Sex and Zika: What You Need to Know

While aedes aegypti mosquitoes like the one here are the main spreaders of Zika, there have been cases — including one in Texas recently — of the disease being sexually transmitted. Credit: MArvin Recinos / AFP / Getty Images

The Zika virus is generally symptomless, can cause birth defects and an autoimmune disease that can lead to paralysis, and is spreading fast. If that isn't terrifying enough, the first case of this outbreak thought to have been transmitted through sexual contact and not via mosquito has been reported in Texas, the Centers for Disease Control says. 

This case throws a wrench into what we thought we knew about how the virus is spread, says Hyeryun Choe, associate professor of immunology and microbial science at the Scripps Research Institute of the University of Florida. Zika is in the flavivirus family, which also includes West Nile, dengue, and yellow fever — all of which are thought to be transmittable through the aggressive Aedes aegypti mosquito, not from person-to-person contact.

"This is very unusual," Choe says. "If I hadn't known of the two prior cases of sexual transmission of this virus, I never would've believed it."

In one of those cases, a scientist working in Senegal is thought to have contracted Zika there and to have transmitted the virus to his wife in 2008. Zika was also detected in a semen sample of another man in 2013, Choe says. In addition to how isolated those cases were, even up to a few days ago, many experts thought there was little concern about contracting the virus for people other than pregnant women, and that the risk to people in the US was minimal. Zika has spread so quickly throughout Latin America because window screens are seldom used to keep out mosquitoes, air conditioning is much less common, people are less likely to use insect repellent, and sanitation in many of those areas can be poor.


Symptoms are unlikely to present for several days, which raises concern that people could unknowingly pass along the virus through sexual contact, now that the possibility of person-to-person transmission is suspected (Choe said that although circumstantial evidence is strong, they can't say with 100 percent certainty). And current tests for it aren't sensitive enough to detect Zika for several days after infection, Choe says.

"By the time you're detectable, you're contagious already," she adds.

People traveling abroad, especially to Zika-affected areas should probably refrain from close contact with others for at least a week or up to two weeks, and use condoms when they do have sexual contact, she says. "That's the safest measure for now," Choe says.

Zika virus transmission in the US should be limited by the same factors that limit dengue transmission, as outbreaks of both have been limited to tropical regions, says Jared Aldstadt, associate professor of geography at the University at Buffalo. 

"People living in parts of the U.S. that are warm enough for transmission spend much of their time in air conditioned environments," he says. "These are not good environments for the vectors and limit their contact with people. Recent dengue outbreaks in Hawaii or Key West are probably good examples of the magnitude of outbreak that we would expect if a Zika outbreak occurred in the US."

Florida declared a state of emergency after cases were reported in four counties. Choe suspects that's because the aedes aegypti mosquito is present in several southern states. By summer, they're likely to see more Zika cases, similar to how dengue made its way there. Choe also echoes recommendations by the CDC to remove any standing water (such as rain collected atop trash cans) or puddles around your home that could attract mosquitoes, wear long pants and sleeves whenever possible, and wear mosquito repellent. 

"A couple weeks ago most people were less cautious" about recommendations for protecting yourself from the virus, Choe says. "But now, after hearing about this new case of sexual transmission, it makes me nervous. A week ago, we were just thinking more carefully about pregnancy, but I'm not sure now." 

What's Known
Zika is a relatively strange virus that is frantically being studied now. Here are some of the answers and questions that researchers still have about it:

  • The easiest way to get Zika is to get bit by a mosquito in a region where it's present.
  • Symptoms of Zika, including fever and rashes, only show up in one of four people. 
  • Zika usually lasts in the bloodstream about a week.

The Unknowns

  • What form of sex — anal, oral, vaginal — can pass the virus is not known; or if there needs to be blood present for it to be infectious. 
  • It is not known if symptoms need to be apparent for someone to be infectious. 
  • How long people might remain infectious is a big question mark.