Scientists are expressing concern that a newly-discovered type of mosquito could mean larger and less easily controlled populations of insects carrying the deadliest form of malaria.
The previously unclassified mosquito is a member of the anopheles gambiae family, the same mosquito group that carries the Plasmodium falciparum parasite and infects humans with the malaria pathogen when it feeds on their blood.
An estimated 300 million people contract P. falciparum malaria each year. The disease is responsible for about one million deaths annually, mostly of them young children in sub-Saharan Africa.
Larvae of the outdoor-dwelling mosquito were discovered by researchers in standing water puddles over a 400-kilometer swath of land near villages in Burkina Faso.
How disease is spread
It's commonly believed that mosquitoes spread the disease mainly by biting humans indoors, according to Ken Vernick of the Pasteur Institute in Paris, who led the research.
Vernick says that's why most malaria control measures, such as insecticide-treated bed nets, are targeted at indoor environments. Vernick says discovery of the outdoor-dwelling mosquito changes the way experts view malaria transmission in regions where the disease is endemic.
"So one cause for concern about this new sub-group of mosquitoes is they are abundant, they are in the species of anopheles gambiae which like to preferentially bite people, which is why they are efficient vectors of malaria, and they don't rest indoors, and so they are hard for us to control," added Vernick.
After mosquitoes take a blood meal by biting somebody, Vernick explains, they are engorged and usually rest to digest the meal and lay their eggs in a water source.
Researchers compared the mosquitoes that hatched from larvae collected outdoors to these indoor resting mosquitoes, feeding each group blood meals drawn from volunteers in nearby villages who had been naturally infected with malaria.
Vernick says a genetic analysis confirms the outdoor mosquitoes were much more easily infected with malaria from their blood meals than the indoor resting mosquitoes.
Vernick thinks various indoor malaria control measures, like spraying walls with insecticide, may be causing some mosquitoes to flee outdoors.
So far, researchers have been unable to locate adult outdoor resting mosquitoes. Vernick would like to develop a control strategy by finding the insects' outdoor niche during the dry season, when there is no malaria transmission in Burkina Faso.
"It's possible that by finding the resting site of the these outdoor resting mosquitoes, we could find a place that could let us target all of the mosquitoes in the dry season, which is their weakest point, and prevent them from coming back in the next rainy season," added Vernick.
At this point, Vernick says researchers don't know what proportion of malaria transmission is carried out by outdoor resting mosquitoes.
An article describing the new sub-group of malaria mosquito is published in the journal Science.