The World Health Organization on Tuesday cautioned pregnant women against traveling to areas where there is ongoing transmission of Zika virus -- something the U.S. Centers for Disease Control and Prevention advised more than a month ago.

Director General Margaret Chan said reports and investigations from affected countries in the Americas "strongly suggest" that sexual transmission is more common than previously assumed.

During a media telebriefing, WHO officials were pressed on why the organization had not issued a travel warning earlier. In addition to the CDC's guidance, health agencies in numerous countries have urged pregnant women and their sexual partners to avoid travel to regions where the virus is spreading and to practice safe sex or abstain from sex for the duration of a pregnancy.

David Heymann, an infectious disease specialist who is heading the WHO emergency committee on Zika, pointed to the accumulating evidence linking the pathogen to birth defects and neurological disorders in adults. "We felt we needed to make this recommendation," he said, adding that it remains up to individual countries to designate regions where there are ongoing outbreaks "and where there are not."

The WHO declared Zika and its suspected fetal dangers a global public health emergency last month. Since then, Chan said, "substantial new clinical and epidemiological research has strengthened the association" between infection and microcephaly as well as other neurological disorders. Brazil has been especially hard hit by microcephaly, the rare congenital condition in which babies are born with abnormally small heads and often underdeveloped brains.

"The geographical distribution of the disease is wider. The risk group is broader. And the modes of transmission now include sexual intercourse as well as mosquito bites," Chan said.

Microcephaly also has been detected in French Polynesia, with unconfirmed reports in Colombia. Nine countries are reporting an increased incidence of the rare Guillain-Barré syndrome, which can cause paralysis, or laboratory confirmation of Zika infection among those cases, Chan said.

"All of this news is alarming," Chan said. "Women who are pregnant in affected countries or travel to these countries are understandably deeply worried."

As for a link to fetal malformations, she said the virus has been detected in amniotic fluid, and evidence shows it can cross the placental barrier and infect the fetus. Chan said the WHO can now conclude that the virus is neurotropic, meaning it attacks the nervous system, "preferentially affecting tissues in the brain and brain stem of the developing fetus."

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