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Revisiting Zika and Pregnancy in 2018: What Have We Learned?

In 2015, Zika and pregnancy concerns dominated the headlines. Zika had been linked to thousands of babies in Brazil being born with severe brain damage, the news and images shocked and horrified the world.

The mosquito-borne virus had been believed harmless up to this point. But following the harrowing reports out of Brazil, The World Health Organization declared a state of emergency. The U.S. Center for Disease Control issued warnings that pregnant women avoid travel to Zika-ridden areas. Columbia, a majority Catholic country, took the extreme step of asking women of childbearing age to postpone pregnancy for at least two years.

Back then, the unknowns were almost as terrifying as the knowns. We did not know how likely a Zika infection during pregnancy would cause birth defects. Or whether the risk varied depending on when during pregnancy a woman became infected. And we did know not know whether infected partners could pass Zika through sex. Obstetricians and health officials were at a loss for how to advise their potentially exposed patients.

These unknowns and the devastating links between Zika and pregnancy spurred a massive scientific undertaking to understand how Zika affects the fetal brain. Thanks to that research, we now have good answers for many of these earlier questions:

What is Zika?

Zika is a flavivirus virus, a biological relative of the tropical viruses Dengue and Chikungunya. Like these other tropical viruses, Zika is primarily spread by the daytime-biting female Aedes aegypti and Aedes albopictus mosquitoes.

zika and pregnancy in 2018


First discovered in Africa in 1967, Zika spread to several Pacific islands in late aughts and to Brazil by 2014.

Until recently, Zika was believed a largely harmless virus that caused only a mild, temporary malaise. Then, in 2015, the Brazilian government noted a surge in the cases of newborn microcephaly–babies with abnormally small heads–that coincided with an ongoing Zika epidemic. Since then, scientists have conclusively shown that Zika infection can infect the placenta and pass to the developing fetus.


Is Zika spreading in the United States?

Likely, no.

In 2016, there were small Zika outbreaks in Miami, Florida and in Texas along its southern border. A large outbreak in the US territory of Puerto Rico affected over 40,000 of its residents, including at least 3,700 pregnant women.

But by 2017, the number of known cases in the continental US had dropped to one, and the number of new cases in Puerto Rico had declined to a steady trickle. While it is too early to predict how the 2018 mosquito season will go, fewer travel-related cases are expected this summer, because Zika’s spread in South and Central America has also slowed substantially. The CDC is continuing to monitor pregnant women along the southern border for Zika infections, but as yet, no cases have been reported.

So far, for 2018 ,there have only been a total of 28 cases in the US.  Those cases “include reported confirmed and probable Zika virus disease”. California is up there with the “most” confirmed cases – a whopping 12.

map of zika infection in united states in 2018
Credit: CDC


Where should pregnant women and their partners avoid travel or take precautions?

In 2015 and 2016, Zika rampaged through the Americas, spreading rapidly through Brazil, Colombia, and Central America. Like a mass vaccination, the widespread epidemics of 2015-2016 are thought to have rendered large swaths of these populations immune, and thus dramatically slowed the spread of the virus.

Pockets of outbreaks are ongoing, so travelers who are pregnant or planning to become pregnant should check the CDC country-specific warnings before booking travel. The CDC continues to recommend that women who are pregnant or planning to become pregnant avoid travel to areas with any ongoing Zika activity. Areas above 6,500 ft elevation are generally considered low risk, because the mosquito that transmit Zika does not live above this elevation.

Credit: CDC


Similar caution on travel is warranted for men whose partners are pregnant or planning to become pregnant.


How does Zika affect a developing baby?

Zika can be devastating to the infected fetus, causing fetal death in an unknown percentage of pregnancies, and wreaking havoc on the rapidly developing brains of those that survive the infection. The most striking of form of Zika-induced brain damage is severe microcephaly–an abnormally small head, technically defined as 3 standard deviations or more below the mean.

Zika causes microcephaly by targeting and killing the brains stem cells that normally develop into neurons. When these stem cells are destroyed, brain development stalls. Because fetal brain growth in the first 30 weeks of gestation of spurs growth of the head, babies affected before 30 weeks are born with microcephaly.


travelers who are pregnant or planning to become pregnant should check the CDC country-specific warnings before booking travel.


Though the media have focused on the dramatic pictures of babies born with microcephaly, exposure to Zika during gestation causes a number of other problems, now collectively referred to as Congenital Zika Syndrome (CZS). These include more subtle forms of brain damage, such as enlargement of the fluid-filled spaces of the brain, loss of white matter, and scarring. These may occur even in the absence of microcephaly. A normal-sized head does not indicate that a baby has escaped unscathed.

After birth, babies with CZS often suffer from hearing and vision loss, stiff muscles and joints, difficulty eating, sleeping, and swallowing, severe mental and physical delays, and seizures.


Does Zika actually cause these fetal brain abnormalities?


When the first news out of Brazil reported a potentially Zika-related spike in microcephaly, lots of alternative theories circulated on the internet, including a popular theory that blamed the mosquito larvicide pyriproxyfen. This theory has been thoroughly debunked.


woman during pregnancy and zika in 2018


The evidence is now incontrovertible that Zika causes these birth defects. In the lab, Zika has been shown to infect and kill fetal neural progenitor cells–the cells which develop into neurons (NPC). Elevated rates of brain damage have been shown in studies that follow infected pregnant women through birth, and animal studies show similar in utero effects.


How likely is a Zika infection during pregnancy to cause fetal brain abnormalities?

The best estimates come from the US Zika Pregnancy and Infant Registry, which has so far tracked the outcomes of over 2,500 Zika-affected pregnancies. Between 5-15% of babies suffered brain damage.


So far, for 2018 ,there have only been a total of 28 cases of zika in the U.S.


As with other birth-defect causing viruses like Rubella and Cytomegalovirus, the timing of exposure matters. Infection in the first trimester appears more likely to cause harm: 8-15% of fetuses born to women infected in their first trimester showed neurological abnormalities, compared to 4-5% infected in their third trimester. Microcephaly occurs most often before 30 weeks, the point at which brain growth slows, but babies infected in the third trimester can still suffer significant brain abnormalities, such as enlarged ventricles and scarring.


Will I know if I have been infected with Zika?

Probably not. An estimated 4 out 5 Zika infections cause no symptoms or only very mild symptoms.  But there is a test for Zika available now and the CDC has some very helpful information about it.


zika and pregnancy
Credit: CDC


For those with symptoms, common complaints include a mild fever, headache, rash, red eyes, and joint pain. These usually resolve within 7 days.


Are developing babies less likely to suffer harm if the pregnant mother has no symptoms of Zika?


The data from the US Zika Pregnancy and Infant Registry finds that Zika is just as likely to cause harm to the fetus when a pregnant woman experiences no symptoms as when she experiences symptoms.


Does Zika affect the brains of adults?

We know that Zika can infect and devastate the fetal brain. But what about the brains of infants, children, and adults?


This is still an open question.

Recent lab studies suggest that Zika may cross through the blood-brain barrier in adults, infecting their brain and nervous system.

Clinically, the ability of Zika to infect the adult brain and nervous system may explain why several countries, including French Polynesia, Columbia, and Brazil have reported an uptick in Guillain-Barre Syndrome (GBS) during Zika outbreaks. GBS is a rare autoimmune condition, triggered by viral or bacterial infections, that attacks the nervous system and can lead to temporary paralysis and even death.

Fortunately, GBS following Zika appears to be a rare complication, affecting only an estimated 2 out 10,000 people infected.

We do not know if Zika causes other, more subtle effects on the adult brain. Adult brains no longer have dividing neural progenitor cells, the brain cells that Zika targets and devastates in fetuses.


What about Zika infections in newborns, older infants, and toddlers?

Unlike adults, the brains babies and toddlers until between 18-24 months of age are still undergoing rapid development, and their neural progenitor cells, the key cells that Zika targets and destroys in the fetal brain, continue to create new neurons in brain areas involved in memory and learning.

“We think that after 1 year-and-a-half—after 18 months of age—there are no more actively dividing neural progenitors. Up to 18 months, Zika probably can still affect these progenitors and have some effect in the brain, but this will be very small because most of the brain is already formed.” says Andre Sousa, a neuroscientist at Yale, who has researched the effects of Zika on the brain.

(Personally, I find less than reassuring that the impact of killing some of my baby’s brains cells will likely be “very small”.)


For those staying in the US or not traveling to affected countries, Zika need not be high on your worry list.


But before you panic, like I briefly did upon hearing this fact (OMG, I just went to Cancun were I was bitten by mosquitos, and I could have become asymptomatically infected, passed Zika on via breastfeeding to my 18 month old and permanently damaged her brain!), take a deep breath and reflect on this: No studies have shown any neural or cognitive effects of Zika in this infants or todddlers. So far, their symptoms appear to mirror the mild ones seen in nearly all adults.


Can Zika spread through sex?


According to the Centers for Disease Control, Zika can be transmitted via “vaginal, anal, and oral sex and the sharing of sex toys”. (You know a virus is serious when the CDC goes R-rated). Infectious viral particles have been found in semen and vaginal secretions, and public health officials have documented both male-to-female and female-to-male sexual transmission.


How long after being infected can someone spread Zika via sexual intercourse?

Zika may survive in semen for several months. Hence the CDC recommends avoiding sex or using condoms for 6 months after the male partner was infection, and for 8 weeks after the female partner infected.


Is Zika spread any other ways?


Zika also appears to pass into breastmilk. Public health officials have documented several cases in which infants appears to have been infected via breastfeeding. Blood transfusions can also spread the virus.


Should I worry about Zika?

For those staying in the US or not traveling to affected countries, Zika need not be high on your worry list.

It’s reasonable if you are pregnant or planning to become pregnant to take precautions when you or your partner is traveling. Check the latest advisories and always use mosquito repellent in potentially affected regions.


Overall, the chance of contracting Zika here, in the U.S., remain very low. Breathe easy, mama.


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About Amy

Amy Kiefer is a researcher by training, and earned her Ph.D. from the University of Michigan. She currently lives in the Bay Area with her husband and three children where she writes about fertility, pregnancy, and breastfeeding. Check out her blog,, for more great evidence-based pregnancy and parenting info.


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