According to a study from Senegal mosquitoes are quick to develop resistance to insecticide-treated nets. This has raised concerns 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.
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, “These findings are of great concern.” Despite decades of efforts to beat it with insecticides, indoor spraying, bed nets and combination drugs, malaria still kills nearly 800,000 people a year, most of them babies and young children in sub-Saharan Africa they add.
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.
The researchers assessed 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 bed nets 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 bed nets were introduced, and two and half years afterwards.
Their results showed that during the two years from August 2008 to August 2010 after bed nets 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.
“Strategies to address the problem of insecticide resistance and to mitigate its effects must be urgently defined and implemented,” authors of the study wrote.
In a commentary, Dr Joseph Keating from Tulane University, New Orleans, US, acknowledges the concerns the study raises. “If indeed this is a real trend we are seeing in this part of Senegal then it has very important implications for future malaria prevention and control strategies.” But he says there are a number of important provisos. “I would certainly advise extending the study a couple of more years which would be helpful in determining if this is a true trend or is it something specific to that particular area. We need to be very careful when generalizing these data to the larger continent of Africa as a whole; there is plenty of variation between communities and within communities.”
Dr Keating acknowledges there is a debate within the scientific community on the issue of acquired immunity, the level of resistance to the disease that people get through being bitten. “There is a huge discussion around acquired immunity. And how long does it take for an individual to lose this immunity once they are no longer exposed to parasite? So if you give someone a net he would be less exposed to parasites and it is possible that their immunity would shift to become less - but I think over all the benefits of nets certainly outweigh this potential loss of acquired immunity.”
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