ATLANTA, Feb. 23 (UPI) -- The Centers for Disease Control and Prevention are investigating at least 14 potential cases of Zika virus that were sexually transmitted, the agency said Tuesday.
A new alert from the CDC was released to serve as a "strong reminder" to the public the Zika virus can be be transmitted sexually and might "a more likely means of transmission for Zika virus than previously considered," the CDC said.
Several of the cases the CDC is investigating involve pregnant women, the organization said.
The virus has been spread to humans from mosquitoes, but has begun to be transferred through sexual intercourse and blood transfusions.
At least 100 cases of Zika infection have been diagnosed, according to state health departments. None of the cases are thought to have been caused by mosquitoes infecting people in the United States.
Sexual contact was confirmed as the only risk factor for the women infected in virtually all of the suspected U.S. cases: The woman's male partner had recently returned from one of the infected countries. The women quickly began to show symptoms consistent with the virus.
"We were surprised, given the numbers actively being investigated," said CDC Deputy Director Anne Schuchat. "We were concerned enough that we thought it was important to share that information."
But the CDC wrote in its alert the opposite might not be true.
"At this time, there is no evidence that women can transmit Zika virus to their sex partners; however, more research is needed to understand this issue."
Zika has been associated with the birth defect microcephaly in Brazil, which causes an abnormally small head and brain that often lead to serious developmental problems.
The CDC is advising men who have traveled to Zika-affected areas to abstain from sex or at least use a condom while their partner is pregnant.