When we think of preventing Zika—the virus linked to severe birth defects and that has continued to spread across Central America, South America, and the Caribbean—we likely think about long sleeves, mosquito netting, and insect repellent, as well as the potential for vaccines against the disease. According to new guidelines recently released by the Centers for Disease Control and Prevention (CDC), however, we should also be thinking about condoms: This virus can be sexually transmitted.
The new guidelines reflect many things we have learned about sexual transmission of the virus, as well as some of what we still don’t know—such as how long after infection the virus can be transmitted via semen and vaginal fluids, how big a part sexual transmission may play in the outbreak, and whether herd immunity may kick in to ultimately protect large areas of the world.
As of earlier this month, the CDC reported 4,128 reported cases of Zika in this country’s states. Nearly 4,000 of these cases were contracted elsewhere and 139 (mostly in Southern Florida) were determined to be from local mosquitoes. Another 34 cases were found to be a result of sexual transmission.
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Sexually Transmitted Zika: What We Know So Far
Though the CDC did not start referring to Zika as, effectively, a sexually transmitted infection (STI) until this summer, public health experts have understood for months that it could be transmitted through sex. CDC Director Tom Frieden told CNN back in February: “There have been isolated cases of spread through blood transfusion or sexual contact and that’s not very surprising. The virus is in the blood for about a week.”
As cases of sexually transmitted Zika virus have been reported from around the world, public health experts learn more about the disease. We now know from a case in Spain that occurred in February, for example, that Zika can apparently be transmitted by men who have had vasectomies. This means that the virus lives not just in sperm, but in other fluids that make up semen as well.
In September, health experts learned that men can transmit Zika even if they have no symptoms. That month, a woman in Maryland was diagnosed with Zika. Her only risk factor was having had sex with a man after he returned from the Dominican Republic, where Zika-carrying mosquitoes are more common. He was later diagnosed with Zika as well but would not have known he had contracted the virus had she not gotten sick, because he never experienced any symptoms. There has also been at least one instance of male-to-male transmission.
And though public health experts originally believed that female-to-male transmission might not be possible, a case in New York City suggests it is a risk. Over the summer, a young woman was diagnosed with the virus after visiting a Zika hotspot. After displaying symptoms, she sought medical attention and was diagnosed with the virus. Before her diagnosis, she’d had sex with a male partner who later developed similar symptoms and was also diagnosed with Zika, even though he had not left the city for areas with Zika-carrying mosquitoes.
Some things, however, are still not clear, including how long Zika can live in bodily fluids. The virus has been discovered in semen for as long as six months after initial symptoms, but by then, there may not be enough viral presence to infect a partner. According to the European Center for Disease Prevention and Control, the longest interval between onset of symptoms in a male and onset of symptoms in his female partner is between 34 and 41 days. The Zika virus seems to last for a far shorter period of time in vaginal fluids
—researchers have found it 11 days after the start of symptoms.
How Big a Problem Is Sexual Transmission?
Zika is unique in that it is transmitted through both sex and mosquitoes, which makes it hard to determine just how much of an effect sexual transmission is having on the spread of the disease. In places like the United States, where transmission by mosquitoes is still rare, it is relatively easy to determine when a case is sexually transmitted. In the heart of the outbreak in Central and South America, however, distinguishing those who got bitten from those who had sex with someone else who got bitten is practically impossible.
We cannot dismiss any transmission route as insignificant, however, because one infected person can lead to many more infected people. Those who travel to Zika hotspots are told to avoid mosquito bites for three weeks upon their return to the United States, so as not to introduce the virus into the mosquito population in their own area. By that logic, someone who has sex with a person who traveled to a Zika–infected area might also be a potential vector into the mosquito population in their area. And, of course, anyone who contracts Zika, whether they know it or not, could be a vector for the disease into the population of sexually active people. With all of these uncertainties, it is not surprising that opinions about how notable of a role sexual transmission will ultimately play vary widely.
Writing in the op-ed section of the New York Times this summer, Kelly McBride Folkers, a research fellow at NYU Langone Medical Center, suggested that Zika could become the “millennials’ S.T.D.” Folkers points out that if Zika does stay in bodily fluids for months after infection, and as many as 80 percent of people never realize they are infected because they are asymptomatic, individuals could be transmitting the virus without knowing it “after the mosquitoes go away.” Using mathematical models from other viruses such as dengue that attempt to calculate how many infections one infected individual is likely to cause, Folkers argues that “sexual transmission is likely to be a significant contributor to the Zika virus’s spread.”
Others, however, doubt that it will have that much impact. Mosquitos are very efficient vectors of disease; not only do they bite numerous people in a short period of time but they are, as Julia Belluz put it for Vox, like “syringes with wings,” which poke into a person’s skin, draw blood, and leave a little bit of their own virus-containing saliva behind. Sex is a less efficient method of transmission, and most individuals have few partners in a given week or two. Dr. Nikolaos Vasilakis, a professor and researcher at the University of Texas Medical Branch, told Vox, “Yes, sexual transmission may play a role [in spreading the virus] but not as significant as the bites of a mosquito, which can go serially within a short period of time.” John Brooks, a physician who is leading the CDC’s efforts on this issue, said in an interview with National Geographic that if sexual transmission were as significant a vector as mosquitoes, we would have expected more cases to have shown up in the United States already.
One reason we might never see too many sexually transmitted cases of Zika is the possibility of immunity. Researchers currently think that once a person becomes infected with Zika, they gain lifelong immunity from reinfection. This would mean that during the course of a lifetime, a person could only transmit the virus during one window of time (whether that window is 11 days, 41 days, or six months). Moreover, once enough people in an area are infected and develop lifelong immunity, whole communities could develop herd immunity (similar to what happens when large portions of a population are vaccinated).
This is very different from any of the STI epidemics we are currently facing. Even though some STIs—such as chlamydia and gonorrhea—are curable, people can contract and transmit them multiple times. The same is true of HPV, which often clears the body without treatment: An individual can be reinfected. Other STIs, such as HIV and herpes, stay in your body indefinitely and can continually be transmitted, though medications can suppress them to the point that an individual is not infectious while on the drugs.
Who Should Prevent Sexually Transmitted Zika and How?
Like with any other STI, there are two basic choices for preventing the spread of Zika virus between partners: abstain from sex or use a condom. However, unlike other STIs, Zika is not something that all sexually active individuals have to worry about equally.
For most people, infection with the Zika virus is not a problem. Many people who contract the virus will never know because they’ll have no symptoms. Others may experience mild symptoms including fever, headache, joint pain, or conjunctivitis (commonly known as pink eye). The symptoms usually last a week. Very rarely, Zika infection can lead to more serious health complications, including a condition called Guillain-Barré syndrome, in which the body’s own immune system begins to attack the nervous system, causing weakness and tingling in the limbs. (The CDC reports 13 cases of this disorder linked to the current Zika outbreak.)
For the most part, however, Zika is only a danger to developing fetuses. Though there is still a lot that we don’t know, the virus appears to impede proper brain development. In fact, the severity of the current outbreak only became clear when a number of babies in Brazil were born with microcephaly, a severe birth defect in which babies are born with an abnormally small heads. Other birth defects linked to Zika include eye defects, hearing loss, and impaired growth. Thus far in the United States, there have been 25 live infants born with evidence of Zika-related birth defects and five pregnancy losses as a result of similar birth defects.
Because of this, the CDC’s recommendations are strongest for pregnant women and their partners, as well as anyone who is planning to become pregnant in the near future. The CDC suggests that pregnant women avoid all travel to infected areas. The agency also suggests that pregnant women who live in, have traveled to, or have a partner who has traveled to Zika-infected areas use condoms or abstain from sex for the entire length of their pregnancy. Men and women who live in or have traveled to high-risk areas and are planning to become pregnant should talk with their health-care provider about safe timing for that pregnancy.
The CDC suggests that men and women who are not pregnant or planning to become pregnant but live in Zika-infected areas consider using condoms or abstaining from sex until the virus is gone from the area.
For women who are not pregnant but have travelled to an infected area, the CDC suggests waiting eight weeks from the time of travel if they have no symptoms or eight weeks from the time of diagnosis if they did get Zika before having unprotected sex. Men who travelled should wait six months from return or diagnosis before having unprotected sex.
Health–care providers, note representatives from the CDC, also have to expand the set of questions they ask to those exhibiting potential Zika symptoms to include a sexual history. It is no longer enough to just ask about their travel; they must also inquire about the travel of any sexual partners the patient may have had.
As fall weather settles over much of the United States and the mosquitoes stop buzzing and biting, many of us may have put those annoyingly itchy bumps out of our minds. But we should remember that in some parts of this country and many parts of the world, there is no end to mosquito season, and no matter where you live sex is year-round. We should all consider our risk based on where we live, where we visit, whom we sleep with, and whether we are or are planning to become pregnant. Anyone at risk should use condoms to protect themselves and their partners.