In 2017, malaria killed over 400,000 people in Africa but scientists in Burkina Faso are now stepping up efforts against the disease by deploying a new weapon, genetically sterilized mosquitoes. And in doing so, the scientists have waded into a thorny bioethics debate.

Their experiment is the first outside the lab to release genetically altered mosquitoes in the hope of reducing their ability to spread the often deadly disease.

It works using a technique called a gene drive, which edits and then propagates a gene in a population – in this case, to prevent males from producing offspring.

There have been concerted efforts in Africa to fight Malaria using anti-malarial drugs, mosquito nets and insecticides. These have slowed down the disease over the past two decades on the continent that contributes to more than 90% of global cases.

The World Health Organization acknowledges that progress in fighting the disease is stalling, leading researchers to push for fresh approaches.

“The conventional tools that we have at our disposal today have reached their limit.” said Dr. Abdoulaye Diabate, who is running the experiment for Target Malaria, a research consortium backed by the Bill & Melinda Gates Foundation.

One hot evening in July, Diabate’s researchers released about 5,000 male mosquitoes into Souroukoudinga, a village in western Burkina Faso. The mosquitoes had been injected as embryos with an enzyme that sterilizes them.

“Our objective is not to eradicate mosquitoes.” said Diabate, noting the enzyme targets only the three main species – out of more than 3,500 worldwide – that carry malaria. “The objective is to reduce the density of these mosquitoes.”

Target Malaria is also developing an enzyme preventing male mosquitoes from passing on X chromosomes. This results in male offspring, reducing malaria since only female mosquitoes bite – males mostly feed off plant honeydew.

Researchers in Brazil have also released genetically modified mosquitoes in an attempt to control diseases like yellow fever and Zika, but it is not clear how effective that has been.

Target Malaria says it consults with communities and that research is overseen by national regulatory authorities and an independent ethics committee.