First isolated in 1947 and first described in a paper in 1952, Zika has long been known to occur in Africa and Southeast Asia—but until a decade ago, fewer than 15 cases had been described in the scientific literature. In 2007, the virus caused a big outbreak on Yap, an island group in the Western Pacific that is part of the Federated States of Micronesia; since then, it went on a major tour of other Pacific Islands before it landed in Brazil, from where it started spreading rapidly to other parts of South America, Central America, Mexico, and the Caribbean.
Why has it exploded so suddenly?
There may have been big outbreaks in Africa and Asia in the past that went undetected; scientists weren’t paying much attention. But the current massive epidemic was an event waiting to happen. Latin America has huge numbers of A. aegypti, also known as the yellow fever mosquito, an important vector for Zika. (The Asian tiger mosquito, A. albopictus, which is on the rise around the world, is believed to be a vector as well.) In addition, nobody in the Americas had immunity to the virus. Travel makes it worse. Aedes mosquitoes don’t fly more than a few hundred meters during their lives; Zika travels from city to city and country to country when infected people get on cars, buses, trains, and planes.
These combined factors meant that the virus had the ability to spread far and fast once it had arrived.
Will Zika spread to the United States and Europe?
Both the United States and Europe have already seen “imported cases”—people who arrived from a Zika-affected country carrying the virus. This was widely expected given the size of the epidemic in Latin America. The key question is whether there will be local outbreaks—that is, mosquitoes spreading the virus from person to person. There’s definitely a chance; A. albopictus occurs in several countries in southern Europe (and it may move north), while the southern and eastern United States have populations of both A. aegypti and A. albopictus.
If so, scientists expect outbreaks to be much smaller than elsewhere, based on past experience with mosquito-borne diseases. Recent dengue outbreaks in Florida, Texas, and Hawaii haven’t sickened more than a few hundred people, for instance; an outbreak of a mosquito-borne disease called chikungunya in northern Italy in 2007—which started when a man infected with the virus arrived from India—ended after 197 cases. One reason that outbreaks in these countries tend to be smaller may be that people spend less time outside and live in houses that are more difficult for mosquitoes to enter; mosquito population sizes may play a role as well.
Do we know for sure that Zika is causing a rise in birth defects?
No. There is strong circumstantial evidence that areas in Brazil hit hard by Zika have experienced a sharp increase in the number of babies born with microcephaly, a condition in which the head is much smaller than normal because the brain fails to develop properly. But it will take at least several months before the results from the first case-control studies of pregnant women infected with Zika are available. Doctors in Brazil first noticed an increase in cases of microcephaly during ultrasounds of pregnant women in June and July, a few months after the sudden rise in Zika infections. Fetal medicine expert Manoel Sarno, who works at the Federal University of Bahia, says the pattern of brain damage he is seeing now looks distinct from microcephaly caused by other infections, such as cytomegalovirus (CMV) or rubella. He and his colleagues started a study in August that is following women infected with Zika during their pregnancy; the results could come out late summer. Similar studies are underway elsewhere in Brazil and in Colombia.
Are there other urgent questions that scientists are asking?
Plenty. Scientists have difficulty determining who has been infected and who hasn’t because diagnostic tests have limitations. The most accurate tests—which detect viral RNA in a patient’s blood—only work within a week of the first symptoms appearing. After that time, researchers can test for antibodies in the blood. But current tests for Zika antibodies cross-react with antibodies to dengue, which is so widespread in Brazil—and much of the rest of Latin America—that almost all adults have antibodies to it. That makes it difficult to tell whether the mother of a baby born with microcephaly was infected with Zika earlier in her pregnancy.
Researchers would also like to know how often Zika is transmitted through sexual contact. One U.S. scientist who caught the virus in Africa passed it to his wife after he got home in 2008, and a second case of suspected sexual transmission happened in French Polynesia in 2013. But researchers have no idea what the risk is. (“If I was a man and I got Zika symptoms, I’d wait a couple of months before having unprotected sex,” virologist Scott Weaver of the University of Texas Medical Branch in Galveston recently told The New York Times.)
What drugs are available against Zika?
None. Until last year, Zika was so rare and believed to be so mild, that nobody bothered to look for candidate drugs. Even now that the virus is surging, it’s not obvious that there’s a big market for an antiviral drug, because the vast majority of those infected have very few symptoms or none at all. And it’s not clear that a drug could prevent birth defects when women contract Zika during pregnancy; by the time they become infected and develop symptoms, it may be too late to prevent such damage. A vaccine against Zika may offer more hope of preventing microcephaly.
And when can we expect a vaccine?
That will take years. Several groups have begun to make candidate Zika vaccines, a process that will take at least several months. Most of these vaccine approaches are piggybacking on existing vaccines. For example, many vaccines are made by stitching proteins from a pathogen’s surface into a harmless virus or vector; that is now being tried with Zika using those same vectors. Once a candidate vaccine is made, it will have to be tested in animals before humans.Human trials begin with small safety studies, then move on to larger studies that test whether the candidate product works. All of that usually takes 10 to 15 months. Given the urgency, the timeline could be compressed, but even so, Anthony Fauci, the director of the U.S. National Institute of Allergy and Infectious Diseases, told STAT that it may be at least 5 to 7 years before a Zika vaccine is commercially available.
Then what can we do to stop the spread of the virus?
Stop mosquitoes from biting people. Countries and communities can try to reduce mosquito populations by removing the small water reservoirs—such as flower pots, empty bottles, and discarded tires—in which Aedes mosquitoes like to breed. People can also reduce their personal exposure—especially important for women who are or might become pregnant—by putting screens on windows, covering their skin, and using insect repellant. However, history has shown that the impact of mosquito control on epidemics is modest at best, and they’re difficult to sustain.
There must be better ways to control mosquitoes?
Not yet but they’re in the works. A British biotech called Oxitec—which was recently purchased by Intrexon, a U.S. synthetic biology company—has developed A. aegyptimosquitoes containing a gene construct that will kill their offspring before they reach adulthood. When massive numbers of male individuals of this strain are released in the wild, they will mate with local females, producing offspring that are not viable, which has been shown to make a dent in the population.
In another line of research, scientists are infecting A. aegypti with a bacterium named Wolbachia, which reduces mosquitoes’ ability to transmit diseases. The researchers developing these approaches were mostly thinking about dengue, but Zika’s surge is giving their attempts a new sense of urgency. But again, it will take several years before these strategies are ready for prime time.