Mosquitoes can quickly develop resistance to insecticide-treated nets, a study from Senegal shows, raising fears that a leading method of preventing the disease may be less effective than previously thought.

Researchers who studied malaria infections in a village in the West African country found that growing resistance to a common type of insecticide by Anopheles gambiae mosquitoes, the species responsible for transmitting malaria to humans in Africa, is causing the disease to rebound.
"These findings are of great concern," the researchers, led by Jean-Francois Trape from the Development Research Institute in Dakar, wrote in a study in The Lancet Infectious Diseases journal on Thursday.

Despite decades of efforts to beat it with insecticides, indoor spraying, bednets and combination drugs, malaria still kills nearly 800,000 people a year, most of them babies and young children in sub-Saharan Africa.

Trape pointed to studies from Africa and South America which have suggested resistance to common insecticides is on the rise, and said this could have serious implications for malaria control strategies, particularly since there are few alternative insecticides that are effective, cheap and safe for humans.

Trape's team set out to assess the impact of the introduction of malaria drugs known as artemisinin-combination therapies (ACTs) as the first-line treatment for malaria, and the distribution of long-lasting deltamethrin-treated bednets in a rural west African population. Deltamethrin is one of the main insecticides used to control malaria in Africa and is recommended by World Health Organization.

In the village of Dielmo in Senegal, they analysed data on malaria cases and mosquito populations that were collected one and a half years before the drugs and bednets were introduced, and two and half years afterwards.

Their results showed that during the two years from August 2008 to August 2010 after bednets were distributed, there was a marked reduction in malaria attacks. But between September and December 2010, 27 to 30 months after the nets had been given out, malaria attacks in adults and older children increased to even higher levels than before.

Testing mosquitoes in the village, the team also found that 37 percent were resistant to deltamethrin in 2010, and that the genetic mutation which gives them resistance increased from eight percent in 2007 to 48 percent in 2010.

The researchers said they thought the rebound in malaria attacks, particularly in older children and adults, was the result of a combination of reduced immunity because of a lack of exposure to malaria in the years when the disease rates went down, coupled with the insecticide resistance that increased their exposure to A. gambiae mosquitoes.

"Strategies to address the problem of insecticide resistance and to mitigate its effects must be urgently defined and implemented," they wrote.