Federal and state health officials are investigating 14 new reports of potential sexual transmission of the Zika virus in the United States, including several cases involving pregnant women, the Centers for Disease Control and Prevention disclosed Tuesday.
In at least two of the cases, a Zika infection was confirmed in women whose only known risk factor was sexual contact with an ill male partner who recently had returned from one of the nearly three dozen countries where the virus has now spread. Four other women have tested positive for Zika in preliminary lab tests but are awaiting final confirmation. The CDC said the eight other cases remain under investigation.
“We were surprised, given the numbers actively being investigated,” Anne Schuchat, the CDC’s deputy director, said in an interview. “We were concerned enough that we thought it was important to share that information … We are seeing more than we expected to see.”
Even if all the cases under investigation are not ultimately confirmed, Schuchat said, the growing number of reports suggests that sexual transmission of the Zika virus is more possible than researchers previously had realized. That development could further complicate efforts to stop the spread of Zika, as well as force couples to contemplate abstaining from sex or using protective measures to prevent transmission, particularly when a woman is pregnant.
Officials said the same scenario played out in most, if not all, of the suspected cases: A man who had traveled to a Zika-affected area returned the United States and had sex with a female partner, who soon began to display symptoms consistent with Zika. The agency did not identify the states where these cases are being investigated.
“Like previously reported cases of sexual transmission, these cases involve possible transmission of the virus from men to their sex partners,” the CDC wrote in announcing the new cases. “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.”
Schuchat said that in each case, the man had displayed symptoms of Zika, such as fever, rash or joint pain — a notable fact, given that as many as 80 percent of people infected with Zika never show symptoms. She said researchers still aren’t sure whether only men who have symptoms can transmit the disease to their partners, but it’s a possibility scientists are exploring. In addition, she said that because the Zika virus is not yet being widely spread by mosquitoes in the United States, as in other affected countries, that might make it easier to spot sexually transmitted cases here.
The issue is among a long list of questions that remain about Zika, a once-obscure virus that has spread explosively throughout the Americas in recent months and is suspected of being linked to serious birth defects and a rare autoimmune disease. Scientists do know that the vast majority of infections result from mosquito bites, but signs that the virus can be transmitted through sex continues to grow.
“We have increasing evidence that this is a route of infection, but we don’t know yet how much of an absolute risk it is,” Schuchat said. For instance, she said researchers know that the virus can linger in semen, but they aren’t sure how long it can remain there and potentially infect someone else.
Earlier this month, Dallas health officials reported that a local resident had been infected with the Zika virus by having sex with a person who had contracted the disease while traveling in Venezuela. The Dallas case marked the first report of local transmission in the United States.
Both the CDC and the World Health Organization have issued guidelines urging the use of condoms to prevent the spread of the virus, particularly when a woman is pregnant or might become pregnant. Officials have also recommended couples consider abstaining from sex for the duration of a pregnancy if the male partner recently traveled to a Zika-affected country.
The CDC on Tuesday reiterated those recommendations, saying that “these new reports suggest sexual transmission may be a more likely means of transmission for Zika virus than previously considered.”
The agency also has suggested that women who are pregnant or plan to become pregnant should avoid traveling to Zika-affected areas.
This post has been updated.