MIAMI, United States (AFP) – Donated blood should be tested for the Zika virus, which can cause birth defects, US regulators warned Friday amid a mounting outbreak of the mosquito-borne virus in the United States.

“There is still much uncertainty regarding the nature and extent of Zika virus transmission,” said Peter Marks, director of the Food and Drug Administration’s Center for Biologics Evaluation and Research.

“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.”

The move revises a previous FDA guideline issued in February that recommended active screening of donated blood only in “areas with active Zika virus transmission.”

But now, as evidence of sexual transmission mounts and those infected often show no symptoms, even stricter safeguards are needed across the nation, said the FDA.

“The FDA is updating its guidance after careful consideration of all available scientific evidence, consultation with other public health agencies, and taking into consideration the potential serious health consequences of Zika virus infection to pregnant women and children born to women exposed to Zika virus during pregnancy,” said an agency statement.

More than 2,500 people in the United States have been diagnosed with Zika, along with more than 9,000 in the US territories such as Puerto Rico, according to the US Centers for Disease Control and Prevention.

Most of those cases were brought in by people who were infected while traveling abroad.

There are 584 pregnant women in the mainland United States with lab evidence of Zika infection, and 812 in the US territories.

Last month, Florida announced its first cases of locally transmitted Zika, with 42 infections.

“Testing of donated blood is already underway 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,” said the FDA statement.

Indeed, one unit of donated blood in Florida was recently found to contain Zika virus and was intercepted, Marks told reporters on a conference call.

The expanded testing of the US blood supply “will be in effect until the risk of transfusion transmission of Zika virus is reduced,” said the FDA.

– Sexual spread –

Zika is primarily spread by the bite of an Aedes aegypti mosquito, but it can also be transmitted sexually.

On Friday, US authorities announced the first known case of a man who had Zika but did not know because he showed no symptoms — and then subsequently infected his female partner during unprotected sex.

Four out of five people who get Zika do not show any of the common symptoms, which may include fever, rash, joint pain and red eyes.

“As new scientific and epidemiological information regarding Zika virus has become available, it’s clear that additional precautionary measures are necessary,” said Luciana Borio, the FDA’s acting chief scientist.

The World Health Organization says 53 countries around the world have reported Zika outbreaks since 2015.

Two US lawmakers, Congresswoman Rosa DeLauro of Connecticut and Congressman Lloyd Doggett of Texas, had recently urged the FDA to expand testing for Zika to blood banks nationwide, saying it would cost less than $10 per donor.

On Friday, DeLauro applauded to decision, calling it “a strong step forward in protecting our nation’s blood supply and the American people.”