The release of genetically modified male mosquitoes into a city in Brazil temporarily cut the virus-carrying insects by up to 96 per cent. Although not a permanent fix, periodically releasing such mosquitoes could reduce the burden of infections including dengue, malaria and Zika.
at the biotechnology firm Oxitec in Abingdon, UK, and his colleagues are particularly looking to control dengue. Although usually mild, the infection can be fatal. Already widespread in many parts of the world, climate change may allow dengue-carrying mosquitoes to survive and breed in non-endemic regions.
Existing control methods include sleeping nets, which not all at-risk people have access to, and insecticides, which mosquitoes can develop resistance against.
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To create a longer-term solution, the researchers modified males of the mosquito species Aedes aegypti. Across all mosquito species, only females bite. A. aegypti females can spread dengue, as well as Zika, chikungunya and yellow fever.
The researchers modified male mosquitoes, which they dubbed OX5034, to express a gene that allows their male offspring to live, but prevents female larvae from surviving to adulthood, unless given an antibiotic antidote.
From May 2018 to April 2019, OX5034 males were released into four densely populated neighbourhoods in the city of Indaiatuba in the state of SĂŁo Paulo.
Within two of the neighbourhoods, 100 modified mosquitoes were released, while the remaining test areas were exposed to 500 of the insects. These releases occurred three times a week.
Compared with a nearby community that wasn’t exposed to any of these mosquitoes, the places where the modified insects were released had an 88 per cent to 96 per cent dip in their mosquito population from November 2018 to April 2019, the insects’ peak breeding season.
Releasing 500 modified mosquitoes, as opposed to 100, didn’t substantially improve insect suppression. How long the engineered gene persisted in the local mosquito population depended on the number of mosquito generations that were produced, rather than the initial number of modified males, says at Oxitec.
Using traps to monitor the mosquitoes, the researchers saw the gene persisted in around half of the surviving male offspring of the modified mosquitoes, but disappeared after about six generations, over roughly six months.
“It was exactly what we expected,” says Rose. “We knew that [the gene] would continue for a little while, but we expected it to disappear.”
Oxitec has since launched a box-type device throughout Brazil to release modified male mosquitoes periodically during peak dengue season. The box contains the male mosquito eggs. Adding water causes the eggs to hatch after several weeks. Rose says there are plans to expand the box to dengue-endemic areas elsewhere, such as in Africa and South-East Asia.
The study didn’t look at whether suppressing the mosquitoes led to a lower incidence of dengue. However, similar efforts in Australia, not involving Oxitec, saw fewer cases of locally transmitted dengue compared with previous years. modified mosquitoes were introduced.
Such mosquitoes have great potential in reducing some infections, but will probably not cut transmission completely, says  at Texas A&M University. This would probably require a multifaceted approach that also reduces human-mosquito interactions, such as the installation of insect screens on windows and removing standing water, where mosquitoes lay eggs.
According to at the Tulane University School of Public Health and Tropical Medicine in New Orleans, Louisiana, the effort is a step up from previous insect control strategies where mosquito sex selection wasn’t done via genetic means.
“That’s the beauty of this method,” she says. “You don’t have to find all the habitats. You just let the males loose and they find the females [and the gene is passed on].”
Dengue aside, Oxitec is working on developing modified mosquitoes to reduce other infections, such as malaria, says Rose.
Frontiers in Bioengineering and Biotechnology
