Because of the pathogens they carry, mosquitoes are responsible for more human deaths every year than any other animal, including other humans. But very few of the 3,500 mosquito species actually transmit deadly diseases to humans. So what if we could get rid of the most lethal mosquitoes? Over the last two decades, scientists have begun conducting experiments using engineered technologies called “gene drives” that could theoretically do just that. So, should we?
To begin grappling with this question, we have to get a sense of how the technology works. In the usual process of inheritance, the genomes of each parent recombine randomly. So their offspring end up with the DNA that’s a rough 50/50 mix from their parents. But gene drives thwart this process and ensure they're passed on. Gene drives are found in nature but, using new gene-editing technology, scientists have also begun engineering them in contained labs.
For example, in a 2018 study, researchers injected a gene drive into mosquito eggs that made females sterile when they had two copies of the modified gene. Such a modification would usually disappear quickly. But it spread. The modified mosquitoes passed the gene drive onto some of their offspring. The gene drive, which they inherited on one chromosome, copied itself onto the other chromosome in the offspring’s sperm and egg cells, ensuring it was passed on to their offspring, regardless of which chromosome they received. This process repeated as all males that carried the gene and all females that had one copy of it, continued reproducing, spreading the gene drive. As they did, they produced more females that had two copies of the gene— and would therefore sterile. With a near 100% inheritance rate, the gene spread through the population and within 12 generations almost all females were sterile, and the populations crashed. In 2020, the same team achieved a similar result with a gene drive that made populations male-only.
Gene drives have proven powerful in the lab. So, implementing them in the wild is a big decision— one that’s being considered because of how the fight against mosquito-borne diseases is going. Existing mosquito control measures, like insecticide-treated bed nets, helped reduce the number of deaths from malaria, the deadliest mosquito-borne disease, between 2000 and 2019. But fatalities have begun rising again. Many mosquitoes have developed insecticide resistance— and insecticides kill more than just mosquitoes. In addition to the first-ever malaria vaccine, approved in October of 2021, many see promise in gene drives.
Experts are researching what it would look like to specifically target the deadliest mosquito populations with this technology. Like Anopheles gambiae, for instance: the species overwhelmingly responsible for spreading malaria in Equatorial Africa, which experiences the vast majority of mosquito-related fatalities. The idea is that, when a gene-drive-affected population of Anopheles gambiae drops low enough, it would break the malaria transmission cycle.
But before gene drive mosquitoes are actually released into the wild, some big questions need answers. Like, could gene drives cross into and cause the collapse of non-target species? It doesn’t seem that many mosquito species interbreed, making this unlikely, but scientists are conducting research to be certain. And how might a mosquito population’s collapse affect ecosystems? One team is examining the feces and stomach contents of insectivores in Ghana to gauge the role of Anopheles gambiae in local food webs. And researchers are investigating whether suppressing populations could make other insects more vulnerable or leave a niche open that a harmful species could occupy. Scientists are also exploring alternatives to population collapse, like gene drives that instead make mosquitoes resistant to the malaria parasite. And others are developing countermeasures to reverse the effects of gene drives if needed.
Meanwhile, some people have called for gene drive research to halt out of concern for the possible consequences. This raises another question: who should decide whether to release gene drives? It’s essential that communities, scientists, regulators, and governments of the countries most affected by mosquito-borne diseases be highly involved in the research and decision-making processes. Conversations are currently underway at all levels to establish a system to manage this new area of research— and the ethical questions it carries.
