Engineering Mosquitoes to Spread Health

The promise of transforming flying vectors of dengue fever into preventive-medicine tools

When I eloped to the Caribbean island of Roatan, Honduras, in February 2002, I returned with a new husband, a tan, and a tropical illness.
Dengue fever started as a dull ache in my lower back, and escalated quickly until it felt as if I'd been hit with a baseball bat. My stomach went queasy, and I took to bed, thinking I'd caught the flu. I drifted off to sleep, only to wake up a few hours later, drenched in sweat and shivering under a mound of blankets, elbows burning, hips and knees aching. I crawled to the bathroom, where I spent the rest of the night alternately hugging the toilet and collapsing onto the blessedly cold tile floor.
Morning brought a migraine, searing joint pain, and aching muscles. My fever spiked to 103, and my mouth tasted like metal. I vomited every day for the next week. Blood tests indicated an inflamed liver, which meant I could no longer rely on codeine to dull the unrelenting pain. During those two weeks of agony, I occasionally longed for death, but dengue never truly threatened to deliver.
I was lucky. According to the World Health Organization, of the estimated 50 to 100 million people infected with dengue, also known as "breakbone" fever, each year, about 500,000—mostly children—will require hospitalization for severe dengue, or dengue hemorrhagic fever. Of those, more than 10,000 will die.
In recent decades, the disease has spread dramatically, from the nine countries that experienced epidemics before 1970 to more than 100 today, on every continent but Antarctica. It made its way to France and Croatia in 2010, and caused an outbreak of 2000 cases on Portugal's Madeira Islands in 2012. Florida, Texas, and Hawaii have reported dozens of cases in the past few years, almost all imported by travelers returning from the Caribbean or South America. But as Walter Tabachnic, director of the Florida Medical Entomological Laboratory in Vero Beach, recently told TIME magazine, "Sooner or later, our mosquitoes will pick it up and transmit it to us. That is the imminent threat."
Dengue fever is spread primarily by the aedes aegypti mosquito, and also by aedes albopictus, more commonly known as the asian tiger mosquito. Aedes aegypti is the culprit in nearly every dengue fever epidemic, while aedes albopictus gets credit for most outbreaks of chikungunya, an illness that produces symptoms similar to dengue, but is rarely deadly. Both mosquitoes are adept at transmitting these vector-borne illnesses to humans, as well as yellow fever and encephalitis. Both also thrive in urban environments, particularly indoors. Their eggs can hatch in the smallest bodies of water, including the pools of liquid that collect around shower and sink drains. Only the females bite, but they do so during the day, which means bed nets offer no protection. There is no vaccine for dengue, and no cure. Epidemics overwhelm local hospitals and crash local economies.
In most places where dengue fever and chikungunya are endemic, pesticides are a vital element of local strategies to prevent outbreaks, alongside public education campaigns encouraging people to rid their yards and homes of standing water. Local mosquito-control workers drive trucks down city streets, filling the air with toxic chemicals.
Besides being an environmental and human health hazard, fogging campaigns are not particularly effective, explains Haydn Parry. Parry is the CEO of Oxitec, a U.K.-based company associated with Oxford University that believes it has developed an alternative to pesticides: genetically modified (GM) mosquitoes. Oxitec’s mosquito carries a lethal gene that it then passes along to its offspring. The modified males are bred in a laboratory, then released into the wild, where they mate with local females, who lay eggs that will die before reaching adulthood.
"With insecticides, you have to take the product to the insect, which means you have to take your fog or spray into people's houses, and people don't like that. If a fogger machine is coming down the street, people close their windows and doors,” Parry says. “You know that 50 percent or more of the places mosquitoes are breeding are in people's houses, and you just can't get at them. The beauty of our little mosquito is that they don't have to ask permission to go on to someone's property. They are biologically programmed to find the females."
Oxitec's OK513A mosquito has been released and monitored in the Cayman Islands, Malaysia, and Brazil, with promising results, Parry says.
"In every open-air trial we've done in urban environments—even cities filled with buckets of standing water because there is no running water—the results are the same. We recently crashed the local aedes aegypti population in Mandacaru, Brazil by 96 percent in just six months," he says.
That success has been tempered by criticism, mainly from anti-GMO activists concerned about the unintended consequences of meddling with Mother Nature. GeneWatch U.K. accuses Oxitec of skimping on risk assessments required by the Cartagena Biosafety Protocol and of releasing genetically modified mosquitoes on an unsuspecting public overseas without their informed consent.
"As a U.K. company, Oxitec has legal obligations here to produce risk assessments before releasing any mosquitoes. We are concerned that they haven't been following those requirements, which means people in other countries are being used as guinea pigs," says Helen Wallace, GeneWatch U.K.'s director.
Critics are also concerned about a complex ecosystem response. "Reducing the numbers of aedes aegypti could increase the population of aedes albopictus, and we might not only get more of that species, but we may find that they evolve to be a more effective transmitter of disease," Wallace says.
Furthermore, the disease itself could be affected by population-suppression techniques. "A partial or temporary reduction in mosquito numbers can make dengue worse, because when people are bitten frequently, starting at a young age, they are more likely to develop cross-immunity to different serotypes of the dengue virus," she explains. "So, any method that reduces frequency of biting can make dengue hemorrhagic fever worse."
Dr. Anthony James, mosquito researcher and distinguished professor of microbiology and molecular genetics at the University of California, Irvine, shares Wallace's skepticism of population suppression techniques, but for different reasons.
"In large urban areas, we'll never knock down enough mosquitoes to eliminate dengue. We've tried. It's not sustainable. If you control dengue with population suppression, the mosquitoes will come back and you'll have to start over. So we have to go after the disease, not just the mosquitoes."
He separates vector-borne illness prevention strategies into two distinct camps: bite and no-bite.
No-bite strategies focus on preventing mosquitoes from biting people, either by suppressing the population with pesticides and "sterile" mosquitoes, or by keeping mosquitoes away from people with window screens, insect repellents, and protective clothing.
Bite strategies, on the other hand, "recognize that it is the pathogen and not the mosquito that is important here," James says. Before his funding dried up, he was developing a strain of genetically engineered aedes aegypti mosquitoes that still carried the dengue virus, but couldn't transmit it to humans.
He explains that the dengue virus starts in the mosquito's midgut and moves to its salivary glands, allowing females to transfer the virus to the people they bite. His team was able to insert a gene into the mosquitoes that prevents the transfer to the salivary glands, resulting in "100 percent knockout of dengue 2," one of five serotypes of the virus.
"The target really is the pathogen, and we can use the mosquitoes to get at the pathogen," James says.
That's the strategy also being followed by Dr. Scott O'Neill, medical entomologist, dean of the faculty of science at Monash University in Melbourne, and leader of an international team of scientists working together on the Eliminate Dengue project. They have managed to infect the aedes aegypti mosquito with a naturally-occurring bacteria called Wolbachia that effectively vaccinates the mosquitoes against the dengue virus. And rather than using their technology to suppress the local mosquito population, they release legions of vaccinated males and females to mate with, and eventually replace, their counterparts in the wild.
"We're not using any GM technology,” O’Neill says. “I'm a fan of what can be done with GM, but there is resistance and concern in communities, so that distinction is important. We have high acceptability with what we're doing because Wolbachia is a naturally-occurring bacterium."
In fact, Wolbachia is already present in 60 percent of insects, including many species of mosquitoes. This is one of the reasons Eliminate Dengue's research projects in Australia, Indonesia, Vietnam, and Brazil have met with very little opposition.
Another reason is community outreach. "In Cairns [Australia] we went door-to-door to get people’s individual consent,” O’Neill says. Mosquitoes can only travel a few hundred feet from where they're hatched, and “If they didn’t want it, we didn’t release at or next to their homes. We spent years doing community engagement, and taking all concerns seriously. We undertook additional studies to respect the views of the community and ensure they were being heard and listened to. That is the same approach we have taken everywhere else. We've got 95 percent support in Australia, and the same in Vietnam."
Evidence from field trials suggests that the population-replacement strategy is working. In 2011, the team released vaccinated mosquitoes for 10 weeks in two Cairns communities. A few months after the release, all the mosquitoes in those communities had Wolbachia, and three years later, they still do, according to O'Neill. But the jury is still out on whether this method—or Oxitec's,—can actually prevent dengue outbreaks.
"In order to measure a change in dengue transmission, we'll need large deployments, to cities of perhaps half a million people," O’Neill says.
Oxitec's Parry concurs. "There is no evidence yet that we have cut down instances of dengue, but that is the same for insecticides,” he says. “If you wanted to run a clinical trial to prove the link between this and the disease, you'd need a scale of at least 250,000 people, and that's a bigger trial than we can currently accomplish."
While GeneWatch U.K. points to this lack of evidence as another reason to drop GM mosquito projects in favor of developing a viable vaccine for humans, UC-Irvine's James sees it differently.
"I encourage all of it. It would be a real shame if we shut down any promising research. In the fight against malaria, we focused on DDT and chloroquine and shut off all other research. Then resistance [to chloroquine] kicked in, and we didn't have a viable alternative to replace it with," he says.
James also says he thinks government and public-health officials in the U.S. should increase funding immediately, before dengue fever and chikungunya establish themselves as a serious threat here. "Globalization is coming, whether we like it or not. That means we have to go to the places where the disease is already rampant. We need to address this now, because we don't make good decisions when we're in crisis mode."
In the meantime, even the world's top mosquito researchers must rely on low-tech methods to protect themselves from vector-borne illness when they’re in the field. "For all the science in the world, your best protection is a long sleeve shirt,” Parry says.