Malaria-carrying mosquitoes slip through the net

Malaria-carrying mosquitoes in Kenya and Benin are slipping through the net of current control methods.

Malaria cases in Africa dropped after the introduction of insecticide-treated bed nets, because the malaria-carrying mosquitoes fly and bite by night. It's estimated that the bed nets have prevented up to 1 million deaths in Africa.

But the nets may be useless against a possible new species of malaria-carrying mosquito that bites humans earlier in the evenings ? long before people retire to their beds. It was found in Kenya by a team led by Jennifer Stevenson at the London School of Hygiene and Tropical Medicine.

"We were making collections of mosquitoes in the highlands of western Kenya, and we noticed that we were having difficulty matching the samples that we were collecting," says Stevenson. "So we sent samples for further molecular analysis." The DNA didn't match that of known mosquitoes in the region.

New species?

The researchers set up indoor and outdoor traps and collected hundreds of mosquitoes ? 40 per cent of them belonged to this new type. Stevenson and her colleagues are uncertain if it is a previously unknown species or a new form of a known species.

The mosquito's emergence may be a response to the unprecedented success in malaria control, says Jo Lines, also at the London School of Hygiene and Tropical Medicine, who was not involved in the study.

"The more we suppress the mosquitoes we know, the more we reveal the rare ones that were always there in the background," he says.

"The ability of this new species to transmit the malaria parasite to humans has to be confirmed," says Nicolas Moiroux at the Institute of Research for Development in Benin. He adds that until the mosquito is found beyond a single village it is unclear how much of a threat it poses.

Time shift

But Moiroux's own research suggests mosquitoes on the other side of Africa ? in Benin ? are also changing, perhaps in response to the introduction of bed nets.

With colleagues, Moiroux conducted indoor and outdoor mosquito collections in the villages of Lokohou? and Tokoli before the introduction of insecticide-treated bed nets, and then three years after the nets had been introduced.

The insects were initially most active between 2 am and 3 am, but three years after the villagers began using mosquito nets, the insects had shifted to become most active at 5 am, putting the early rising farmers at risk.

Journal references: Stevenson study: Emerging Infectious Diseases, doi.org/jd9; Moiroux study: The Journal of Infectious Diseases, doi.org/jfb

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