Fourteen more people 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 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."
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.
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.
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.
"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.
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.