Blood donations in the U.S. aren't routinely screened for the Zika virus.
There's been a looming fear that mosquitoes would start spreading the Zika virus in the U.S. Now that possibility seems increasingly real.
On Thursday, senior officials at the Food and Drug Administration said they have asked blood donation centers in two Florida counties, Miami-Dade and Broward, to stop collecting blood for the time being.
The move came after investigators ruled out travel as the cause of four cases of Zika virus in those counties. Florida health officials announced the cases last week. The people hadn't traveled to places where Zika is endemic and don't appear to have contracted it through sex, leaving a possibility that they got the virus from being bitten by infected mosquitoes in the U.S.
If they did acquire the virus from domestic mosquitoes, it could mean that others in the area also may have acquired Zika virus locally, and may have donated blood without knowing it was infected.
In an effort to "help ensure the safety of the nation's blood supply," Dr. Peter Marks, Director of the FDA's Center for Biologics Evaluation Research, said in a statement that blood collections shouldn't resume in the area until the centers can test each incoming blood donation for Zika virus.
The agency suggested that neighboring counties do the same, and that at-risk counties across the U.S. screen donors before collecting blood.
"These may be the first cases of local Zika virus transmission by mosquitoes in the continental United States," said the statement. The Florida department of health has yet to confirm how the people acquired the virus.