Mosquitoes remain the primary way Zika is spread, and preventing bites is the best way to avoid infection. But the CDC said it was issuing the interim recommendations to stop sexual transmission, however rare, because of concerns over Zika's potential link to birth defects. Abstaining from sex during a pregnancy is one option. The other is for men who live in or have traveled to Zika-affected regions to consistently and correctly use condoms during sex with a pregnant partner.

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The recommendations are for vaginal, anal or oral sex. They do not address kissing.

In a briefing with reporters, CDC Director Tom Frieden said he was aware of reports from Brazil that traces of the virus had also been found in saliva and urine samples from sick patients. But "we need more information," including the methodology behind those findings, he said.

"We're still learning more about [the virus in] saliva and how it works in the body," Frieden said. "There's been a total of three cases in the world literature of Zika being present in male secretions."

The precautions are targeted at pregnant women because of Zika's suspected association with a rare congenital condition known as microcephaly, where babies are born with head and brain abnormalities. In Brazil, the epicenter of the outbreak in the Americas, Zika has been linked to a surge of such cases.

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"We're not aware of any mosquito-borne disease associated with such birth outcomes on a scale anything like what is occurring now," Frieden said. "Because the phenomenon is so new, we are quite literally discovering more about it each and every day."

Yet with each passing day, he said, the association between the virus and microcephaly has become stronger. So, too, has a link between Zika and Guillain-Barré syndrome, which can lead to paralysis in adults. Several South American countries have identified cases of that rare condition.

Colombia’s health minister Alejandro Gaviria told reporters Friday that three patients with the syndrome died last week at a clinic in Medellín, the country’s second-largest city. All three were infected with Zika, Gaviria said, and authorities believe the virus is to blame for their deaths. Two individuals were from the town of Turbo on the country’s north coast; the third lived on the Colombian island of San Andres in the Caribbean.

The World Health Organization said Friday that the virus is spreading in at least 33 countries. Seven countries have reported an increase in cases of microcephaly and/or Guillain-Barré syndrome.

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In its latest report, the CDC said current information about possible sexual transmission of Zika is based on the three cases. The first is "probable sexual transmission of Zika virus from a man to a woman, in which sexual contact occurred a few days before the man's symptoms began." The second is the Dallas case. The third is an instance of virus isolated from semen at least two weeks and possibly up to 10 weeks after illness began.

The virus remains in a person's blood for about a week. But researchers don't know how long it lingers in semen. "We know semen may have a large amount of viable virus for at least a short time after viral infection," Frieden said.

The agency is advising additional screening for pregnant women who live in areas with ongoing spread of Zika. Those with symptoms should be tested at the time of illness, CDC recommends.

If Zika behaves like other infectious diseases, the first trimester and early second trimester would be when the fetus is most at risk, Frieden said. But it's also possible the virus has the ability to target the developing brain, which means adverse consequences could occur at any time during pregnancy, he added.

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Pregnant women without symptoms can be offered testing two to 12 weeks after returning from Zika-affected regions, according to the CDC guidelines. They should begin testing when they begin prenatal care, with follow-up testing around the middle of a pregnancy's second trimester. "An additional ultrasound may be performed at the discretion of the health care provider," the guidelines state.

It's often difficult to accurately determine microcephaly during the 20-week ultrasound that many women get as part of routine prenatal care, said Cynthia Moore, a CDC expert on birth defects. But based on reports from Brazilian physicians, she said, it appears that around 30 weeks is when microcephaly is best detected along with other ‎abnormalities of the developing brain.

The timing and extent of fetal brain damage could vary case to case and probably depends on when in pregnancy the infection occurs, Moore said.

The agency's recommendations also address women who are not pregnant and have a sexual partner who lives in or has traveled to an area affected by Zika. These couples also may consider using condoms or abstaining from sex, the CDC said. "The science is not clear on how long the risk [of infection] should be avoided," it noted.

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Whether infected men who never develop symptoms can transmit Zika to their sex partners is unknown, according to the CDC. Sexual transmission of the virus from infected women to their sex partners has not been reported.

The Texas case, the first known instance of a person becoming infected while in the United States, immediately raised a whole new set of concerns about the rapid spread of the pathogen. Officials did not release the genders of the two people involved there. The CDC only acknowledged that "there was no risk to a developing fetus."

More than four dozen Zika cases have been confirmed in 14 states and the District of Columbia -- with six pregnant women infected -- plus at least another 21 cases in U.S. territories. Frieden said Friday that one U.S. case of Guillain-Barré may be linked to Zika.

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The symptoms of Zika infection are typically mild and only seen in about 20 percent of people with the virus. They include fever, rash, joint pain and red eyes, which usually last a week at most. There is no vaccine to prevent infection, as well as no treatment for it.

The CDC recommended two weeks ago that pregnant women postpone trips to countries with Zika outbreaks.

Nick Miroff contributed to this report from Rio de Janeiro.

This post has been updated.

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