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The Centers for Disease Control and Prevention is investigating 14 new US cases in which Zika virus may have been transmitted sexually. Some of the suspected cases involve pregnant women.

Zika infections in two of the 14 are already confirmed by laboratory tests. Up till now there have only ever been two known cases of sexual transmission of Zika, both in the United States.

This suggests sexual transmission of the virus may not be as rare as has been previously thought, acknowledged Dr. Anne Schuchat, the CDC’s principal deputy director.


Given the early evidence, the agency felt it needed to reissue its warning about the risk of sexual spread of this virus.

“Sometimes we wait until every ‘i’ is dotted and every ‘t’ is crossed before we disseminate information. But we thought this was actionable, even if it’s kind of messy, the numbers may change, and some of these [cases] — a lot of these — may not pan out,” Schuchat told STAT.


“We had enough concern that we thought it was important to release [the information] in the midst of following up on each of these.”

The first reported case of sexual transmission of Zika virus occurred in Colorado in 2008. That was thought to be a medical oddity until health officials in Dallas reported in early February that they had recorded a case of sexual transmission there.

Earlier this month the CDC issued guidance aimed at preventing the virus’s spread through sex to pregnant women.

Schuchat said the CDC believes these 14 potential transmissions occurred before the agency issued its warning about the risk of sexual spread of the disease.

Are you at risk for contracting Zika virus? Your level of risk depends in part on your living conditions. Alex Hogan/STAT

Read more: CDC outlines guidance to reduce sexual transmission of Zika

There have been nearly 100 cases in which travelers have been diagnosed with Zika after their return to the United States in recent weeks, she said.

The advisory the CDC issued Tuesday did not reveal how many of the new cases involved pregnant women, saying only that there were “several.” Likewise, the agency did not specify the states in which the cases occurred.

“In two of the new suspected sexual transmission events, Zika virus infection has been confirmed in women whose only known risk factor was sexual contact with an ill male partner who had recently traveled to an area with local Zika virus transmission; testing for the male partners is still pending,” the agency said in a statement.

“For four additional suspected sexual transmission events, preliminary laboratory evidence … is available for the women, but confirmatory tests are pending. For eight other suspected events, the investigation is ongoing.”

Zika infection during pregnancy is suspected to be linked to congenital birth defects in infants; it is not clear, though, what percentage of women infected in pregnancy will give birth to affected babies.

Read more: US reports first case of Zika transmitted through sex

Brazil, which has experienced a large Zika outbreak that began sometime in 2014 or early 2015, has reported a surge in cases of babies born with microcephaly — small heads and underdeveloped brains. An increase in microcephaly cases was also seen during a Zika outbreak in French Polynesia in 2013-14.

The virus mainly spreads to people from infected mosquitoes.

The CDC and other health agencies have urged men who have traveled to Zika-affected areas to use condoms during sex upon their return home if they are having sex with a pregnant woman — or abstain from sex for the duration of the pregnancy.

In cases where there isn’t a pregnancy involved, people concerned about contracting Zika virus through sex can consider applying the same advice, the CDC said.

It is not currently known how long men who have had Zika infections continue to emit the virus in their semen. But a recent report from Britain disclosed that testing showed a man infected a couple of years ago while traveling in the Cook Islands had traces of virus in his semen 62 days after the onset of his symptoms. It is not known if that virus was infectious at that point in his recovery.

Public Health England has recommend condom use for men for six months if they had laboratory-confirmed Zika or symptoms compatible with Zika infection. Returning men with no symptoms should use condoms for 28 days, the agency said. This advice relates to men who have a partner who is pregnant or may become pregnant.

Also on Tuesday the CDC added two new locations to the list of nearly three dozen countries or territories where the Zika virus is known to be spreading. New to the list are Trinidad and Tobago in the Caribbean, and the Marshall Islands in Micronesia.

The CDC has advised pregnant women to avoid travel to any location where Zika is known to be transmitting, if they are able.