U

S health officials said Thursday that pregnant women should consider avoiding travel to 11 countries in Southeast Asia because of concerns about the Zika virus.

Because Zika has circulated in the region for years, health officials believe many people there might be immune to the virus, which is primarily spread by mosquitoes. But several countries in the region have seen an increase in cases recently. The Centers for Disease Control and Prevention said it made the new recommendation because most American travelers do not have immunity to the virus.

“Pregnant women should talk to their health care provider and consider postponing nonessential travel to Southeast Asia,” the agency said.

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Zika typically causes just a mild illness or no symptoms at all. But an infection during pregnancy can lead to devastating birth defects, including a condition called microcephaly in which the head and brain do not develop fully.

The agency also said that pregnant women who have traveled to the Southeast Asian countries recently should have access to Zika testing. The countries are Brunei, Burma, Cambodia, Indonesia, Laos, Malaysia, the Maldives, the Philippines, Thailand, East Timor, and Vietnam.

The CDC has issued stronger travel advisories for pregnant women to avoid traveling to much of Latin America and the Caribbean — as well as a portion of Miami Beach — because of the spread of Zika. Scientists have known about the virus since 1947 and have occasionally documented cases in Africa and Asia, but they only discovered the risks it poses to fetuses when it started to spread rapidly in the Americas in 2015.

Zika can also be spread through sex — oral, anal, and vaginal sex — so the CDC said Thursday that pregnant women and their sexual partners who live or travel in the region should abstain from sex or use condoms during the pregnancy.

The virus spreading in the Americas appears to be related to the strain found in Asia, but scientists are still sure exactly how much immunity exists in Asia. Health officials in the region have never before associated Zika and microcephaly, so it’s not clear if the virus in the Americas changed or if the virus did not cause enough birth defect cases to be noticed.

Health officials in Thailand announced this week they are investigating four suspected cases of Zika-related microcephaly, Reuters reported. They would be the first documented Zika-related cases of the birth defect in Southeast Asia if confirmed.

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