WASHINGTON (AP) — The government told all U.S. blood banks Friday to start screening for Zika, a major expansion intended to protect the nation’s blood supply from the mosquito-borne virus.

Previously, blood testing was mostly limited to parts of Florida and Puerto Rico, where Zika is spreading. Screening will initially extend to states along the Gulf Coast and a few others.

“There is still much uncertainty regarding the nature and extent of Zika virus transmission,” Dr. Peter Marks said in a Food and Drug Administration release. “At this time, the recommendation for testing the entire blood supply will help ensure that safe blood is available for all individuals who might need transfusion.”

Blood banks already test donations for HIV, hepatitis, West Nile and other blood-borne viruses.

The Zika virus stays in the blood for about one week, but is thought to remain in other bodily fluids longer.

While Zika is primarily spread through mosquito bites, there have been reports in Brazil of Zika transmission through transfusion.

No such cases have been reported in the United States. One Zika-positive blood donation, though, was recently intercepted in Florida, Marks said Friday.