WASHINGTON (REUTERS) - The US Food and Drug Administration recommended on Tuesday (Feb 16) that blood should no longer be collected from regions where the Zika virus is circulating, and that blood needed for transfusions be obtained from areas of the country without active transmission.

The agency said blood banks can continue collecting and preparing platelets and plasma if an FDA-approved pathogen-reduction technology is used. Current pathogen-reduction technology is not approved to treat whole red blood, which is used for most transfusions.

The guidelines come as Zika is spreading rapidly through the Americas, with more than 30 affected countries in Latin America and the Caribbean. The virus has been linked to a spike in cases of a rare birth defect known as microcephaly in Brazil, prompting health officials to declare a global health emergency.

Researchers have begun to study the consequences of Zika transmission through the blood, but that work could take six to 12 months to produce results. In the meantime, the FDA said people at risk of having been infected with the virus defer donating blood for at least four weeks.

"We believe the new recommendations will help reduce the risk of collecting blood and blood components from donors who may be infected with the Zika virus," Dr Peter Marks, director of the FDA's biologics division, said in a statement.

The Centres for Disease Control and Prevention (CDC) has identified Puerto Rico, the US Virgin Islands and American Samoa as areas with active Zika transmission and health experts expect some localised outbreaks may occur in the south-eastern United States later this year.

There is considerable evidence from prior Zika outbreaks that the virus can be transmitted in the blood. What is less clear is whether that transmission causes the recipient to become ill.

Dengue and chikungunya viruses, which are carried by the same mosquito as the Zika virus, do not typically cause illness in patients when transfused.

Much remains unknown about Zika, including whether the virus actually causes microcephaly.

Brazil is investigating the potential link between Zika infections and more than 4,300 suspected cases of microcephaly. Researchers have confirmed more than 460 of these cases as microcephaly and identified evidence of Zika infection in 41 of these cases, but have not proven that Zika can cause microcephaly.

The studies under way in Brazil to further understand the clinical consequences of Zika transmission through the blood could take six to 12 months to complete.

"No one feels comfortable waiting for the results of those studies to be concluded," said Dr Michael Busch, director of the Blood Systems Research Institute. "There's a lot of work going on to understand the value and need for blood screening."

Dr Busch, who is widely considered one of the world's leading experts on transfusion medicine, added that it is possible for private companies to develop a test for investigational use by the time the mosquito season moves into full swing in the summer.

The two companies the government is relying on to make such a test are Hologic Inc and Roche Holding AG. Each has platforms that it can turn to making a virus test quickly.

Roche spokesman Nicole Rueppel said the company is testing prototypes in its laboratories but that further evaluations with clinical samples need to be conducted.

The FDA said its guidance is for immediate implementation in areas with active Zika virus transmission for those at highest risk, including pregnant women. It recommends implementation within two weeks for the remainder of the population in such areas.

The FDA recommended that areas with active Zika transmission fulfill blood orders from areas in the US without transmission except when the blood is tested with an FDA-licensed or investigational screening test.

The agency also suggested exceptions could be made for platelets and plasma subjected to pathogen-inactivation treatment.

The FDA also formally endorsed recommendations made by the American Association of Blood Banks that donors at risk for Zika virus be deferred from donating blood for four weeks.

That includes people who have had symptoms suggestive of Zika virus during the previous four weeks, those who have had sexual contact with a person who has travelled to, or lived in, an area with active Zika virus transmission during the prior three months, and those who have travelled to areas with active transmission of Zika during the past four weeks.