Photograph by Mario Tama, Getty Images
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Doctors worry that the Zika virus may be behind an increase in the number of Brazilian babies diagnosed with a form of permanent brain damage called microcephaly.

Photograph by Mario Tama, Getty Images

Sexual Transmission Is Just the Latest Zika Surprise

Either the virus is becoming more treacherous, or scientists are just learning about dangers they had missed before.

Information about the Zika virus now “spreading explosively" through the Americas seems to be changing every day—and little of it is comforting.

Tuesday, researchers reported that a Dallas resident contracted the virus during sex, rather than from a mosquito bite, raising new questions about the virus, which has been carefully followed for only a few years.

“This virus is very frightening,” said Didier Musso, who studied a 2013-2014 outbreak of Zika on the French Polynesian islands.

Many details about the virus remain mysterious, including its evolution, the likelihood of sexual transmission, and whether this risk will complicate the fight against it.

(Update: The U.S. Centers for Disease Control and Prevention has recommended that men who travel to areas with Zika wear condoms or abstain from sex with pregnant partners.)

“It’s a very new disease. It would be possible to answer all the questions in 10 years, but not today,” said Musso, director of the emerging infectious disease unit at the Institut Louis Malardé in Tahiti, which is in French Polynesia.

Though Zika was first discovered in 1947 in Uganda, only 14 human cases have been described in Africa and in Southeast Asia, where it apparently also spread to travelers.

Zika transmissions had been reported in recent months on Puerto Rico and in the U.S. Virgin Islands, but Tuesday’s report of sexual transmission is the first on the U.S. mainland.

Most of what we know about the mosquito-borne disease comes from the outbreak in French Polynesia in 2013, where an estimated 32,000 were infected out of a population of 270,000. Although Zika had been considered a mild disease—with 80% of victims showing no symptoms at all—cases in French Polynesia suggested that the virus can cause very severe complications and has been more threatening than believed all along.

In October, Brazilian pediatricians began to worry that the increase they were seeing in brain-damaged newborns, born with a condition called microcephaly, might be due to Zika. On January 19, The U.S. Centers for Disease Control and Prevention warned pregnant women to avoid traveling to more than 20 countries and territories in the Caribbean, and Central and South America.

Although that link has yet to be officially confirmed, there also appear to be higher rates of other neurological problems, including Guillain-Barré  disease, which can cause temporary paralysis. Musso, who believes the virus jumped from Polynesia to Brazil during an international canoe race in August 2014, said he is convinced there is a connection.

Researchers are now trying to figure out whether the virus became more dangerous as it migrated across the world, or if the dangers were always there and simply overlooked.

“The big question is, has the change actually led to a change in pathology?” asked Dr. Leslie Lobel of Ben-Gurion University of the Negev in Beer-Sheva, Israel, adding that genetic analysis should eventually reveal the answer.

Sexual Transmission

It’s not yet clear how common sexual transmission may be. But it took some researchers by surprise.

“It’s something I don’t think would have been expected,” said Lobel, who also works at the Uganda Virus Research Institute in Entebbe.

Lobel said he doesn’t think sexual transmission happens easily, but if it does, public health officials will likely have an even harder time stopping the virus.

This is not the first time Zika has been contracted via sex. In 2008, an American scientist studying mosquito-borne viruses in Senegal, apparently infected his wife with the disease when he returned, according to a study published in 2011.

Musso said he also reported sexual transmission in rare cases in Polynesia, though he thinks Zika remains primarily a mosquito-borne disease.

The fact that sexual transmission is possible suggests the need to change public policy, said Dr. Davidson Hamer of the Boston University School of Public Health and School of Medicine.

“I think this means we need to counsel travelers to have safe protected sex,” he said. “Until we know more about how long the risk persists after infection, we need to be cautious about sexual activity, especially if [someone] might become pregnant.”

And it’s now clear that Ebola remains in semen and can be transmitted for as long as six months after clear blood tests. Viruses similar to Zika, including dengue and West Nile, have been seen to linger in some bodily fluids—mostly urine—longer than in blood.  

Semen and a few other parts of the body are preferentially protected by the immune system. It is harder for infections to reach this area, and harder for the body to clear them, the scientists said. For instance, in one Ebola survivor, months after his recovery the virus remained in his eye, which is another protected area, and a number of sexual transmissions have been reported in West Africa since the outbreak largely wound down.

If Zika can linger in semen, it likely also remains for a while in urine and even saliva, though it’s not clear if the virus can be transmitted through contact with either, said Hamer, who recently published a paper tracing the spread of Zika and its characteristics.

Hamer said he thinks that Zika was probably always present in bodily fluids, but that our detection methods are now better and thus able to find evidence of the virus.

Lobel said he thinks researchers should immediately begin testing other bodily fluids to determine their levels of contagion. “We can’t assume anything unless we’ve actually tested everything,” he said.

Calculating Risks

What we’re learning about Ebola and Zika could teach us more about viruses more broadly, Lobel said. At least 10% of early pregnancies are lost, for instance, and it’s possible that viral infections are behind many of those, he said.

Zika also poses a potential risk to the blood supply, particularly in places like Brazil, where the virus is raging, Lobel said. It is unlikely to be a major problem in the U.S., where the disease remains uncommon, he said.

Blood donors in this country are asked about their travel history before donating. It is not yet clear if the Red Cross will change its screening of donors in response to the new findings about Zika.

“We will continue to work with the U.S. Food and Drug Administration, Centers for Disease Control and Prevention and state health departments to track Zika and to update donor eligibility criteria as necessary,” the Red Cross said in an emailed statement.

Both Lobel and Hamer urged caution but not panic. The threat to Americans from Zika remains very low, they said, though pregnant women should avoid traveling south and—in light of the Dallas case—avoid unprotected sex with someone who has traveled there within the last two weeks.

The fact that Zika is occasionally transmissible via sex is not a game-changer, Lobel said.

“We don’t need to rethink what we do about Zika just because of that potential mode of transmission.”