers in Miami-Dade, Broward and surrounding areas to stop accepting donations until the blood can be tested for the virus.

In a statement Friday, OneBlood, the main organization collecting blood in Florida said that, effective immediately, all blood collections in its service area in Florida, Georgia, Alabama and South Carolina will be tested for the virus using an experimental donor-screening test. It said operations remain business as usual.

Florida, Texas and other parts of the Gulf Coast are considered at highest risk for local spread of the mosquito-borne virus. The region is home to the Aedes aegypti mosquito, the primary vector for transmitting Zika. Both states have had local cases of dengue and chikungunya, related viruses that are spread by the same mosquito species.

Florida already has 386 travel-related Zika cases, more than any other southern state. Of those, 55 involve pregnant women. Miami-Dade has 99 travel-related cases, the most in the state.

Stephen Higgs, director of Kansas State University's Biosecurity Research Institute, explains the anatomy of a mosquito. (Kansas State University)

Philip said transmission is confined to a one-square-mile area, which falls largely within the 33127 Zip code. Its boundaries are NW Fifth Avenue on the west, U.S. Route 1 on the east, NW/NE 38th Street on the north, and NW/NE 20th Street on the south. It is the only place where state officials are testing for local transmission.

According to Census Bureau data, the area is home to about 30,000 people, many of them black and Hispanic. More than one-third of residents there are foreign-born, and more than 40 percent live below the poverty level.

Officials have been spraying in targeted neighborhoods and trapping mosquitoes, as well as going door to door to interview and collect blood and urine samples from more than 200 people to test for the virus. They also distributed Zika-prevention kits and repellent to local obstetrician offices and at local county health offices.

Karen Harris, an obstetrician who heads Florida's chapter of the American Congress of Obstetricians and Gynecologists, said the news in recent days has been worrisome to clinicians and their patients.

"There is huge concern," said Harris, who practices in northern Florida. One of the biggest challenges is that four out of five people infected with Zika won't have symptoms. "All the obstetricians have been very proactive with patients, but now we really have to step it up," she said.

Adam Putnam, Florida's agriculture commissioner, said Friday that officials have tested more than 19,000 mosquitoes and have not found Zika. Even so, experts say finding a virus present in specific mosquitoes can be a needle-in-a-haystack endeavor.

Widespread transmission is less likely because "there are significant differences in the residential neighborhoods between Florida and some of the other impacted countries,” Putnam said. He cited air conditioning and well-sealed houses with screens on windows.

“Let’s be very clear about that," he said. "The opportunity for [mosquito] habitat in Florida, while Florida is a warm, wet, subtropical climate, is very different than the nations that have seen much, much higher incidence of Zika spread — largely because higher standards of living in the state of Florida."

Scott said President Obama called him Wednesday and said the administration was going to send $5.6 million in aid to Florida to help in its Zika fight. To date, CDC said it has provided Florida more than $8 million in Zika-specific funding and about $27 million in emergency preparedness funding that can be used for response efforts.

Still, Scott said the lack of federal aid thus far on Zika has been "disappointing."

“I went to Washington and met with members of Congress to talk about the funding. I talked to the White House, the [health and human services] secretary," he said. "Congress didn’t fund, and they went on recess.”

Congress left town in mid-July without finalizing legislation to combat the virus, much to the dismay of public health officials, infectious disease experts and children's advocates. Health officials have warned that the $589 million the Obama administration recently redirected from fighting Ebola to combating Zika is insufficient and that lawmakers' failure to approve new funding is delaying work on a vaccine, improved diagnostics to test for Zika and research on the long-term consequences of the virus during pregnancy.

"I would expect that Congress can do more to help us, as I’ve said all along,” Scott said. “The federal government needs to do their part. This is not just a Florida issue. This is a U.S. issue, it’s a national issue. We’re just at the front of it.”

White House spokesman Erik Schultz said Friday that the president had been briefed on the finding in Florida and directed federal agencies to not only monitor the ongoing situation but provide resources and support. Schultz added that Congress should approve the administration's $1.9 billion request to combat Zika outbreaks in the United States.

Senate Minority Leader Harry M. Reid (D-Nev.), who said the reports highlight the seriousness of the Zika threat, called for the House and Senate to return to Washington immediately to provide the needed funding. "Things will only get worse if Republicans continue their refusal to work with Democrats on a bipartisan response," he said.

Republican Rep. Vern Buchanan, who represents a congressional district in southwest Florida, reiterated his call for full emergency funding for Zika. “We should be laser-focused on protecting the most vulnerable among us," he said. "Florida is ground zero for Zika.”

Advocacy groups and others also urged Congress to act.

“This is the news we’ve been dreading," said Edward McCabe, chief medical officer at the March of Dimes. "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."

In comments that seemed to be aimed as much at tourists as residents, Scott stressed how the state has successfully dealt with previous local transmission of dengue and chikungunya and how it will use the same approach now.

"Florida is an outdoors state with pristine beaches, award-winning state parks and world-class fishing," he said, adding that the state continues to welcome visitors and remind them to take proper precautions and wear insect repellent.

The virus is linked to microcephaly and other serious birth defects. Zika spreads to people primarily through the bite of an infected Aedes species mosquito (Aedes aegypti and Aedes albopictus), but it can also be spread during sex by an infected person. Most people with Zika won’t have symptoms, but for those who do, the illness is usually mild.

Federal health officials have said they would send a rapid-response team to any community on the mainland and in Hawaii where Zika begins to be transmitted locally — even if only a single case of infection is confirmed. The CDC also is prepared to deploy experts to help state and local authorities in monitoring cases, performing laboratory tests and increasing mosquito control as part of a multilevel response plan. The teams of 10 to 15 people will go if invited by the state.

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