A View from Zach Adelman
When Extinction Is a Humanitarian Cause
The mosquito spreading Zika doesn’t belong in our environment. Soon we may be able to eradicate it with new technology.
Humans have driven species to extinction through our hunger, our ignorance, our desire for economic growth, and our indifference. Will one species of mosquito be the first we eliminate for humanitarian reasons?
The mosquito is Aedes aegypti. It is commonly known as the yellow fever mosquito and, in more recent years, has become known as the dengue mosquito for spreading a hemorrhagic disease that disproportionately affects small children and has high public health costs. Now it's the Zika mosquito, suspected of transmitting Zika virus with a potential association to microcephaly in newborns.
Aedes aegypti provides an ideal distribution network for dangerous viruses because of its strong preference for feeding almost exclusively on people and living in and around our houses. Once confined to a small region of sub-Saharan Africa, Aedes aegypti has invaded the Americas, Asia, the South Pacific, and Australia within the last few hundred years, due entirely to human activity.
While people have long waged war on this invasive mosquito, with repeated calls for its eradication, the recent development of "gene drive" technology raises the theoretical possibility of winning it. Unlike an ordinary gene, which is passed on to just half of all offspring, a gene drive construct could be passed on to virtually all offspring. It can be used to spread genes that destroy female mosquito chromosomes, prevent female mosquitoes from flying, or determine whether a mosquito becomes a male.
By releasing a small number of gene-drive mosquitoes, the number of wild females could be reduced each generation until they disappear completely. Without any females to produce the next generation of eggs, the surviving males would have a very lonely last few weeks until they died out, too, along with the genetic modification that caused their disappearance. While similar strategies are in the works for controlling malaria, these are complicated by the fact that malaria transmission is mediated by a large number of diverse mosquito species, varying tremendously in different parts of the world. For dengue, chikungunya, and now potentially Zika, Aedes aegypti is the common thread, and potentially the weakest link.
Though there are certainly technical and regulatory milestones ahead, what would happen if it worked? First off, some very good things: the extinction of this mosquito would save more than 20,000 lives per year due to dengue alone and prevent millions of cases of illness. In addition to stopping dengue, Zika, and other viruses such as chikungunya, the elimination of this mosquito would prevent the spread of other obscure viruses that have been cataloged and may be waiting their turn to cause the next epidemic.
What about the ecosystem? As a species we are guilty of repeatedly taking actions without thinking about the effects on the environment. Considered in isolation, anything that damages "the ecosystem" sounds universally bad. In this case, the ecosystem in question is cans, buckets, pots, water storage jars, trash, tires, and whatever else is lying around collecting rainwater. Aedes aegypti does not breed in ponds, marshes, swamps, or wetlands, and thus there are no frogs and no fish to eat these mosquitoes—one of the reasons they have done so well as a species. Currently, our ability to control dengue transmission (and now Zika) is dependent on our ability to remove the places where Aedes aegypti lives and breeds. If we are already willing to destroy an entire ecosystem (i.e. clean up garbage, screen-over water storage containers), why not eliminate just this mosquito?
Gene-drive-based approaches differ from traditional vector control methods such as insecticides and removing breeding sites. With a gene-drive system, the population of the target species could be massively disrupted, without directly affecting any other species (a very desirable trait!).
But would getting rid of this mosquito open the door for another mosquito to occupy the same niche, making things even worse? Once Aedes aegypti is gone, other mosquitoes might move in and exploit its absence. However, given that the majority of suitable breeding sites surveyed are already devoid of growing larvae, even in cities with large Aedes aegypti populations; it appears there are already plenty of vacancies for newcomers. While not thought to be as important in spreading disease as Aedes aegypti, the Asian tiger mosquito has had no trouble pushing Aedes aegypti out of most of the southern U.S. through its ability to effectively sterilize Aedes aegypti females without any help from advanced genetic technologies. It may very well keep invading new territory no matter what we do at this point. There's simply no evidence that an even more dangerous mosquito is lurking in the shadows, waiting patiently for the day when the yellow fever mosquito finally disappears. I’d feel safer taking my chances on the next mosquito than the next virus.
By the mid-1960s, the hemispheric eradication of Aedes aegypti—led by the Pan-American Health Organization using chemical insecticides such as DDT and malathion—appeared to be within reach and was still considered possible even by the late 1990s. For a multitude of reasons, that goal has been slipping farther and farther away. The development of gene-drive approaches, combined with all of our current mosquito control practices, holds the promise of reversing this trend and bringing us closer to the goal of eradication (continental, hemispheric, or even worldwide) of this mosquito and the terrible pathogens that depend on it.
Zach N. Adelman is an associate professor at the Fralin Life Science Institute and Department of Entomology at Virginia Tech.