By ROBYN DIXON
As mosquitoes buzzed about on her veranda in Johannesburg, South Africa, one recent evening, Maureen Coetzee didn’t reach for bug spray or a swatter. She dashed inside, grabbed a device resembling a drinking straw and sucked four mosquitoes into a specimen jar.
She quickly identified them as Aedes aegypti, the villain in the Zika crisis a continent away.
The next day, in her laboratory at the National Institute for Communicable Diseases, Coetzee peered at her new captives as they squatted in their containers.
To her delight, one had laid eggs, meaning she could breed them.
Coetzee has devoted her life to understanding mosquitoes – in order to kill them.
No other animal has done so much harm to the human race. Each year, they infect millions of people with malaria, yellow fever, dengue fever and other viruses and parasites, killing at least 600,000, the vast majority of them children in Africa. Which raises the question: why not try to wipe mosquitoes off the planet?
Coetzee and other scientists said that would be extreme, given that only about 150 of the 3,500 species of mosquitoes are carriers of deadly pathogens. It would also be wildly impractical to try infiltrating every mosquito breeding ground.
But more thorough control may be possible through a combination of methods. After decades of fighting mosquitoes – and mostly losing – technology is bringing new biological weapons to the battle.
Coetzee, an entomologist at the University of Witwatersrand and an international expert on mosquito control, is conducting research on an anti-malaria strategy that involves breeding male mosquitoes, sterilising them with radiation and releasing them into the wild.
The concept was pioneered in the 1950s when US scientists used it to eradicate the screwworm fly. Applied to mosquitoes, it relies on two basic facts: they mate only once, and only females bite.
Rigging the mating game
The strategy depends on releasing enough mosquitoes to crowd wild males out of the mating game, letting the current generation die out without reproducing. The life span of a mosquito is two to four weeks. Coetzee said that effective control would require releasing millions monthly during malaria season across vast areas.
One advantage of the strategy over other methods is that it allows targeting of individual species of mosquitoes. “It makes sense to target only those mosquitoes that are involved in transmission of disease,” Coetzee said.
Though she and other scientists have no qualms about trying to eliminate those species, even that goal is “highly unlikely”, she said.
The four species that transmit malaria in Africa breed in rain pools. “If you think about the whole of Africa, you are never going to get to every rain pool,” she said. “Inevitably you will have pockets of mosquitoes.”
Mosquito populations have grown increasingly resistant to pesticides, a global problem that Coetzee places on par with the rise of bacteria resistant to antibiotics.
Mosquito breeding sites are also easy to miss, said Laith Yakob, a vector control expert at the London School of Hygiene and Tropical Medicine. For example, Aedes aegypti, which carries the Zika virus, is especially hard to reach because it often breeds in houses and can reproduce in thimble-size pools of water.
“Any of them can be viable breeding sites, and imagining that citywide, it would be impossible to eliminate all breeding sites,” he said. “And even if you did, the next time it rained, they would be back.”
That’s where newer technologies can help.
Programmed to die
One of the newest is a variation of the sterilised male strategy. It involves genetically modifying male mosquitoes so their offspring are programmed to die before they mature and are able to reproduce.
In the months before the Zika outbreak in the Americas, a British company announced that it had conducted a successful trial in the Brazilian city of Piracicaba, cutting the number of mosquito larvae by 82%.
The company, Oxitec, announced last month that it planned to build a facility in Brazil soon to produce genetically modified Aedes aegypti mosquitoes in batches large enough to protect a town of 300,000.
The mosquitoes are puffed out the window of an SUV in bursts of 1,000, and can be directed at hotspots or blanket an entire town. “A male will always find a female,” Oxitec chief executive Hadyn Parry said. “A male will find its way into a house.”
Like the sterile insect method, the approach is best used in conjunction with other methods, such as insecticides.
“For us it’s a numbers game,” Parry said. “We need to outnumber the wild males, so the lower the population you have to start with, the quicker and easier it is to get rid of it.”
But the company faces strong opposition from groups like the Britain-based group Gene Watch, which argues genetically modified mosquitoes have no proven benefits and that killing one species may result in another more invasive species filling the vacuum.
“They’re intriguing little organisms,” said Coetzee, who has been studying mosquitoes for more than 40 years. “Three and a half thousand species is a lot of variation and some of them are very beautiful,” she said.
She took a mosquito she had pinned to a board decades ago: Toxorhynchites brevipalpis, better known as the elephant mosquito. It has a wingspan of almost 2.5cm and a striking blue thorax flecked with gold and fuzzy antennae.
It lives off nectar and doesn’t bite humans. – Los Angeles Times/Tribune News Service