The US has its first case of sexually transmitted Zika virus related to the recent outbreak

The first case of the untreatable Zika virus being transmitted sexually has just been confirmed, NBC reports.

This is the first time the Zika virus has been transmitted in the US. The Zika virus has been detected in at least 31 travelers returning to the US in 11 states and Washington, DC, the Centers for Disease Control and Prevention said last week on a conference call with reporters, adding that it expected those numbers to rise.

The case happened in Dallas County, Texas after the patient had sexual contact with a person with Zika that had just returned from a country where the virus is being locally transmitted. Currently, Dallas County is reporting no cases of local transmission via mosquito.

There is no vaccine or treatment for the virus, though the National Institute of Allergy and Infectious Diseases expects early trials of a potential Zika vaccine to start by the end of this year. The virus is mainly transmitted through mosquitoes, though it's been documented as a potentially sexually transmitted disease as well.

Once infected, only about one in five people with Zika ever shows symptoms, which most commonly include fever, rash, joint pain, and red eyes.

Zika and sexual transmission

Although the Zika virus is mostly transmitted when a mosquito bites a human infected with the virus, then goes on to bite another human, some rare cases of sexual transmission via semen have been documented in the past.

In December 2013, during a Zika outbreak in French Polynesia, a patient that was seeking treatment for hematospermia (blood in the semen) was found to have Zika in his semen. And a 2011 paper documents a man who unwittingly infected his wife with Zika after they had sex upon his return from Senegal where he was studying the mosquitoes responsible for Zika transmission.

Zika and pregnancy

One reason Zika is troubling is that it has been linked to birth defects. After some mothers showed symptoms of the virus during their pregnancy, their babies were born with abnormally small brains, a condition known as microcephaly. The CDC recently published a set of working guidelines for pregnant women traveling to areas where local transmission is happening. These include monitoring themselves and their unborn children.

The CDC and other government organizations are working to determine whether this link is a causal relationship or whether the birth defects are caused by something else. Government agencies including the CDC are working to develop better ways to diagnose the virus in people who no longer have Zika symptoms.