Rows of steel cages in a building on the National Institute of Communicable Diseases campus in eastern Johannesburg stand empty. But they will be buzzing with mosquitoes if a bold project to boost malaria control by breeding at least 100,000 sterile males a week takes off. Male mozzies, which don't bite humans, are being bred and sterilised as a smart weapon against the major mosquito species that transmits malaria in South Africa, Anopheles arabiensis. Malaria-spreading females that mate with lab-bred sterile males will not reproduce.
This is not as sci-fi as it may sound: sterile male mosquitoes in Australia have dropped the population of a dengue fever-carrying species by 80% in a Queensland town, scientists reported this month.
Another recent trial, in Brazil, found that genetically modified sterile males reduced mosquito larvae of the species that spreads the Zika virus- also by over 80%.Closer to home, the citrus industry has hammered the false codling moth population in the Western, Northern and Eastern Cape over the past decade by releasing millions of sterile males.Nevill Boersma, Citrus Research International's programme manager, said about 40 million sterile males were being reared and released every week in five river valleys so far.
"In the Citrusdal area, the false codling moth population is down by 95%. Farmers are using less pesticides and it is sustainable," he said. "But this needs an area-wide approach and it is a long-term programme."
In Johannesburg, the 24 mosquito colonies being bred at the NICD in two humid, cramped insectaries near the rearing facility were started in 2011.
The research got a boost when scientists discovered that their lab-bred males could fly with fitter wild insects after being released in KwaZulu-Natal.
An experimental study in Australia shows that technology using artificial intelligence can sort males and females speedily. But institute researchers had to suck a bunch of mozzies out through a pipette and observe their genitals under a microscope, at the pupal stage, to identify males. At the adult stage, they can check for bushy antennae.Dr Maria Kaiser, a medical scientist at of the NICD's vector control reference laboratory, said: "We recaptured our marked males in mating swarms, which is exciting because it means our laboratory-reared males can locate and participate in swarms of wild mosquitoes.
"This is important because for the sterile insect technique to work the males must be able to recognise and mate with wild females."
Dr Givemore Munhenga, head of the programme, said: "If sterile releases are consistently maintained over seasons, they might eventually result in the extinction of the targeted mosquito population, thereby disrupting the malaria transmission cycle."The Bill and Melinda Gates Foundation is funding interventions that will have a high impact on malaria. They include gene modification that would make mosquitoes immune to malaria or lead to a reduction in the population, said Dr Trevor Mundel, the foundation's president of global health.
"In the next couple of weeks, we want to release about 10,000 sterile males in Burkina Faso to test our ability to track them and see the fitness of these kinds of modified mosquitoes," he said.
In South Africa last year malaria cases increased by almost 50%, from 6,375 reported in 2016 to 9,478, due to delays and gaps in spraying, and inadequate resources.
Limpopo, Mpumalanga and northern KwaZulu-Natal were worst affected.
Professor Lucille Blumberg, deputy director of the NICD, said this week: "Malaria control is fragile, and malaria bounces back if there are gaps. But our latest report is that KwaZulu-Natal is moving in the right direction - towards elimination."
Finding alternative tools for malaria control is important, given emerging resistance to insecticides among mozzies in KwaZulu-Natal. Sterile males could accelerate the process of eradication.
Mozzies cannot fly below 4ºC. The plan is that, in time, tens of thousands of them will sink to the floor in the cold room of the Joburg rearing facility, to be packaged neatly for export to malaria hotspots...