Mirroring what happened with the latest Ebola outbreak, the World Health Organization (WHO) has been criticized for dawdling over the Zika outbreak. But it seems it's now stepping up its game, having issued advice on ways to curb the situation. Most notably, the organization has recommended the use of mosquito control techniques, which are considered by some to be controversial. Namely, the release of genetically modified or even irradiated insects.
Zika virus is predominantly spread by mosquitoes, specifically a species called Aedes aegypti that also transmits dengue, yellow fever, and chikungunya. In the absence of drugs or a vaccine, the best way to curb Zika spread is therefore to attack this vector. Unfortunately for us, this insect is frustratingly adaptable and has coped with almost everything that has been thrown at it, including dramatic changes in habitat due to urbanization. Ae. aegypti will opportunistically take advantage of anywhere water collects to lay eggs, be it plastic cups or toilets, creating extensive breeding grounds that must be laboriously destroyed.
But it’s not just the resourceful nature of this species that makes it problematic. While males don’t bite, the females have unfortunately developed a preference for humans over other animals, feed during the day, and stealthily attack from behind. All in all, these characteristics make Ae. aegypti a particularly troublesome species. And with insecticide resistance on the rise, there is a clear need for alternative control methods, something that the WHO recognizes.
Perhaps the most controversial, one technique the WHO has recommended investigating further involves releasing genetically modified insects. Produced by U.K. biotech company Oxitec, male mosquitoes contain a lethal gene that kills off offspring before they can reach reproductive maturity. The idea is that by releasing a large enough number of these, they will out-compete normal males in the wild for females, and the population will crash. And it works: Trials have consistently shown suppression rates of greater than 90 percent in release zones, compared with 50 percent typically achieved by insecticide spraying.
While the GM label often evokes fear, long-term studies have shown the gene is stable, and it can’t hop into other species to make some kind of Franken-animal. In addition, the environmental impacts are negligible. In light of the evidence supporting their use, “the WHO Advisory Group has recommended further field trials and risk assessment to evaluate the impact of this new tool on disease transmission,” the WHO said in a statement.
The modified mosquitoes also have fluorescent genes so that they can be monitored in the environment. Oxitec.
Another way to achieve a similar goal is to sterilize male mosquitoes with radiation and then release them into the environment, where they again mate with females but the eggs don’t survive. While the WHO recognizes that this approach has shown success in large trials concerning crop pests, it should be noted that sometimes the irradiated mosquitoes aren’t as fit as their wild counterparts and thus can be out-competed.
A less controversial tactic involves calling upon the “gender-bending” bacteria Wolbachia, so named due to their ability to transform male crustaceans into females. While these infect many insects in the wild, Ae. aegypti isn’t a natural host, but when infected in the lab once again males can’t produce viable offspring.
All of these approaches aim to crash mosquito populations, and while some may be concerned with the potential knock-on effects, experts have pointed out that we probably won’t miss them.
As Jo Lines from the London School of Hygiene and Tropical Medicine highlights: “This is an invasive species, so getting rid of these mosquitoes would, if anything, restore the natural ecology, not destroy it.”