CDC: Zika could affect 10,000 pregnancies in Puerto Rico by year's end

The Zika virus could affect up to 10,000 pregnant women in Puerto Rico this year, putting hundreds of babies at risk of catastrophic birth defects, according to the Centers for Disease Control and Prevention.

Zika is spreading so quickly on the island that it's likely to infect one in four people by the end of the year, CDC director Thomas Frieden said. The greatest danger from Zika is microcephaly, in which infants are born with abnormally small heads and incomplete brain development, he said.

"That's horrifying," Frieden said. "This is a silent epidemic that is rapidly spreading through Puerto Rico."

About 41% of pregnant women in Puerto Rico with symptoms of Zika — such as a rash, fever, joint pain, pink eye, headache — tested positive for the virus, Frieden said. But just 20% of people with Zika develop symptoms. About 5% of pregnant women without Zika symptoms also tested positive for the virus, according to a CDC report published Friday.

Tests of the blood supply in Puerto Rico also indicate the virus is widespread. About 1.8% of blood donations tested positive for Zika, meaning that the person who gave the blood was infected in the last week or so, according to the report.

"That may not sound like a lot," Frieden said. "But that means that about 4% of people are being infected every month."

Tests also found that 21 people with Guillain-Barre syndrome — a condition in which a person's immune system attacks the nerves of the body, causing paralysis — were infected with Zika, according to the report. A Puerto Rican man with Guillain-Barre syndrome died earlier this year after suffering a severe loss of platelets, the cells that help the blood clot. The condition can lead to people bleeding to death.

About 5,582 people in Puerto Rico have been diagnosed with Zika, including 672 pregnant women, according to the CDC.

More than 1,600 people in the continental U.S. also have been diagnosed with Zika, mostly due to travel to places with outbreaks. Zika has affected 20 pregnancies in the U.S. so far. Thirteen women in the U.S. have given birth to babies with Zika-related birth defects; seven have lost pregnancies due to miscarriage or have aborted babies with brain damage.

Brazil has confirmed 1,749 cases of microcephaly related to Zika, according to the World Health Organization. Colombia has reported 21 cases of microcephaly; along with nine in Cape Verde, located off the coast of Africa; eight cases in the Caribbean island of Martinique; and five in Panama. A handful of Zika-related microcephaly cases also have been reported in French Guiana, French Polynesia, the Marshall Islands and Paraguay.

Scientists say they don't know why microcephaly is more common in Brazil than in other countries and territories hit by Zika. Some researchers say it's possible that something about the environment of Brazil, particularly the hard-hit northeastern part of that country, may contribute to microcephaly. Doctors say it's also possible that fetuses are more likely to be harmed by Zika if their mothers were previously infected with other mosquito-borne diseases.

Governor: 4 new Zika cases likely came from Fla. mosquitoes

Florida's governor, Rick Scott, said Friday that four people with Zika in Miami-Dade and Broward counties were infected not by travel, but by local mosquitoes. That's a major development in the disease, because it means people could be risk not just from traveling to foreign countries, but from sitting in their own backyards.

In Puerto Rico, Zika appears to be following the same pattern seen with another mosquito-borne disease, chikungunya, which arrived in the Western Hemisphere in 2013. Within a year, the disease infected one in four Puerto Ricans, Frieden said. While chikungunya can cause excruciating joint pain, it's not known to harm fetuses.

Fighting Zika in Puerto Rico will be challenging.

Few people have air conditioning or screens on their doors and windows, Frieden said. Standing water collects on many areas of the island after heavy tropical rains, which provides breeding grounds for mosquitoes.

The mosquitoes that spread Zika, including the Aedes aegypti species, are mostly resistant to common pesticides, according to the CDC report.

The U.S. government has taken several steps to help Puerto Rican women. The federal Women, Infants and Children program has offered mosquito control services to infected pregnant women. Those services include removing standing water from around their homes, applying larvicide to kill immature mosquitoes and spraying insecticides that are still effective against mosquitoes. Puerto Rican authorities are installing screens in the homes of 350 pregnant women and have removed 1.6 million discarded tires, which can collect water.