The US Food and Drug Administration has ordered blood banks in Florida's two most densely populated counties to stop collecting blood as health officials determine whether Zika has begun transmission in continental US.
Florida has been investigating four possible cases of local transmission in Miami-Dade and Broward Counties. It is the first US state to report cases that may not be related to travel to other countries with active outbreaks.
Zika has struck hardest in Brazil, where the outbreak was first detected last year, and has since spread through the Americas. The virus can cause a rare birth defect, microcephaly, in newborns whose mothers have been infected, and is believed to be linked to Guillain-Barre syndrome in adults.
Zika most commonly infects people via mosquito bite. But reports of the virus being transmitted through sex and blood transfusions has prompted public health officials to recommend additional precautions for sexual partners and blood banks.
In a statement posted online on Wednesday, the FDA said blood centres in the two Florida counties should stop collecting blood until they could test each unit or put in place technology that could kill pathogens in the blood.
The FDA also recommended nearby counties implement the same measures as it moved to prevent transmission of the virus through the blood supply.
OneBlood, Florida's biggest blood collection centre, said it would begin testing all of its collections for Zika virus, effective July 29, using an investigational screening test.
The FDA has authorised the emergency use of several investigational Zika screening tests, including products made by Hologic Inc and Roche Holding AG.
The agency has also approved a pathogen inactivation technology made by Cerus Corp that kills the virus in blood platelets and plasma. The company is conducting clinical trials to show it can also kill pathogens in red blood cells.
Unlike oxygen-carrying red blood cells, which can be kept for 42 days in a refrigerator, or plasma, which keeps for a year if frozen, platelets have a shelf life of just four to seven days.
Dr Richard Benjamin, chief medical officer for Cerus, said because of platelets' short shelf life hospitals typically did not keep much surplus.
"All we need is a few more Zika hotspots and there will be a shortage of platelets across the country," Benjamin said.
The FDA's action follows Florida's announcement on Wednesday that it had identified two additional Zika cases - one more in each county - that were not related to travel to an area where the virus was being transmitted.
A Centres for Disease Control (CDC) spokesman said on Wednesday "evidence is mounting to suggest local transmission via mosquitoes" in South Florida, noting the cases fitted transmission patterns seen with prior mosquito-borne outbreaks such as Chikungunya.
