The number of people infected with Zika in Puerto Rico is rising at an alarming rate, putting pregnant women at even greater risk of their babies suffering severe birth defects, a top U.S. public health official said Friday.

The latest data show that the most accurate, real-time indicator of Zika infection suggests that thousands of pregnant women there could contract the virus in the coming months. That, in turn, could lead to "dozens to hundreds of infants born with microcephaly in the coming year," Tom Frieden, director of the Centers for Disease Control and Prevention, said during a media briefing.

The data come from a CDC report showing the percentage of Puerto Ricans who are testing positive for Zika during blood donor screening. The numbers are the closest approximation for a representative sampling of infection rates on the island as a whole. "These numbers are increasing faster than we had anticipated," Frieden said in a separate interview, referring to the blood-test results. He noted that Puerto Rico has not yet reached the typical peak of mosquito-fueled disease outbreaks, which occurs over the summer and into the fall.

About 1 percent of the island's blood donors tested positive for Zika in the week ending June 11, the highest level since testing began in April, according to the agency's report. The test measures whether someone is infected at that moment. A 1 percent positive rate translates to roughly a 2 percent infection rate each month, Frieden said -- which in turn would mean an annual infection rate of about 25 percent for the commonwealth's 3.5 million residents.

Women in Puerto Rico give birth to about 32,000 babies a year. That projected rate of Zika infection would put thousands of pregnant women at risk before they deliver, he said.

His estimate that dozens to hundreds of infants could suffer microcephaly is based on recent research from Brazil. There, doctors found that women who were infected during their first trimester faced as much as a 13 percent risk of giving birth to infants with the rare congenital condition, which is characterized by an abnormally small head and often underdeveloped brain.

Even for the thousands of other Puerto Rican infants who would escape microcephaly, there are additional concerns because "we simply don't know if there will be long-term consequences on brain development," Frieden said.

Blood-collection centers in Puerto Rico began screening for the virus in April, using a new, highly sensitive test, to ensure the safety of the blood supply. Public health officials say there is a strong possibility that Zika virus can be spread through blood transfusions, with at least one reported case in Brazil. On the island, donations that test positive are pulled.

No Zika transmission through blood transfusions have been confirmed in the United States, but the Gulf Coast Regional Blood Center in the Houston area also began testing for the virus May 23. The center has tested over 9,000 donations with no positives, CDC officials said. Other blood centers along the Gulf Coast, which is considered at the highest risk for Zika, are expected to soon start similar screening on their own initiative.

Matthew Kuehnert, CDC's director of blood and organ safety, said Friday that no blood collected in the continental United States has tested positive for Zika. In February, the Food and Drug Administration said anyone infected by -- or even potentially exposed to -- the virus should wait at least four weeks before donating blood. The agency said that included individuals with recent travel to Zika-affected regions, those with symptoms suggestive of a Zika infection in the past month and those who have had sex with a person who'd traveled to or lived in Zika-affected areas in the previous three months.

An Aedes aegypti mosquito at a lab of the U.S. Centers for Disease Control and Prevention's Dengue Branch in San Juan, Puerto Rico. (Alvin Baez/Reuters)

In areas of active Zika virus transmission -- which includes U.S. territories such as Puerto Rico, the U.S. Virgin Islands and American Samoa -- blood used for transfusions should be obtained from areas of the country where the virus isn't present unless locally collected donations are being screened.

Puerto Rico has 1,726 confirmed Zika cases, which include 191 pregnant women, according to its health department. Only one case of microcephaly has been reported there to date. (In the continental United States, which has far fewer Zika cases, at least six pregnancies involving infected women have been affected by microcephaly or other serious birth defects.)

Frieden said officials and the community can take measures to reduce the risk of mosquito exposure.

"I want to try and make sure all of us involved in the response don't get to a situation three, six or 12 months later where we look back and say we wish we had done more back in June," Frieden said. Even if officials are able to reduce the number of infections by 10 to 30 percent, "every one of those infections is a tragedy prevented."

When Puerto Rico was hit by its first chikungunya outbreak in 2014, testing for the virus in a small proportion of blood donors suggested that nearly 25 percent of the general population becoming infected during the course of the epidemic.

There was greater awareness of chikungunya, a related virus spread by the same mosquito that transmits Zika, because most people develop symptoms, such as fever and joint pain, which can be quite severe and persist for months. By comparison, most people infected with Zika virus won’t even know they have the disease because they won’t have symptoms. The most common symptoms of Zika are fever, rash, joint pain and conjunctivitis (red eyes), and the illness is usually mild, lasting for several days to a week.

Scientists have concluded that Zika has spread to more than 40 countries, mostly in South America and the Caribbean, and that it causes microcephaly as well as an array of other neurological problems.

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