In all the cases the C.D.C. is examining, women in the continental United States had sex with men who had traveled to countries where the virus is circulating, and developed symptoms associated with the virus within about two weeks of their male partners’ symptoms.
Officials at the C.D.C. reported the potential cases in an alert to health care providers on Tuesday.
The agency did not say exactly how many of the women were pregnant, but it reiterated its recommendation that people returning from Zika-infected areas use or abstain from sex for the duration of their partner’s The alert said there was no evidence that women could transmit Zika virus to their sex partners, but added that more research was needed to be sure.
This country has become a laboratory of sorts to test the sexual transmission of Zika, as scientists race to understand the disease. Transmission by mosquitoes is not yet happening in the continental United States because it is still winter, so health officials say they believe that any infection of an American resident who has not traveled to a place where Zika is circulating has probably been contracted through sex.
“In the U.S., where most people aren’t traveling to these areas, we may be able to uncover the potential risk,” Dr. Schuchat said.
In all, the United States has around 90 cases of Zika, according to the most recent count from the C.D.C., most of them contracted by people who had traveled to Latin America, currently the center of the virus. If confirmed, the new reports of sexual transmission would represent about 15 percent of that total.
“It’s beginning to look as though Zika can be more readily transmitted sexually than we first anticipated,” said Dr. William Schaffner, professor of preventive medicine at Vanderbilt University Medical School. “These data are illuminating some of the things we don’t know.”
Zika was originally identified in the 1940s in Africa. For most people, it is a relatively mild virus, causing rashes, red eyes and joint pain; many people have no symptoms at all. But the association with a condition known asmicrocephaly, in which babies have been born with unusually small and deformed heads to women who had Zika during pregnancy, has raised global alarms. On Feb. 1, the World Health Organization declared the virus and its link to the birth defects a public health emergency.
Questions about how frequently Zika can be transmitted by sex and how long the virus can stay in semen are particularly urgent here, given the large volume of travel between the United States and Central and South America. There were about 5.5 million visitors from South America to the United States in 2014, and nearly a million from Central America, according to figures from the Department of Commerce.
And with the season for mosquitoes — still believed to be the primary mode of infection — nearing in the United States, Tuesday’s report is likely to further complicate preparations in states across the country.
“This suggests that along with virus in the blood, Zika is gaining access to other fluids, including semen,” said Dr. Peter J. Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine. “Anyone who is pregnant and lives in an area where the Zika virus is circulating will need her male partner to use condoms. In the coming weeks, that may include the U.S. Gulf Coast.”
Testing of semen may be difficult. Patients at real risk in the United States need to be first tested by standard blood testing before the testing of semen would even be considered, said Dr. Gary W. Procop, a professor of pathology at the Cleveland Clinic. Only the C.D.C. and state laboratories do such testing, and only for people determined to be at high risk, he said.
Scientists have suspected for several years that Zika could be transmitted sexually.
Two mosquitoes of the species aedes aegypti, which can spread Zika virus. The C.D.C. reported additional cases of sexual transmission of the disease on Tuesday. CreditOscar Rivera/European Pressphoto Agency
In 2008, a malaria specialist who caught the Zika virus while gathering mosquitoes in Africa passed the infection to his wife shortly after his return to northern Colorado. Because his wife had not left the state and there were no mosquitoes in the region capable of carrying Zika — and because the couple did not infect any of their four children — experts concluded the only logical explanation was transmission through sex.
Last year, French scientists described finding viable Zika virus in the semen of a 44-year-old Tahitian man who had recovered from an infection during a 2013 outbreak in French Polynesia.
And the health authorities in Britain recently described a case of a 68-year-old British man who contracted Zika in the Pacific islands in 2014. After the man recovered, the researchers conducted follow-up tests for the virus. It could still be found in the semen 62 days after the man’s illness started,according to a report in Live Science.