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Scientists have discovered a new mosquito sub-type that could become a key malaria transmitter out of doors, thus avoiding the mainly indoor control measures, according to a paper published in Science today (4 February).

Malaria rates have decreased in many African countries following the widespread use of insecticide-treated bednets and indoor pesticide spraying. But research in Burkina Faso now suggests that up to half of mosquitoes may never enter households and therefore cannot be controlled by nets or sprays.

The finding comes at a time when other researchers are expressing fears that increasing bednet use could drive those mosquitoes that bite indoors into outdoor biting behaviours.

In the Burkina Faso research, scientists took mosquito larvae from ponds near houses and used genetic analysis to identify a recently evolved, sizeable genetic subgroup of Anopheles gambiae s.s. — the most effective transmitter of malaria across Africa. This group has not been detected before, indoors or outdoors.

They found lab-grown adults of this subgroup were highly susceptible to infection by the malaria parasite.

The group does not yet know whether these mosquitoes bite humans: "We're trying to catch them outdoors to see whether we can establish the extent of human feeding," co-author Michelle Riehle, a researcher at the University of Minnesota in the United States, told SciDev.Net.

The research adds to a growing literature that suggests current mosquito control measures may be inadequate. For example, George Christophides, reader in infection and immunity at UK-based Imperial College London, recently co-authored two papers in Science suggesting that the indoor-resting strain of A. gambiae s.s. is diverging into two separate species.

He told SciDev.Net: "Malaria mosquitoes are evolving fast … possibly due to man-made pressures. We know this is happening with insecticides and it may happen with bednets. The more bednets we apply in Africa, the more we may push the mosquitoes to bite outdoors."

Steve Lindsay, professor of public health entomology at the UK's London School of Hygiene and Tropical Medicine, said: "We have very effective controls against A. gambiae s.s., but they're all directed against mosquitoes coming indoors. We're not very good at controlling outdoor biting ... Compliance with repellents is a problem."

Christophides said, "It's very likely we will need a suite of methods depending on what type of mosquitoes we find in each place. Maybe bednets will be functional in one place but not in another."

But transmission-blocking interventions [which interrupt the life cycle of the parasite, which passes from mosquito to human and back again] could be "a universal solution to stop malaria transmission", he said.

Transmission-blocking vaccines or drugs would be given to people to ensure that a mosquito that had bitten a treated person would no longer be able to transmit the disease.

Link to full paper in Science

References

Science 331, 596 (2011)

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