The Food and Drug Administration has asked all blood banks in Miami-Dade County and Broward County, Florida to stop collecting blood immediately until all individual donations can be tested for the Zika virus.

In a statement posted on its website late Wednesday, the FDA said it was taking this precaution because the counties are investigating four cases of the Zika virus that are not related to travel and may be the first instances of transmission by local mosquitoes in the United States.

The agency recommended that the two South Florida counties halt donations until they can test each donation for Zika or until they use an approved method to inactivate the virus. The FDA also recommended that “adjacent and nearby counties” implement the precautions. For blood collection centers outside the area, the agency suggested that people who have traveled to Miami-Dade and Broward Counties in the previous four weeks be deferred.

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Blood banks in the area are scheduled to begin testing for Zika on Aug. 1, the Miami Herald reported. Experts have said they expected transmission of Zika within the continental U.S. this summer. There have been 1,658 cases of Zika in the U.S. and 4,750 cases in the U.S. territories as of July 27, according to the Centers for Disease Control and Prevention. Despite Florida’s Zika concerns, the CDC said Thursday they are not adding South Florida to the list of restricted travel areas for pregnant women.

Zika is primarily spread by mosquitoes and can cause birth defects if women are pregnant while they have the virus. It has also been linked to other brain and autoimmune problems, including Guillain-Barré syndrome, a disorder that can cause nerve damage.

Write to Abigail Abrams at abigail.abrams@time.com.