All blood donations should be tested for the Zika virus, the Food and Drug Administration said Friday.
Zika can spread invisibly, so it's safest to test all blood donations for the virus, which causes severe birth defects if pregnant women get it and which can cause a paralyzing condition called Guillain Barre syndrome.
"There is still much uncertainty regarding the nature and extent of Zika virus transmission," said Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research.
"The potential for sexual transmission also adds another layer of complexity to the Zika virus outbreak. Work is ongoing to better understand the duration of the risk, but based on the available evidence, the possibility exists that men may transmit Zika virus to sexual partners for several months and women may transmit to their partners for a few weeks," Marks told reporters.
"Individuals who contract Zika virus through sexual transmission might be unaware and present to donate blood as well."
And just hours later the Centers for Disease Control and Prevention said a man showing no symptoms of Zika likely passed the virus to his sexual partner in Maryland — in what is believed to be the first case of its kind in the U.S.
The man had traveled to the Dominican Republic but the woman he infected had not traveled, the CDC said in a report. She was not pregnant.
There was another instance in which a man with no symptoms is believed to have passed on Zika through sex, but in that case both had traveled in a country where Zika infections were occurring, making it impossible to rule out a mosquito bite, the CDC said.
Zika is spreading at epidemic rates in Puerto Rico, and it's caused at least two local outbreaks in Florida, including in Miami Beach.
Aedes mosquitoes carry the virus, and it can also be transmitted sexually.
"Testing of donated blood is already under way in Florida and Puerto Rico, as well as in other areas, and it has shown to be beneficial in identifying donations infected with Zika virus," the FDA said.
"Expanded testing will continue to reduce the risk for transmission of Zika virus through the U.S. blood supply and will be in effect until the risk of transfusion transmission of Zika virus is reduced," the FDA added.
The FDA's guidance is not binding, but it is routinely followed by blood centers.
Two experts said adding a Zika test would not affect the U.S. blood supply.
"I don't think it will impact blood donation," said Dr. Louis Katz, chief medical officer of America's Blood Centers.
"If anything, this is going to make the blood supply safer in the whole country," agreed Joshua Buckley, Media Relations Manager at Houston's Gulf Coast Regional.
Blood is routinely tested for hepatitis B, hepatitis C, HIV, a virus called HTLV, the bacteria that causes syphilis, West Nile virus and the parasite that causes Chagas disease.
There is a device that can filter plasma and platelet donations for viruses, including Zika, the FDA said. It's sold under the brand name Intercept and uses a drug called amotosalen combined with ultraviolet light.