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Zika Virus Outbreak

4 Zika Cases In Florida Likely ‘Homegrown’

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Aedes aegypti mosquitos are seen in a lab at the Fiocruz Institute on June 2, 2016 in Recife, Brazil. Microcephaly is a birth defect linked to the mosquito-borne Zika virus where infants are born with abnormally small heads. Mario Tama / Getty Images

4 Zika Cases in U.S. Likely Caused By 'Homegrown' Mosquitoes 2:24

The four cases of Zika infection in Miami, likely the result of bites from local mosquitoes, may be just the beginning of the spread of the virus in the continental US, a health official said Friday.

The Florida Department of Health reported Friday that the four people — a woman and three men — infected earlier this month in Miami are the first known "homegrown" cases of Zika virus in the US.

"We anticipate that there may be additional cases of 'homegrown' Zika in the coming weeks," Dr. Lyle Petersen of The Centers for Disease Control and Prevention said in a statement.

Although no mosquitoes have tested positive for the Zika virus yet, Florida health officials believe that active transmission of the Zika virus is occurring in a one-square-mile area just north of downtown Miami.

"Zika is now here," CDC director Dr. Tom Frieden, confirmed in a call Friday. "It's important for pregnant women to avoid exposure," by using DEET, wearing long sleeves and avoiding mosquito bites.

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The Florida Health Department is going door-to-door in the area, collecting urine samples. Anyone living in the area who wants to be tested for Zika should contact the Miami-Dade county department of health.

Related: Florida's 4 Zika Cases Likely Came From Local Mosquitoes

Once summer hit, experts have been anticipating the first cases of local transmission — especially in hot, humid areas like Florida where a big population of the Aedes aegypti mosquito is able to spread Zika.

The Miami cases apparently were infected in early July; they became sick a week later and were diagnosed shortly after, Frieden said on the call. Immediately after testing — before the diagnosis was confirmed — Florida health officials began mosquito eradication, spraying and getting rid of standing water in the area.

The Aedes mosquito doesn't travel more than 150 meters (450 yards) in its lifetime, Frieden said. While more cases are possible, a country-wide outbreak in the US is not expected.

"We're not taking this lightly, we're preparing as best as we can for the worst," says Dr. Anthony Fauci, head of the National Institute for Allergy and Infectious Diseases. "But we do not believe that there will be a widespread outbreak. There will be local transmission as we're seeing right now with these four cases."

Related: Bracing for Zika: A Tale of Two Cities

Pregnant women or women who could become pregnant are advised to be extra cautious because of the possibility of devastating birth defects in babies born to infected mothers.

"We continue to recommend that everyone in areas where Aedes aegypti mosquitoes are present—and especially pregnant women—take steps to avoid mosquito bites," Frieden said Friday.

Staying away from areas where mosquitoes are circulating is the best way to avoid being bitten. Repellents and sprays containing DEET are among the most effective, the CDC advises. DEET is safe for pregnant women and for children to use.

Zika guidelines for pregnant women updated by CDC 0:19

The most common symptom of Zika is a raised rash. People also report fever, muscle aches and red eyes while others don't remember any symptoms at all. There's no cure for Zika infection but fortunately for most people, the infection is mild and clears up in about a week. Doctors also say evidence suggests that once infected with the Zika virus people develop an immunity.