The Food and Drug Administration Thursday approved the first blood donor screening test for Zika virus. The agency last year recommended blood centers screen donors for the virus using an investigational test until a licensed test was available. The new licensed test was found effective by several blood centers that used it as well as studies performed by the manufacturer, FDA said. Zika is transmitted primarily by mosquitos but also can be spread through blood transfusions and sexual contact. Infection during pregnancy can cause serious birth defects.