Fourteen more people may have caught the Zika virus in the U.S. without traveling to affected zones, federal health officials said Tuesday — strong evidence that the virus is sexually transmitted fairly often.
Some of those suspected of having been infected sexually have been pregnant women, the Centers for Disease Control and Prevention says.
"CDC and state public health departments are now investigating 14 new reports of possible sexual transmission of Zika virus, including several involving pregnant women," the CDC said in a statement.
"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."
Evidence is growing that Zika can cause serious birth defects, especially microcephaly, which results from a damaged brain that stops developing in the womb. There are also suspicions that Zika is causing a paralyzing condition called Guillan-Barre syndrome.
The CDC's Dr. Anne Schuchat said only two cases are confirmed, but all of the suspected cases involved men who had traveled potentially infecting women who had not.
"In each of the episodes, a man was traveling to a Zika affected area, developed symptoms that were consistent with Zika, and within two weeks…a female partner developed symptoms consistent with the virus," Schuchat told NBC News.
Earlier this month, Dallas health officials reported the first known case of sexual transmission of Zika in the current epidemic.
Doctors had known Zika could be spread by sexual transmission. In 2008 a U.S. researcher was infected in Africa and infected his wife back in Colorado. Zika has been found in semen.
"Although this is an investigation in progress, we thought it was important to get the word out to people to remind them that we think pregnant women should avoid traveling to Zika-affected areas," Schuchat said. And women who are pregnant and whose sex partners have been to affected areas should abstain from sex or use a condom throughout their entire pregnancy, just to be safe, she said.
Zika's spreading fast across the Americas and the Caribbean and the World Health Organization has declared it a public health emergency of international concern. The virus itself is relatively harmless to most people, but what's worrying is the potential that it causes severe birth defects.
The CDC's already advised travelers to be aware of the risk, recommending that men who have traveled to Zika-affected zones should use a condom if they want to be absolutely sure they don't infect sex partners.
"We are not changing the guidance today but we are really reinforcing it," Schuchat said. "For the time being we are telling women to avoid sex or to be careful during sex with a partner who is coming back from an area where Zika is."
CDC issued a travel advisory last month telling pregnant women to stay away from countries where Zika is circulating.
The Zika epidemic is a real-life science experiment. The virus is infecting millions of people who have never had it before and it's giving doctors a chance to use modern tests and techniques to see how new infections move across a population.
Zika's clearly a mosquito-borne virus, spread as female Aedes mosquitos sip blood from one person after another, often in the same room. Other viruses are spread this way, too: yellow fever, dengue, West Nile and chikungunya. And the malaria parasite is also spread by mosquitoes.
"It's not likely that sexual transmission is anywhere close to the frequency of mosquito-borne transmission. The mosquito is the most dangerous animal on the planet," said Dr. William Schaffner of Vanderbilt University, past president of the National Foundation for Infectious Diseases.
Common wisdom has been that these viruses don't usually spread sexually, but since sex partners generally live together, it has been impossible to tell whether, say, a married couple both get infected by mosquito bites or through sex. And because most people will have been infected by mosquitoes, it hasn't mattered much.
But with Zika, right now the only way the virus is getting to unaffected countries like the U.S. is in the bodies of travelers. So the cases of sexual transmission stand out. CDC says 30 million to 40 million Americans fly to Latin America and the Caribbean every year and even more go by land.
One big question is how long men might be able to transmit Zika in semen. "The science is not clear on how long the risk should be avoided. Research is now underway to answer this question as soon as possible," the CDC said.
Tests have found evidence of Zika in saliva and urine as well. But Schuchat says there is little evidence the virus spreads that way. "While we don't have specific information that proves this was sexual transmission, we know that some other viruses are found in semen as well as in saliva and we know that the sexual route is the clear route of spread," she said. "We are learning more every single day about this virus and its effects in people."
Experts almost all agree that Zika is unlikely to spread much in the U.S., in part because the Aedes aegypti mosquito that carries it isn't common except in parts of the far south and Hawaii, and also because Americans live indoors mostly, with air conditioning and little chance for the mosquitoes to live and breed inside homes.
"I think it's a balance. The average person is really at very low risk of developing Zika infection or getting really sick from it," Schuchat said. "But pregnant women, we believe, are at a very substantial risk of complications. It can be really scary to have something new."
Zika is actively spreading in more than two dozen Latin American and Caribbean countries. CDC added two more destinations to its caution list Tuesday: Trinidad and Tobago in the Caribbean, and the Marshall Islands in the Pacific.