(Joern Pollex/Getty Images)

The U.S. Food and Drug Administration is asking blood centers in two Florida counties to immediately stop collections. The counties are investigating possible local transmission of Zika virus.

In a notice sent to blood centers and posted on the agency's website Wednesday evening, the FDA said it is requesting all blood centers in Miami-Dade or Broward counties to "cease collecting blood immediately" until those facilities can test individual units of blood donated in those two counties with a special investigational donor screening test for Zika virus or until the establishments implement the use of an approved or investigational pathogen-inactivation technology.

The action by the FDA comes as health officials in Florida said Thursday they were continuing to investigate two Zika cases that could have been spread by local mosquitoes, in addition to two similar cases they announced last week. Health officials have not confirmed whether any of the infected individuals acquired the virus from local mosquitoes, but it seems increasingly likely.

"These may be the first cases of local Zika virus transmission by mosquitoes in the continental United States," the FDA said in its notice and in a media statement Thursday. It said it was making the request of blood-collection establishments "in consideration of the possibility of an emerging local outbreak of Zika virus, and as a prudent measure to help assure the safety of blood and blood products."

Everything you ever wanted to know about the Zika virus and its spread across North and South America. (Daron Taylor,Claritza Jimenez/The Washington Post)

The FDA is also recommending that nearby counties also put these precautions in place as soon as possible to maintain the safety of the blood supply. For blood-collection establishments outside of this region, FDA is recommending that donors who have traveled to Miami-Dade and Broward Counties during the previous four weeks defer on donating blood.

The FDA alerted the Florida's surgeon general and the major blood-collection industry organization Wednesday night. On Thursday morning, the FDA also reached out to blood-collection establishments in Florida, starting with the state’s largest blood collectors, according to Tara Goodin, an FDA spokeswoman.

The FDA also said Thursday that it is working closely with companies that are making blood-screening tests available to ensure that these companies are ready to expand testing as needed. Blood-collection establishments in the rest of the United States may also choose now or in the future to participate in that testing, even if there is no local mosquito-borne transmission of Zika virus in their region.

The main organization collecting blood in Florida, OneBlood, said last week that it had been alerted by public health authorities about the suspected non-travel-related Zika case under investigation in South Florida. OneBlood said that if the case was confirmed, it will stop collections in the affected Zip code and bring in blood from unaffected areas to supply needs in that region.

The organization also said it had received approval to use the investigational tool and planned to start testing donated blood for Zika virus starting Monday.

The organization did not immediately respond to a request for comment on the FDA's request for blood centers to immediately stop collection in the two counties.

Its website says it plans to test a portion of its collected blood for the Zika virus and to allow hospitals to have access to Zika-screened blood products from unaffected areas to use with their patients at high risk of Zika complications, such as pregnant women. Hospitals that want Zika-screened products will request what they need on an on-demand basis.

Additional proactive steps OneBlood implemented earlier this year to protect the local blood supply from the Zika virus remain in effect, including enacting additional donor-deferral guidelines, updating the donor history questionnaire to include Zika-specific questions and issuing educational materials to donors.

This post has been updated.

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