WHY YOU SHOULD CARE
Mosquitoes carry a variety of deadly diseases. It’s time for creative defense, including turning the bugs against themselves.
By Melissa Pandika
One of the deadliest creatures on Earth is clawless, toothless and no bigger than a grain of rice — yet it kills one million people each year, mostly children in developing countries. The mosquito can spread a variety of deadly diseases, including malaria and dengue fever, and as warming temperatures create more mosquito-friendly habitats, the threat is only going to increase.
So scientists have proposed a radical new solution: to engineer mosquitoes that fight illness instead of transmitting it. Some researchers are designing mutant mosquitoes that wipe out their own species or lack the ability to spread disease. Others are developing insects that are resistant to disease-causing microbes. Although some scientists warn of the unintended and possibly dangerous consequences of releasing custom-made insects, others argue for innovative strategies to defeat mosquito-borne illnesses. Governments could start folding GM technologies into existing mosquito-control programs within the decade — if public opinion warms up to the idea.
Releasing engineered mosquitoes ’can uniformly reduce the [disease-carrying] population’ even where prevention isn’t effectively implemented.
Malaria and dengue are the most common mosquito-borne illnesses. Malaria results in high fever, vomiting and anemia. About 219 million cases of malaria and 660,000 deaths are reported each year. Dengue, which causes flu-like symptoms and joint pain, sickens 50 million to 100 million people each year and kills about 25,000. There are no vaccines for either disease, and no treatment for dengue.
Scientists first began pitting mosquitoes against each other in the 1970s.Many countries have implemented mosquito-control programs, which often involve spraying pesticides, as well as advising residents to hang bed nets and drain water from gutters and other breeding sites. These measures “help but they’re just not enough” to eradicate disease, mainly because many residents don’t take the recommended precautions, said Mark Benedict, a molecular biologist at the University of Perugia in Italy. Releasing engineered mosquitoes “can uniformly reduce the [disease-carrying] population even where the measures in place aren’t being implemented very effectively.”
Scientists first began pitting mosquitoes against each other in the 1970s. In the so-called sterile insect technique, large numbers of mosquitoes are bred and sterilized, and the males are released into the wild. The sterile males compete for females, reducing the number available for impregnation by wild males. The problem is that mosquitoes are often sterilized by zapping them with radiation, which sometimes leaves them too “frail and puny” to compete for mates, said geneticist Luke Alphey, co-founder and chief scientific officer at Oxitec, a biotech company that’s based in the U.K.
Why not insert a gene into the flies that kills only females, leaving behind only males — which can’t bite or spread disease.
Alphey first learned about the sterile insect technique in the 1990s from a colleague at Manchester University who was designing a more targeted version by breeding Mediterranean fruit flies, or medflies, to carry a gene that rendered the males sterile. But he still needed to painstakingly distinguish males from females, and the breeding scheme worked only in medflies.
In 2009, Oxitec scientists unleashed 3.3 million of its mosquitoes in a field trial in the Cayman Islands. The ratio of wild mosquitoes to GM mosquitoes steadily dropped week after week. Five months later, the wild mosquito population had plummeted by 80 percent — a level sufficient to eliminate dengue fever in the area. “It really exceeded my expectations,” Alphey said. Something clicked in Alphey’s mind. Why not insert a gene into the flies that kills only females, leaving behind only males — which can’t bite or spread disease. Eventually the number of females in the population will drop to zero, causing the population to perish.
Since then, Oxitec has also completed successful trials in Malaysia and Brazil. The company reported in May that it had reduced the wild mosquito population in Mandacaru, Brazil, by 96 percent. It’s awaiting federal approval to launch a trial in Key West, Florida, where 22 cases of dengue have been reported this year.
Some scientists are designing mosquitoes that replace, rather than kill, the resident population. A team of Imperial College London and University of Washington researchers, for example, has equipped mosquitoes with a fungus gene that disrupts key genes involved in transmitting malaria.
Other groups are taking a more direct approach, targeting the actual microbes that cause disease instead of the mosquitoes that carry them. In June, Johns Hopkins University scientists reported engineering mosquitoes to carry a modified gene that impairs the development of Plasmodium, the malaria-causing parasite. Meanwhile, scientists at Monash University in Australia are field-testing mosquitoes infected with GM bacteria that seem to fight the dengue virus by priming the insects’ immune systems.
At best, “designer mosquitoes” may offer an effective complement to traditional mosquito-control methods — and a safer, more eco-friendly alternative to pesticides, which often contain chemicals that are toxic to humans and other animals. But some scientists worry that the technology could have long-term consequences that are difficult to predict.
One possibility is that wild mosquitoes and/or the disease-causing microbes they carry could develop resistance to attack. Local females may evolve to recognize and avoid mating with Oxitec-engineered males that carry the lethal gene, for example. Likewise, Plasmodium or dengue virus could learn to overcome the defenses scientists have devised against them.
It’s also unclear how the proteins encoded by modified genes will affect human health. If millions of mosquitoes are set loose, even a small percentage of females could boost disease spread, critics say. And no one knows exactly how GM mosquitoes will affect local ecosystems. Wiping out the disease-carrying population could make room for a more dangerous species.
Cost is another concern, especially in the developing countries where disease-carrying mosquitoes are endemic, Benedict added. So far, there are no cost estimates for the proposed GM approaches. Even if the technology proves effective, it may end up being too expensive to implement.
Right now, public opinion remains “the biggest hurdle” to rolling out designer mosquitoes for widespread use, said biomedical engineer Ryan Smith. “A lot of people fear genetically modified organisms,” he explained. For now, the most scientists can do is to educate the public about their work. But opinions could change if trials like Oxitec’s prove successful.
“I think in maybe the next five to 10 years we’ll have the technology and ability to release these mosquitoes,” Smith said. “But if we can convince the public or governments about these releases — that’s a whole new question.”
- Melissa Pandika Contact Melissa Pandika