Zika Sexual Transmission in US Prompts Health Warning
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After a person in Dallas was confirmed to have contracted the Zika virus through sex, U.S. health officials are warning men who travel to countries where Zika is spreading to take steps to prevent spreading the virus through sex.

If a man has a pregnant partner, and has traveled to any of the more than 20 countries where Zika virus is spreading, he should either abstain from sex, or use condoms, until the end of his partner's pregnancy, officials at the Centers for Disease Control and Prevention (CDC) said today (Feb. 5).

The warning comes because health officials are concerned about a strong link between Zika virus infection during pregnancy and a birth defect called microcephaly, in which babies are born with abnormally small heads and face lifelong cognitive impairments.

"We know that this is anxiety-provoking for women who are pregnant and [for] their families," Dr. Tom Frieden, director of the CDC, said in a news conference today (Feb. 5). But there are things that people can do to lower their risk of contracting the virus, including rigorously protecting against mosquito bites, which are the usual mode of Zika transmission, and following the new guidelines to prevent sexual transmission, Frieden said. [Zika Virus FAQs: Top Questions Answered]

Although the Zika virus stays in a person's blood for only up to a week after the individual is infected, it's unknown how long the virus could stay in a man's semen. That's why the new recommendation for precautions during sex is for a woman's entire pregnancy. In one study, researchers found that the virus may have been present in a man's semen for up to 10 weeks after he got sick.

"We are quite literally discovering more about Zika each and every day," Frieden said.

The CDC also said that women who aren't pregnant, but who are concerned about contracting the virus through sex could consider using condoms during sex or abstaining if their partner has traveled to an area with Zika cases. The agency noted that Zika infection is usually mild, and about 80 percent of people who are infected with the virus have no symptoms at all.

In the Dallas case, a person contracted Zika virus after having sex with a man who had recently returned from Venezuela, and had symptoms of the infection. Still, sexual transmission of Zika virus is thought to be rare; there are just three known reported cases of the virus spreading through sex.

Frieden emphasized that the primary way Zika spreads is by mosquito bites.

It's not known whether Zika virus is also present in a person's saliva when the individual is infected, but the CDC is not currently issuing any recommendations about avoiding kissing, Frieden said. It's also not known if a woman who is infected with Zika can spread the disease to a man through sex, the agency said.

The CDC recommended that all pregnant U.S. women consider postponing travel to the areas where the Zika virus is spreading, including 24 countries in Central and South America.

So far, there have been 51 cases of Zika virus in the United States (not including cases in the territories such as Puerto Rico or American Samoa), and of these, 50 concerned people who caught the disease while traveling.

Pregnant women who travel to areas where Zika is spreading should be tested for the virus within two to 12 weeks after they return, even if they don't have symptoms, the CDC said.    

Follow Rachael Rettner @RachaelRettner. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.