How Zika Virus Works

By: Nicholas Gerbis

Researchers at the Fiocruz Institute in Recife, Pernambuco, Brazil are studying the Aedes aegypti mosquitoes behind the spread of the ZIka virus.
Researchers at the Fiocruz Institute in Recife, Pernambuco, Brazil are studying the Aedes aegypti mosquitoes behind the spread of the ZIka virus.
Mario Tama/Getty Images

In late 2015, American media began buzzing with rising concern over a mosquito-borne disease that was old news in Africa and Asia but new to the West. To doctors familiar with this relatively wimpy cousin of dengue fever, chikungunya and yellow fever, the concern might have seemed misplaced, were it not for a series of seemingly related cases of microcephaly that coincided with the virus's arrival in Brazil.

Fears rose as images of children with abnormally small heads began appearing in the news, accompanied by words like "unusually severe" and "incomplete brain development." And as it became clear that the Americas, whose lack of Zika immunity was matched by their abundance of mosquito-friendly habitats, were facing an epidemic, it no longer mattered that the virus typically caused only mild flu-like symptoms in a small percentage of infected adults. Proven or not, understood or not, the threat of children being born with microcephaly — or with Guillain-Barré syndrome, another neurological disorder linked to Zika — soon drove calls for "nuclear" options. The mosquitos, said some, had to die, and devil take the consequences.


But wait a minute, said others. What's actually going on here? Are these neurological disorders truly caused by Zika? If so, what mechanism links the two phenomena, and why have we never heard of this connection before? Is it possible that the link was mere coincidence, or that we are overlooking some vital factor, such as diet or environment? And what about the fact that Zika can be spread through blood, semen or possibly other bodily fluids?

Whatever the case, where Zika has led, dread has followed. On Feb. 1, 2016, the World Health Organization declared Zika a Public Health Emergency of International Concern because of its wide and fast spread in the Americas and its possible link to neurological complications. One week later, the Obama administration said it would ask Congress for more than $1.8 billion in emergency funds to support testing, surveillance, response and containment, and to pursue a vaccine. Some countries, including the U.S., have recommended that their athletes skip the Summer Olympics in Rio de Janeiro. Some, too, have suggested that women put off having children for a few years.

As of early February 2016, Zika had reached epidemic levels in Mexico, the Caribbean, and Central and South America [source: Ungar]. The disease is spreading through 33 countries, home to a total of 600 million inhabitants, suggesting the potential infection of tens of millions [source: McNeil et al.]. There's no vaccine on the immediate horizon, so, as usual, our best weapon is knowledge.


Zika Origins

The female mosquito, who has to suck lots of blood in order to lay her eggs, spits the Zika virus into each her new snacks when she bites them.
The female mosquito, who has to suck lots of blood in order to lay her eggs, spits the Zika virus into each her new snacks when she bites them.
Universal History Archive/UIG via Getty images

Zika is a mosquito-borne arbovirus (short for arthropod-borne virus) of the genus Flavivirus, family Flaviviridae — the same genus as yellow fever, dengue fever and West Nile virus [source: CDC]. It comes in two main flavors, or lineages — African and Asian, each with multiple strains. The Americas play host to the Asian lineage. Zika can infect a number of animals, such as elephants, lions, rodents and zebras, but humans and nonhuman primates seem to be its main reservoirs [source: Rogers].

Like many of its Flaviviral cousins, Zika's mosquitos of choice come from the Aedes genus, particularly the aegypti species native to Africa. In the U.S., these skeeters can be found in the Gulf Coast states, Georgia and South Carolina, the southern half of Arizona, and peppered across California and New Mexico. The virus likely makes a second home in aegypti's Asian counterpart, Ae.albopictus, which has American digs as far north as New Jersey and as far west as west Texas, with ranges extending into the southern third of Arizona and bits of California and New Mexico [sources: CDC, McNeil et al., Rogers]. Other aedine mosquitoescan carry the disease, too. For example, some believe that the 2013 French Polynesian Zika outbreak spread through the proboscises of Ae. polynesiensis [source: Rogers].


Mosquito-borne Zika begins its cycle when a female, which requires blood to lay eggs, pokes a meal source and sucks in the virus along with her plasma Slurpee. Zika travels in the blood to the female's gut, later to return via her circulatory system to her saliva. Her spit's anti-clotting properties come in handy when she squirts it — and the virus — into a later meal [source: McNeil et al.].

In the tradition that that did so much to boost tourism on the Ebola River, Zika was named for the Ugandan forest in which it was isolated in 1947, first in a rhesus monkey and later in mosquitos. Zika antibodies were first found in humans in the early 1950s [sources: McNeil et al., Rogers]. Unfortunately, naming convention is not all Zika shares in common with Ebola. Like that hemorrhagic fever, Zika might "hide" and replicate in areas of the body — such as the brain, eyes, placenta and testes — that block immune response [source: Steenhuysen].

Once confined to Africa and Asia, in 2007 Zika began moving across the Pacific. By early 2015, it had already knocked around Brazil for at least a year. Although the outbreak there began in May 2014, cases had already shown up in Natal, capital of the state of Rio Grande do Norte, earlier that same year. This timing roughly lined up with the 2014 World Cup, which was partly held in Natal, although numerous other proposed sources are equally possible [sources: McNeil et al., Wade].

Now in a country with conditions ripe for mosquito population explosion, situated in a hemisphere with virtually no immunity, Zika spread quickly in 2015. And that's when the trouble really began.


Zika and Microcephaly

Estafany Perreira holds her nephew David Henrique Ferreira, 5 months, who has microcephaly. Microcephaly results in newborns with abnormally small heads and is associated with various disorders including decreased brain development.
Estafany Perreira holds her nephew David Henrique Ferreira, 5 months, who has microcephaly. Microcephaly results in newborns with abnormally small heads and is associated with various disorders including decreased brain development.
Mario Tama/Getty Images

Mild concern over Zika's advent in the West turned to dismay when Brazilian doctors began to find record numbers of microcephaly cases in the maternity wards of Pernambuco. The northeastern state of 9 million people typically saw just nine cases of microcephaly out of 129,000 births each year. In November 2015, they reported 646 such births, with neighboring states Bahía and Paraíba soon following suit [sources: McNeil et al., Wade]. Brazilian health officials were facing 4,000-plus possible cases; by mid-February 2016, they had verified around 400, compared to the country's average rate of 150 to 163 microcephaly cases per 3 million annual births [sources: Berkrot and Boadle, McNeil et al., Rampton and Hirschler, Wade].

The sheer prevalence was enough to shock physicians, but doctors were taken aback by the severity of the cases as well. Beyond the characteristic small heads and brains, they found eye malformations, intracranial calcifications (aka "brain stones"), malformed cerebral cortexes, or abundant spinal fluid suggesting that the brain had grown and then abruptly shriveled [sources: CDC, Berkrot and Boadle, Rogers]. In many cases, mothers were confirmed to have Zika or Zikalike symptoms while pregnant [sources: CDC, Wade]. Zika had also been found in placentas, amniotic fluid and fetal brain tissue, proving that the virus could cross the placental barrier [sources: Steenhuysen, Wade].


Meanwhile, another disorder began spiking in suspected Zika patients — Guillain-Barré syndrome (GBS), a slowly paralyzing attack on the body's nervous system by its own immune system [sources: CDC, NINDS, Rogers]. One Pernambuco neurologist reported 50 patients in 2015, compared to 14 the year before [source: McNeil et al.]. A similar uptick had occurred before, during the Polynesian outbreak of 2013, but the Zika link in that case, if true, remains unknown [sources: CDC, Rogers].

Indeed, tracing a clear causal link between Zika and microcephaly has proven tricky — in part, because it's baffling. As one Pan American Health Organization epidemiologist told The New York Times, no one has ever seen a "congenital malformation by mosquito before." Moreover, the microcephaly link is unconfirmed and largely circumstantial — a matter of co-occurrence in some regions but not others. Further blurring the picture: false positives, varying diagnostic standards and spikes in diagnoses thanks to heightened awareness [sources: CDC, McNeil et al., Wade].

Fire or no, there is plenty of smoke. In 17 of the 400 cases mentioned above, Zika has been confirmed in mother or baby. In a different study of 35 Brazilian babies born with microcephaly, all of the mothers involved had spent time in a known Zika area while pregnant [sources: Berkrot and Boadle, Rampton and Hirschler].

Clearly, caution is called for, as well as a good set of guidelines for avoiding spreading the disease to women who were, or might become, pregnant. If only it were that simple.


Sex and Pregnancy

A pregnant woman gets an ultrasound at the maternity of the Guatemalan Social Security Institute (IGSS) in Guatemala City. Guatemala increased the monitoring of pregnant women amid the rapid spread of Zika virus.
A pregnant woman gets an ultrasound at the maternity of the Guatemalan Social Security Institute (IGSS) in Guatemala City. Guatemala increased the monitoring of pregnant women amid the rapid spread of Zika virus.

Zika's second avenue of transmission has less to with bugs than with the birds and the bees. Evidence suggests that a man can transmit the virus to his sex partners, although this is probably not common, and in most known cases the men experienced symptoms such as genital pain and bloody semen. We know that the virus persists in semen longer than in blood (a few months versus 7 to 10 days), but we don't know how long semen remains infectious. We also don't know whether a woman can transmit Zika to a sexual partner, or what types of sexual contact (e.g., anal or oral) could spread it [sources: CDC, McNeil et al., Steenhuysen].

It's also possible that Zika can spread through blood transfusions and that the virus lives on in saliva and urine, although its transmissibility via these fluids remains an open question [sources: CDC, McNeil et al., Rampton and Hirschler].


Although cases remain rare, mothers can transmit Zika to fetuses, typically at or near the time of birth. Infection might also occur during pregnancy, but the jury is still out on this [sources: CDC, pregnancy; CDC, transmission]. If true, then infection earlier in pregnancy is likely more dangerous [source: McNeil et al.]. Researchers have detected Zika RNA in breast milk but have yet to see transmission through that route, so the benefits of breastfeeding still outweigh the risks [source: CDC].

According to the CDC, EPA-registered insect repellents that contain DEET, picaridin and IR3535 are safe for use during pregnancy.

Given the sexual knowns and unknowns, the safest plan for people at risk of having, getting or giving Zika lies in abstinence or in using condoms the right way every time. Along similar lines, countries such as Brazil, Colombia, El Salvador and Honduras have called for women to put off having children for a few years [sources: CDC, McNeil et al., Wade]. Initially, Latin American Catholic bishops balked at the mention of contraceptives, but Pope Francis has since relaxed the church's stance in cases of likely Zika infection [sources: Goodstein, NPR]. The Pope still strongly denounces abortion, however, which means that pregnant women affected by these issues will face significant resistance from both the Catholic Church and, in many countries, the legal system if they decide to terminate their pregnancies.

Further complicating matters, Zika is hard to diagnose in adults, let alone fetuses. There's no widely available test, and procedures such as amniocentesis carry risks of injury to the baby and rendering false positives. In part, this is because we can only detect Zika in the first week — after that, we're detecting antibodies, which closely resemble the antibodies of Zika's cousins (and their vaccines) [sources: CDC, McNeil et al.].

Bottom line: If you got pregnant before/during recent travel to a country with Zika, see a doctor and get a blood test 2 to 12 weeks after returning. Have ultrasounds done, too, but prepare yourself for the fact that your doctor won't be able to spot microcephaly before the end of the second trimester. If you get pregnant after travel to a country with a Zika outbreak, your risk is much lower, but you should still have your newborn tested. Even if your child does not have microcephaly, other birth defects (e.g., vision and hearing) might still occur [sources: CDC, McNeil et al.].


Protecting Yourself — and Fighting Back

Health workers fumigate in an attempt to eradicate Zika-carrying mosquitoes in Recife, Pernambuco state, Brazil.
Health workers fumigate in an attempt to eradicate Zika-carrying mosquitoes in Recife, Pernambuco state, Brazil.
Mario Tama/Getty Images

We said above that the obvious answer to avoiding mosquito-borne viruses is to avoid mosquito bites. This is easier said than done with Aedes, which bite aggressively, mainly (but not exclusively) during the day, and live and feed both indoors and out [source: CDC]. But mosquitos are only half of the equation. The other half is their preferred meal — us [source: CDC].

In many areas, including the U.S., mosquitos do not yet carry Zika, and all cases come from human travelers. We want to keep it that way. So if you think you have Zika, see a doctor, and try to avoid being bitten by any mosquitos, especially during the first week of illness [source: CDC]. Also, as mentioned previously, you should really abstain from sex. But if you can't fight the urge to merge, at least inform your partners, and use condoms the right way every time [source: McNeil et al.].


Bite avoidance is mainly a matter of wearing the right clothes, controlling your environment and living better through chemistry. Make long-sleeved shirts and long pants your new fashion statement, and treat your togs with permethrin. Spend some quality time in air-conditioned spaces, deck out your domicile with screens and screen doors, and sleep under a mosquito net when camping or traveling in Zika-infested countries [source: CDC].

Obviously, travelers should pay attention to health notices and avoid traveling to places where Zika exists, if possible [sources: CDC, McNeil et al.].

States and countries are fighting Zika through a combination of tracking populations and denying the critters their preferred habitat. You can help by organizing your neighborhood to cover, get rid of and dry out areas where water collects. Aedes mosquitos lay their eggs in standing water, and even a bottle cap's worth can be enough. During mosquito season, officials will sample adult mosquitos for evidence of infection and apply adulticides around homes known to have Zika [sources: CDC, McNeil et al.].

More radical solutions with potentially unforeseeable and dangerous consequences involve wiping out the mosquitos themselves. One company is offering to spread genetically modified mosquitos to wipe out Aedes vectors. These male mosquitoes are engineered to have offspring that die before adulthood [source: Barker]. But the wrong approach could have disastrous ecological consequences. Whether that will stay our hands for long remains to be seen.


Lots More Information

Author's Note: How Zika Virus Works

In many ways, Zika is a nightmare for epidemiologists, and not only because it is spreading through an area of the world with plenty of mosquito habitat and no immunity to the disease. No, what makes Zika truly awful is its horrifying, yet unproven, link to microcephaly. Emotion makes for a poor companion to crisis decision-making and — after an Ebola outbreak that many viewed as poorly handled, and faced with images of suffering babies and mothers — we're primed for overreaction. The question is, do we know enough about Zika to measure our approach, to put resources where they can do the most good, or will we let our fears get the better of us?

For now, Zika will spread — or, perhaps not. It's possible the virus has already peaked, at least in the region where it hit most intensely [source: McNeil et al.]. The only thing that is truly clear is that scientists need to understand how Zika affects developing fetuses, whether through mouse models, "brain balls" grown from stem cells or some other means [source: Wade]. We simply lack too many essential answers.

Related Articles

More Great Links

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