Zika virus is no Ebola, but neither was Ebola

For anyone who remembers the millions of Americans dying in the streets from Ebola in 2014, the news about the Zika virus this week must have conveyed a chilling sense of déjà vu: another obscure virus originating in the forests of Africa, “spreading explosively,” this time not an ocean away but right in our own hemisphere. Even for those who don’t remember America’s Ebola epidemic — because it didn’t happen — there were echoes of that nondisaster: the cautiously hedged official warnings, the dots spreading ominously over the maps in newspapers, and the fifth horseman of every 21st-century apocalypse, a sell-off in airline stocks. Who will be the first Republican candidate to blame it on Barack Obama?

For most Americans, what Zika — at least in our current understanding — has in common with Ebola is this: a remote risk of a horrendous outcome, which, in a quirk of human psychology, may translate into an instinctual, not entirely rational, panic. In Zika — generally a disease so mild that it may not even be recognized by those infected — the risk is of a particularly ghastly birth defect: microcephaly, characterized by an abnormally small head and impaired brain development. (In an unknown fraction of cases, infection with Zika may also lead to Guillain-Barré Syndrome, a severe but usually temporary paralysis.) When the number of newborns with microcephaly unexpectedly shot up in Brazil last year, public-health officials found a statistical, and later a clinical, correlation with an epidemic of Zika, a virus that was first identified in East Africa in 1952 and named after the Uganda forest where it was found. The relationship is still unproven, and it is almost impossible to quantify the danger. But the evidence was enough for the Centers for Disease Control and Prevention to warn pregnant women to “consider postponing” travel to the areas of Central and South America affected by Zika.

In a move virtually unprecedented in the annals of public health, the government of El Salvador — a country with one of the most stringent antiabortion laws in the world — advised women to put off having babies for at least the next two years.

The other thing about Zika that reminds people of Ebola is that there is no vaccine or specific treatment for the infection (although that may be changing in the case of Ebola). But whereas Ebola spreads with frightening ease by direct contact, Zika is transmitted largely, if not entirely, by way of mosquitoes. “One, or possibly two, cases of sexual transmission have been reported,” says Justin Lessler, PhD, associate professor of infectious disease epidemiology at Johns Hopkins University, “but if it happens, it is massively less efficient than by mosquito.” That’s bad news — because mosquitoes fly, infecting people over a wide area — but good news in the sense that even lacking drugs to attack the virus directly, we do know how to kill mosquitoes.

Slideshow: Battling the Zika virus >>>

Right now, with much of the Midwest and Northeast covered with snow, the threat to Americans from the two species implicated in spreading Zika — Aedes aegypti and Aedes albopictus, better known as the Asian tiger mosquito — is probably confined to the Florida Keys, southern Texas and possibly the southernmost parts of California. That’s the assessment of Kenneth J. Linthicum, who as director of the U.S. Department of Agriculture’s Center for Medical, Agricultural and Veterinary Entomology is — along with his counterparts at the CDC —a major player in the fight against Zika. “Peak transmission periods in the Northeast and Midwest would probably be May to September,” he says, adding that “this peak transmission period could be expanded if temperatures in a given year warm early and/or stay warm longer.” In a warming world, “this is likely to occur more in the future.” There’s some evidence that aegypti is expanding its range into hot, drier climates — as long as it can find a small container of water in which to lay its eggs — and by this summer, around 60 percent of the American population may be living in areas where Zika has the potential to spread.

But, as Lessler points out, Aedes mosquitoes also transmit other viral diseases, notably dengue and chikungunya fevers. So far, he notes, outbreaks of Zika seem to track the spread of those two viruses through South America and Central America. But even without a wall at the southern border, dengue and chikungunya haven’t made much progress beyond the Rio Grande, a puzzle that mosquito experts think may reflect cultural and economic disparities: greater use of air conditioning and window screens, less time spent outdoors in the summer and a predilection for dwellings surrounded by lawns and pavement rather than ceramic pots and planters that hold water. And, of course, intensive disease surveillance and aggressive mosquito control by local, state and national officials.

So while epidemiologists agree that caution is warranted — most especially in women who are pregnant — based on what we know so far, Zika doesn’t seem to pose an existential threat to the United States any more than Ebola did. The unanswerable question is whether we have just been lucky so far, and how long our luck can hold. “We need to have a better infrastructure for checking people coming into the country,” says Joseph Conlon, a retired Navy entomologist who is a spokesman for the American Mosquito Control Association. “We’ve got the mosquitoes; they’ve got the disease. Malaria, yellow fever, even dengue — these used to be common in the United States, all up and down the East Coast. We don’t need global warming for this to happen.

“There are viruses we don’t even know exist. Ecotourism is bringing stuff back here, and people think, if I get sick, I’ll go to the hospital and they’ll cure me. Not with these things: Either you get better on your own, or you die.”

If you ask a mosquito-control expert what keeps him up at night, there’s a good chance he’ll bring up Rift Valley virus, endemic to sub-Saharan Africa, where it primarily infects livestock and wild animals. “Fatal [sometimes] to people, extraordinarily fatal to cattle and horses,” says Conlon. “If it got loose here, where there’s no natural immunity, it could be devastating.” In recent years, it has spread beyond its traditional range into the Middle East and Madagascar. Like Ebola, it could jump borders in the body of an unsuspecting host, or even a mosquito. “I’ve flown out of the airport in Nairobi any number of times,” says Linthicum, “which is right in the middle of a Rift Valley endemic zone. Aircraft sit on the tarmac all evening with the lights on and the doors open, and then they take off and they go to Europe, all over the world.”

So it’s probably only a matter of time. Linthicum’s USDA lab has worked for the last decade to correct that precise vulnerability on which the health of millions of people, and animals, may depend: a mosquito a few millimeters long traveling undetected in a 747. It is, he says, “no easy task.”