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.

Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, said Friday, “all the evidence we have seen indicates that this is mosquito-borne transmission that occurred several weeks ago.”

Officials believe the local transmission is confined to a small area north of downtown Miami within a single ZIP code. However, local, state and federal health officials are continuing their investigation, which includes going door-to-door to ask residents for urine samples and other information in an effort to determine how many people may be infected. Additional cases are anticipated.

It is possible that someone could have Zika without knowing, since 80% of those infected have no symptoms. When symptoms occur, they can include fever, rash, joint pain and red eyes, and they can last from a few days to about a week.

There is no treatment or vaccine for Zika. None of these four unidentified patients, which include a woman and three men, has needed hospitalization.

To date, there have been 386 cases of Zika in the state of Florida, including 55 pregnant women. The counties with the highest number of cases are Miami-Dade with 99 and Broward with 55.

Blood donations halted

The main way people become infected with the virus is through the bite of an infected mosquito. The female Aedes aegypti and its sister, Aedes albopictus, are the primary vectors, but people can also become infected through other methods, including sexual transmission. Babies can become infected in utero, and there are confirmed cases of transmission from blood transfusion and laboratory exposure.

Blood donation centers in the two affected Florida counties have stopped accepting blood from donors in the affected ZIP code until officials implement measures to screen donated blood or institute a process to deactivate the virus in blood.

In addition, individuals who have traveled to these two counties are asked to delay blood donation for four weeks after their return. The FDA said in a statement that it is “a prudent measure to help assure the safety of blood and blood products.”

More than 60 countries and territories are reporting local transmission of the virus. U.S. health officials had warned that there would be local transmission of the virus from mosquitoes but don’t expect it to be widespread, as has been seen in Puerto Rico and throughout the Americas. That’s based on outbreaks of two similar mosquito-borne diseases, dengue fever and chikungunya.

The reason is largely because of living conditions, including mosquito-control efforts and regular use of air-conditioning.

On Thursday, 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.”

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.