Mosquitoes now wait for early risers to beat bed nets

Malarial mosquitoes may be able to adapt their behaviour in response to bed nets, according to a new study.

The widespread use of insecticide-treated nets has been credited with a sharp drop in the number of malarial deaths, especially in sub-Saharan Africa.

But researchers in two African villages where every household was given a net found that the insects changed their period of “peak aggression” from the middle of the night to the pre-dawn hours. The study in the West African nation of Benin found that three years after the nets were introduced two-thirds of mosquito bites were happening outdoors compared to 45 per cent when the French researchers began.

The finding was “worrying since villagers usually wake up before dawn to work in crops, and as such they are not protected by mosquito nets,” researcher Vincent Corbel from the Institute of Research for Development in Montpellier told Reuters. However, Mr Corbel warned against extrapolating wider conclusions from such a geographically-limited study.

The dissemination of millions of treated bed nets has been one of the most heavily-touted wins in global development in recent years. A report last week from the UN children's agency, Unicef, which showed that child mortality had been halved in the last 20 years, named anti-malarial mosquito nets among the most successful contributions of overseas aid. Estimates by the respected Cochrane Collaboration, which conducts reviews, stated that between six and seven lives are saved each year for every 1,000 children put under a net.

Scientists have also noted that monitoring the “biting behaviour” of mosquitoes is notoriously difficult as it involves letting the pest land on the researcher's skin and then catching it. There were also concerns that the Benin research was conducted over just three years.

An infectious disease transmitted by parasite-carrying mosquitoes, malaria kills 650,000 people every year, according to the World Health Organisation (WHO), most of them in Africa. The toll from the infectious disease has come down from close to one million deaths in 2000 but the death rates in some countries have started to climb again recently.

Until now concern over the long-term viability of nets coated in insecticide has centred on mosquitoes developing resistance to the chemicals. Researchers in a separate study found that in 64 countries the insects had begun to develop resistance to insecticide. The French research will now lead to concern that mosquitoes may evolve to bypass the protection offered by nets.