Why the Zika mosquitoes are so damn hard to kill off
There are more than 3,000 mosquito species around the world, but the chief villain in the Zika outbreak appears to be a pesky little critter called Aedes aegypti. This mosquito has a particular taste for human blood, and it's capable of carrying viruses like dengue, yellow fever, chikungunya, and Zika. If one of these winged syringes bites an infected human and then bites a non-infected human, the disease spreads.
The crucial thing to know about A. aegypti is that it's really, really hard to kill off. The species was originally native to Africa, hitched a ride over during the slave trade, and rapidly infested Latin America. During the 1950s, many countries launched massive control programs, using DDT to eradicate the pest. But these campaigns were costly, tough to sustain, and eventually subsided. The mosquitoes that still remained began breeding — and quickly overran the Americas again, spreading dengue, yellow fever, and other assorted miseries:
Today, things are worse than ever. The Aedes mosquito has become particularly well-suited to modern cities because, unlike other species, it doesn't need large bodies of open water to breed. It can lay eggs in virtually any nook or cranny it finds: buckets, tires, cups, plastic bags — anything that will later fill up with rain, allowing the larvae to hatch.
"The diversity of breeding sites means it's tough to find them all, which makes them harder to kill with larvicide," explains Catherine Hill, an entomology professor at Purdue. It doesn't help that heavy rainfall driven by El Niño seems to be boosting Aedes populations throughout South America this year.
Right now, our best strategies involve trying to avoid contact by telling people to use repellent, wear long sleeves, and use window screens. (The Aedes is hard to avoid because, unlike many mosquito species, it bites during the day rather than evening.) Local officials sometimes attempt costly campaigns to remove potential breeding sites and spray larvicide. But these campaigns only wipe out a fraction of the insects, and it doesn't help that Aedes mosquitoes are becoming resistant to pyrethroids and other favored pesticides. We're losing the battle.
Despite all the hype, we still don't know if GM mosquitoes are the answer
This explains why everyone's so jazzed about using genetic engineering to kill off mosquitoes. The most realistic idea to date is being tested by a British firm, Oxitec. The catch, however, is no one yet knows if this is an effective solution.
What Oxitec's scientists have done is modify A. aegypti mosquitoes so that they contain a self-destructive gene — these mosquitoes will eventually die unless they receive antibiotics in a lab. The company then releases genetically modified males into the wild. The GM males mate with wild females and pass on the self-destructive gene to their offspring. The offspring, in turn, die before reaching adulthood. Boom, population crash.
Field tests have been encouraging so far. In 2015, Brazil's government gave Oxitec the go-ahead to release hundreds of thousands of GM mosquitoes in the southeastern city of Piracicaba. Results suggest this technique killed off 80 percent of mosquitoes in places it was tried — way more effective than conventional treatments:
Hence the hype. Oxitec's "weaponized" mosquitoes kill way more insects than pesticides do, without the environmental downsides. They're safe. And, frankly, they're pretty neat.
So I was surprised to find that several mosquito experts I contacted urged caution about getting too excited here. Thomas Scott is a professor of entomology at the University of California Davis who has collaborated with Oxitec in testing (other) modified Aedes mosquitoes. He thinks the technology is extremely promising, but also pointed out a few important practical hurdles.
For starters, we still don't know if Oxitec's mosquito rampage will actually lead to a reduction in diseases like dengue or Zika — which is, after all, our ultimate goal here. It's entirely possible that Oxitec could kill 80 percent of A. aegypti mosquitoes but the remaining 20 percent would are still enough to sustain an outbreak. Or it's possible that another mosquito species, like the closely related Aedes albopictus, could take over start transmitting the disease instead. We still don't know.
"If you had an 80 percent reduction in mosquitoes, you'd expect to see some effect on transmission," Scott says. "But you have to demonstrate that." Doing so requires large-scale randomized control trials that are complex and slow — they'll take years. But those epidemiological tests are crucial. Deploying GM mosquitoes will cost money, and poor countries with finite health budgets need to be sure they'll see results.
A related question is whether Oxitec can scale up its GM mosquito strategy at reasonable cost. Because the modified males quickly die in the wild, the company will need to keep manufacturing and releasing thousands and thousands of modified mosquitoes over and over to suppress populations. (The second you stop, Scott notes, the mosquitoes come back with a vengeance.) Is that cost-effective? Is it feasible in major cities with millions of people? No one has any idea yet.
Bottom line: Oxitec's GM technology is very cool. But the need for further testing means it probably won't be much help against the current Zika crisis. And even in the future, it may just end up being one (valuable) tool among many in fighting mosquito-borne disease. No expert I talked to thought it would be the answer.
Advanced genetic editing could one day wipe out mosquitoes for good — but we're not there yet
Oxitec's scientists aren't the only ones working on this problem, meanwhile. The really futuristic technology that could someday revolutionize mosquito control involves something called "gene drives."
The basic idea goes like this. Using advanced genetic tools, scientists have been able to modify fruit flies so that they contain a desired gene — but also are more likely to pass that gene on to the next generation. In other words, rather than a 50-50 chance that a fly will pass on its modified genes to offspring, scientists can increase the odds to nearly 100 percent.
The possibilities here are astounding. Imagine if you modified some mosquitoes to be resistant to malaria and released them into the wild. With gene drive, in theory, all of their offspring would soon be incapable of spreading malaria. And all of their offspring's offspring.
More radically still, you could program mosquitoes to, say, only produce male offspring. If this gene modification eventually propagated throughout the entire population, mosquitoes would go extinct. Forever.
I called Zach Adelman, a molecular geneticist at Virginia Polytechnic Institute and State University in Blacksburg, to walk me through the possibilities here. He explained how gene drive systems could one day prove enormously powerful.
But he also had this caveat: "You need to be careful about the timeline here." We're not going to develop these tools in time to stop the current Zika crisis.
So far, gene drive systems have only been described for fruit flies. It will take a lot more research to develop a system for Aedes mosquitoes, our hateful bloodsucker. Scientists would then need to demonstrate in a lab that they could — for example — program mosquitoes to only produce male offspring and get that gene to propagate for many generations, through an entire population. (Adelman is doing research in this area.) They would also likely need to develop "failsafe" mechanisms to stop the process if something goes wrong — there are some ideas for failsafes being kicked around, but nothing concrete yet.
Once the lab tests are done, there's another bar to clear. You need field tests to see how many mosquitoes it would take to push the modified genes out to the entire population. Do you just need to release five mosquitoes? A hundred? Thousands? In one place? Many? After all, if you just wipe out mosquitoes in one area, others could rush in to fill the gap. Then, of course, regulatory agencies like the Environmental Protection Agency will want to scrutinize the technology to make sure they don't cause unforeseen problems.
The most optimistic estimates predict that this is all at least three to five years away. Maybe longer. We're not wiping out Zika with this stuff tomorrow.
Of course, Zika won't be the last outbreak. Even if this crisis subsides, we'll still have dengue and yellow fever and malaria, and god knows what else is coming. Mosquito-borne diseases, particularly malaria, kill nearly a million people each year and cause untold suffering. So it'd be nice to have a sweeping solution someday.
That's where we get into the interesting ethical debates. Would it be worth it to wipe out mosquitoes once and for all? Quite possibly! The health benefits would be staggering. Though, of course, we'd want to consider unintended consequences, such as what might happen to all the birds and fish that feed on mosquitoes each year. (Ecologists seem to be divided on whether it would all turn out fine or not.)
(Also, for those squeamish about TOTAL EXTERMINATION, there's always the less radical possibility that we could use gene drive systems to make mosquitoes resistant to malaria or dengue or Zika. But Adelman points out that doing this is actually harder from a research perspective — we don't know enough about mosquito immune systems yet. There's also the risk that pathogens might evolve in response.)
So where does that leave us? Gene drive systems could one day prove invaluable for fighting diseases. But we're not there yet. It's going to be some time before we need to have a serious ethical debate about wiping out mosquitoes for good.
But in the meantime, our best ideas for fighting Zika are still pretty low-tech
So that brings us back to the present-day Zika crisis. On this, the experts I spoke to were mostly in agreement on what should be done. And it's not very futuristic.
First, there's public education. People need to be aware of A. aegypti and its awful ways and know how to combat it. Wear protective clothing, use insect repellents, and put up screens on windows and doors. Get rid of anything that could create a pool of standing water outside, whether a bucket or Styrofoam cup or whatever else.
The next step would be for governments to launch mosquito control campaigns in earnest. That means treating potential breeding sites with larvicide and organizing cleanup crews to get rid of trash and old tires and other places where water can pool. (See more details here.)
"You have to go back to past successes," says Peter Hotez, dean of the National School of Tropical Medicine at the Baylor College of Medicine. "In the 1950s, we eradicated Aedes through brute force — insecticides, drainage control. The problem is that these campaigns are labor-intensive, they're not cheap, and they're difficult to sustain." Once an outbreak fades, governments lose the political will and the mosquitoes return stronger than ever.
Thomas Scott of UC Davis cautions, however, that it may be tricky to replicate past eradication campaigns. The man who led those earlier mosquito wars, Fred Soper, worked closely with authoritarian governments in Latin America to force inspectors into every home and inspect for breeding sites. That heavy-handedness wouldn't go over well in today's democracies. "Plus, Soper didn't have to deal with massive cities of 8 to 10 million people, or all the plastic trash we have now," Scott notes. "So we're dealing with things he didn't have to deal with."
Another key difference: In the 1950s and '60s, countries were spraying lots of DDT, a cheap insecticide that lingered on surfaces for months, making it particularly effective at killing insects. But in the years since, many Aedes mosquitoes have developed resistance to DDT, making the pesticide a less suitable option today. (The compound can also kill birds and other wildlife, which is why the US banned it in 1972.)
Ultimately, beating back Zika is going to be incredibly arduous — especially in developing countries. "People are reluctant to accept that there is no magic bullet," concludes Uriel Kitron of Emory University. "It's a tedious process, it involves a lot of commitment, it involves community participation."
Sadly, that stuff isn't nearly as enthralling to write about as GM mosquitoes or futuristic gene drives. But for now, it's the best we've got.
- 9 questions about the Zika virus you were too embarrassed to ask
- Sharon Begley wrote a terrific explanation of gene drives. And Elizabeth Pennisi's dispatch from a recent gene drive "workshop" details many of the practical challenges still facing the technology.
- In the Lancet, Laith Yakob and Thomas Walker describe another potential control strategy for Aedes mosquitoes — infect them with the Wolbachia bacteria, which can prevent dengue transmission. We'll need to see if this works for Zika as well.
- In general, many of the popular fears about GMO mosquitoes are wildly overblown (though they're likely to be a real obstacle to testing and deployment). Christie Wilcox dismantles a particularly stupid conspiracy theory that's sprung up around Brazil.
- Malcolm Gladwell's profile of Fred Soper, who eradicated Aedes mosquitoes from many countries in the 1930s and saved countless lives, remains essential reading.