【Watch Friends】

Every week,Watch Friends doctors find out something new about the Zika virus, which has spread across 48 countries since 2015 and is poised to cross into the southeastern United States this summer.

On Friday morning, the Centers for Disease Control and Prevention (CDC) announced that doctors in New York City have documented the first known instance in which the Zika virus, which can cause birth defects and neurological damage, was spread from a woman to her male partner during sexual intercourse without a condom.

According to the CDC as well as the World Health Organization (WHO), all previous cases in which the virus was spread sexually occurred when an infected man had sex with a female or male partner.

Studies have shown that the virus can be present in semen for up to two month or more after Zika symptoms dissipate, while there is a dearth of studies on how long after Zika infection a woman might be able to transmit the virus, and in what ways.

As of July 13, there were 310 cases of Zika virus in New York City. In each of these cases, the virus was contracted while patients were visiting other countries where active Zika transmission is taking place via mosquito bites.

In particular, a large number of cases were contracted in the Dominican Republic, which is one of the countries where a CDC Zika-related travel advisory is in place.

New York City's Health Department reports that in 36 of the 310 cases, women were pregnant at the time of their diagnosis.

Findings don't significantly alter doctors' advice

The new findings do not immediately alter health agencies' guidance for couples, which includes practicing safe sex for at least eight weeks after returning from a country where Zika is spreading, or abstaining from sex entirely.

The guidelines were developed in order to prevent women from becoming pregnant while infected with the virus, since the virus can cause microcephaly, a particularly devastating condition in which babies are born with unusually small heads and brain abnormalities.

The new finding also raises the possibility that women could spread Zika to a female sexual partner, though no documented cases of such transmission exist.

The virus can also cause neurological disorders and the Guillain-Barré syndrome in adults, which can result in paralysis.

On Wednesday, the CDC reported that seven babies in the U.S. have been born with birth defects that were related to Zika, while five women who had Zika while pregnant lost their babies during their pregnancy.

Doctors will now need to re-examine health recommendations to ensure that women who return from regions where the Zika virus is being spread do not infect their male partners.

"Although no cases of woman-to-woman Zika transmission have been reported, these recommendations now also apply to female sex partners of pregnant women," the CDC said in a statement on Friday.

"[The] CDC is currently updating recommendations for sexually active people in which the couple is not pregnant or concerned about pregnancy and for people who want to reduce personal risk of Zika infection through sex."

Sexual contact is not the main way that Zika has been spreading, however. The primary means of transmission is mosquito bites, specifically from the Aedes aegypti mosquito, which also carries yellow fever and the dengue virus. This mosquito is found across much of the southern and eastern U.S., but the virus has not yet been detected in U.S. mosquito populations.