Scientists discover chemical trap for malaria mosquito

A mother and her baby sleep under a mosquito net. Icipe and others say naturally occurring Cedrol to support other methods. PHOTO | FILE  NATION MEDIA GROUP

By SARAH OOKO

In Summary

• The female anopheles mosquito, that transmits malaria, mates only once in her lifetime.

A partnership of local and international researchers has for the first time discovered a naturally occurring chemical that attracts pregnant mosquitoes to certain breeding sites, ranking as a major development in efforts to control malaria.

Female Anopheles gambiae mosquitoes were more likely to lay eggs in water containing a chemical known as Cedrol than in those without it, the chemical’s scent pulled them to those sites, the scientists found.

“This paves the way for the development of new attract-and-kill strategies for malaria control,” says the study published in the Malaria Journal by the OviART research group, a team comprising of researchers from Icipe, the London School of Hygiene and Tropical Medicine, the Swedish Royal Institute of Technology and the UK’s Durham University.

The female anopheles mosquito, responsible for transmitting malaria, mates only once in her lifetime. The sperms she receives are then stored within her body in a special sac known as spermatheca, which will be used to fertilise eggs that the mosquito will lay over her lifetime.

Each time the mosquito wants to fertilise the eggs, it needs to feed on a blood meal which is sought from human beings or animals.

“So at this point, if they suck blood from a person infected with malaria it will pick the malaria parasites and pass it to other healthy individuals they bite afterwards,” says Dr Njagi Kiambo from the Ministry of Health Division of Malaria Campaign.

It is for this reason that malaria prevention strategies, such as the use of insecticide treated bed nets (ITNs) and Indoor Residual Sprays (ITNs) aim of killing mosquitoes before they reach human beings. But these indoor malaria control interventions have not been 100 per cent effective due to various factors.

For instance, even though ITNs are distributed to high-risk populations free of charge by the government, the 2010 Malaria Indicator Survey statistics show that only 42 per cent and 41 per cent of children (below five years) and pregnant women sleep under one respectively.

Statistics also show that only about 10 per cent of households in malaria endemic regions have their interior walls sprayed with pesticides targeting malaria-carrying mosquitoes.

In other instances, mosquitoes may bite people outside, thereby transmitting malaria parasites to them before they get into their households.

To address this challenge, the OviART researchers focused on what the Anopheles gambiae mosquitoes do after they feed on a blood meal rather than before that.

“To improve vector control and work towards malaria elimination, we need to look beyond blood-feeding to better understand mosquito behaviour at other times in her life,” said Mike Okal, an Icipe researcher and PhD student at teh the London School of Hygiene, who co-authored the study.

The researchers discovered that after feeding on blood from a person, the mosquitoes lay eggs in a pool of water. But the team noticed that some pools would be full of larvae while others remained empty.

“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,” said Mr Okal.

The research team in Kenya— at Icipe’s Thomas Odhiambo Research Station in Mbita on the shores of Lake Victoria — set up a number of pools of water with various infusions such as grasses, different soils, and rabbit food pellets.