Officials in Dallas said a local resident was infected with Zika after having sex with a person who contracted the virus during a trip to Venezuela. This is the first known case of a person who became infected while in the United States and raises a whole new set of concerns about the rapid spread of the pathogen which has been linked to babies being born with head abnormalities. Here's what we know -- and don't know -- about sexual transmission of the virus.

1. The Centers for Disease Control and Prevention conducted lab tests that confirmed the Zika virus in someone who had not traveled outside the continental United States. Dallas County Health Department officials said this person had a sexual partner who had traveled out of the country to an area where the virus is spreading and that this person also tested positive for Zika.

2. Dallas said this appeared to be a case of Zika virus "acquired through sexual transmission." The county's health services director said that “Now that we know Zika virus can be transmitted through sex, this increases our awareness campaign in educating the public about protecting themselves and others."

3. But the CDC was more cautious in its statement and did not emphasize a definitive link. However, the CDC did update its guidance to include information about possible sexual transmission.

4. While the CDC said the best way to avoid Zika remains protecting yourself against mosquito bites, it is now encouraging condom use as a way "to prevent spreading sexually transmitted infections."

5. The CDC said that in this case"there was no risk to a developing fetus." But officials have not released the genders of the two people involved in the possible sexual transmission in Dallas. It appears from this statement that the person who may have been infected through sexual transmission either wasn't pregnant or is not a woman. So we don't know yet whether semen or another bodily fluid was involved.


People read zika virus fliers from an information campaign by the Chilean Health Ministry at the departures area of Santiago's international airport on Jan. 28. (Reuters/Ivan Alvarado)

6. However, previous reports about possible sexual transmission have involved males who were infected possibly passing it on to female sexual partners. On Tuesday, the CDC seemed to be focusing on that type of transmission, saying it would issue guidance in the coming days "on prevention of sexual transmission of Zika virus, with a focus on the male sexual partners of women who are or who may be pregnant."

The CDC also reiterated its previous guidance that pregnant women postpone travel to areas where Zika virus transmission is ongoing and added that "pregnant women should also avoid exposure to semen from someone who has been exposed to Zika virus" and that "women trying to become pregnant should consult with their healthcare professional if their partner has had exposure to Zika virus."

7. The past research on sexual transmission of Zika shows one case of a man possibly passing on the virus to a woman and confirms the possibility that the virus can exist in the semen, possibly for longer periods than in the blood. This may be similar to what scientists have seen recently with Ebola persisting in semen.

2008: Two American scientists, both men, appeared to have contracted Zika virus infections while working in Senegal. Soon after their return to the United States they started experiencing mysterious symptoms such as a rash and extreme fatigue. One of them, Brian Foy, noticed that his semen had a red-brown tinge. A few days after Foy got sick so did his wife. His wife had not traveled outside the United States for at least a year and had never been to Africa where the mosquitoes carrying the virus were concentrated at the time. The scientists sent their blood and that of Foy's wife to a colleague who confirmed that all three were infected with Zika. Foy's semen wasn't tested for Zika but based on circumstantial evidence, the researchers theorized that he had transmitted the virus to his wife through sexual contact.

The full paper from Emerging Infectious Diseases can be found here.

2013: During an outbreak in French Polynesia, a 44-year-old man came in to the hospital in December after finding blood in his semen. The man said he had had symptoms of Zika, including a low-grade fever, earlier in the year but that he did not seek treatment because they went away after three days. Eight weeks after that he said he had a second episode of sickness involving a fever, headache and rash. It was two weeks after that that he found blood in his semen.

Researchers found Zika in semen samples, which they said added more evidence to the idea that it might be possible to transmit the virus through sexual contact.

The paper can be found here.

Previously: Other similar viruses such as West Nile, dengue and yellow fever have been found in the urine of patients who infected others. There was also an animal study, on boars, that showed that Japanese encephalitis virus could be transmitted to female boars through artificial insemination with infectious semen.


Pedestrians walk past smoke lingering after workers from the Chacao municipality fumigated a building to eradicate mosquitoes that can cause the Zika virus in Caracas, Venezuela, on Feb. 2. (Wilfredo Riera/Bloomberg)

Read more:

Zika virus: WHO declares global public health emergency, says causal link to brain defects ‘strongly suspected’

WHO: Zika virus ‘spreading explosively,’ level of alarm ‘extremely high’

FAQ: What is Zika, and what are the risks as it spreads?

CDC issues interim Zika guidelines for testing infants

Why the United States is vulnerable to the alarming spread of Zika virus

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