(CNN)Four individuals in Miami-Dade and Broward counties have been infected with the Zika virus by local mosquitoes, Florida health officials said Friday.
These are the first known cases of the virus being transmitted by mosquitoes in the continental United States.
"While no mosquitoes trapped tested positive for the Zika virus, the department believes these cases were likely transmitted through infected mosquitoes in this area," according to a statement from the Florida Department of Health.
The reason is largely because of living conditions, including mosquito-control efforts and regular use of air-conditioning.
Frieden said Florida has taken aggressive mosquito-control measures in the affected area, which should also limit further transmission. He said there is no cause for concern unless additional cases are identified after those measures had begun.
As ofThursday, the CDC reported 1,658 cases of the virus in the continental United States and Hawaii. None of those cases is a result of local mosquito transmission.Fifteen of those individuals were infected by sexual transmission, and there is one case of a laboratory-acquired infection. (The CDC's numbers do not always include the most current cases reported by states.)
Nearly every state is reporting cases of the virus. Only Idaho, South Dakota, Wyoming and Alaska have not reported it.
Risks for pregnant women
Pregnant women are at greatest risk because the virus can have devastating consequences for an unborn baby, including the birth defect microcephaly and other neurological deficits, as well as miscarriage and stillbirth among women who were infected while pregnant.
The exact risk that an infected woman will have an affected baby remains unknown, but some studies have shown that it is between 1% and 13%.
Frieden called this unprecedented, saying, "never before in history has there been a situation when a bite from a mosquito can result in such a devastating scenario."
Local transmission "is the news we've been dreading," said Dr. Edward McCabe, chief medical officer and senior vice president of 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. Our nation must accelerate education and prevention efforts to save babies from this terrible virus."
Frieden restated that it's important for pregnant women in any area where the Zika-carrying Aedes aegypti mosquito is found to protect herself against bites with the use of repellant and by wearing long pants and long-sleeve shirts.
At least 13 infants have been born with Zika-related birth defects in the continental U.S. and Hawaii, and there have been six sudden or voluntary Zika-related pregnancy losses reported. There are more than 400 pregnant women with the virus in the United States.
In February, the World Health Organization declared a "public health emergency of international concern" because of an alarming increase in cases of microcephaly linked to the virus.