"Based on the best info available, Zika is increasing rapidly in Puerto Rico," Frieden said. "The importance of this is that thousands of pregnant women could become infected, which could lead to dozens or hundreds of babies born with microcephaly."
Microcephaly is a birth defect in which the baby is born with a small head and brain, which often leads to serious developmental delays or even death.
Though Frieden has sounded the alarm about projected cases of microcephaly in Puerto Rico in the past, the current warning is based on data coming in from a newly developed nucleic acid blood screening test, authorized by the Food and Drug Administration under an investigational new drug application.
The test, which has been in place since April 3, is "very sensitive, able to identify even a few copies of virus per milliliter of blood," and has shown no false positives to date, said Dr. Matthew Kuehnert, director of the CDC's Office of Blood, Organ and Other Tissue Safety.
Between April 2 and June 11, the new test discovered 68 active cases of Zika out of a total of 12,777 blood donations, and officials removed those from the blood supply. Though that number may seem small, the CDC says the number has been consistently rising. Just last week, 1.1% of the blood donor supply in Puerto Rico tested positive for Zika.
"An increase in active Zika infections in Puerto Rico's blood donor supply likely means Zika infections are rising among the general Puerto Rican population," Frieden said. "While the blood supply is safe, our priority is to protect pregnant women from becoming infected."
Frieden emphasized the need for residents in areas of active transmission of the Zika virus to use protective measures
such as DEET and long-sleeve clothing, and to work to reduce mosquito breeding sites, adding that it "takes an entire community" to protect women.
"We believe that the highest risk for microcephaly is in the first trimester of pregnancy," Frieden said, but he pointed out that even babies born without microcephaly to infected mothers are still at risk. "We do not yet know what the long-term impact may be on the babies."
There is no government mandate to do so, but one independent blood center in Texas has been using the new test to screen blood for Zika even though the virus is not active in the United States. Gulf Coast Regional Blood Center, which is based in Houston and serves most of eastern Texas, has screened 9,000 blood donations since May 23 and found no positives. That's good news for the test's accuracy, said the CDC's Kuehnert.
"So if a test was 99% accurate, that means that one out of a 100 times, you'd get a false positive," he explained. "In Houston, there has been 9,000 tests, and there's so far not one positive, and that's in an area where Zika is not active and spreading. So far, you'd have to give this test high marks."
Other blood centers in Texas and Florida and along the Gulf Coast are considering adding the test to their screenings, said Kuehnert, as those are the primary geographic areas inhabited by the Aedes aegypti, the mosquito that transmits the virus.