RESEARCHERS at the Nairobi-based International Center for Insect Physiology and Ecology (ICIPE) have discovered a new way that could be used to control malaria in Africa.
ICIPE Researcher Mike Okal said a naturally occurring chemical, cedrol, found in mosquito breeding sites near Lake Victoria that attracts pregnant malaria-transmitting mosquitoes, could be used in traps to “attract and kill” the female mosquito, preventing reproduction before she lays eggs.
“For the past six years, we have been studying how the major malaria-transmitting mosquito in Africa selects which pool to lay her eggs in, and asking how that choice could be manipulated so we can intercept and kill her before she lays hundreds of eggs,” Okal said in Nairobi on Friday.
The researcher said while much research has been done into repellents and attractants of malaria-transmitting mosquitoes, this is the first chemical confirmed to attract female mosquitoes while they search for a place to lay their eggs. “The research offers a new way to control mosquitoes and will also boost malaria control efforts in tropical Africa,” Okal noted.
Beyond vector control
Vector control, or preventing contact between mosquitoes and people, and reducing the number of mosquitoes, is considered to be the best strategy to tackle malaria in sub-Saharan Africa. Many studies have been done to find out how the blood-thirsty mosquito finds human targets for a blood meal, and many successful control methods focus on protecting people from bites.
Several chemicals have been identified that attract hungry mosquitoes, and odour-based traps have been designed to attract the mosquito away before biting.
Okal noted that to improve vector control and work towards malaria elimination, researchers and organisations that are involved in the research need to look beyond blood-feeding to better understand mosquito behaviour at other times in her life.
He revealed that working at a research station on the shore of Lake Victoria, they set up a number of pools of water with different infusions, such as grasses, different soils, even rabbit food pellets, and judged which pools the mosquitoes preferred to lay in by counting the number of mosquito larvae in each.
And they quickly settled on one particular soil, which they dubbed their “magical mud”. “We found the mosquitoes were more than twice as likely to lay eggs in water infused with this particular soil than in water fresh from Lake Victoria,” said Okal. After various studies to confirm that it was an odor released from the soil infusion, rather than the look of the turbid water that was attracting mosquitoes, the challenge was to isolate the precise chemical that drew them in.
Researchers at the Swedish Royal Institute of Technology used gas chromatography coupled to mass spectrometry to identify a number of chemicals released from the soil-infused water and compared these with over 100 samples taken from natural mosquito breeding sites around Lake Victoria.
They quickly settled on one, the sesquiterpene alcohol cedrol, which was present in their soil infusion and was also found in more than 50% of their natural habitat samples.
The team confirmed that the mosquitoes were two times more likely to lay eggs in water with cedrol in the laboratory and a controlled field environment. And during their field test, the team showed that wild mosquitoes were three times more likely to be caught in traps baited with cedrol than in traps with lake water alone.
Ulrike Fillinger, the project team leader, said the next step is to show how to use cedrol in traps as part of an “attract and kill” strategy to complement current vector control methods and to protect people from the deadly malaria parasite carried by these mosquitoes.