Mosquitoes show insecticide resistance22nd August 2011
Researchers in Senegal say that mosquitoes can develop resistance to bed nets treated with insecticide relatively quickly.
Nets soaked with insecticide have been one of the front-line methods use in malaria prevention in recent years, especially in African countries.
Writing in the journal Lancet Infectious Diseases, the research team said the nets might also play a role in reducing immunity to malaria infection among older children and adults.
But their conclusions have been called into question by other experts, who say the study was too small to come to any firm conclusions about the long-term effectiveness of mosquito nets in malaria prevention work.
Nets soaked with insecticide are one of the cheapest ways to cut infection rates.
According to the World Health Organisation, the nets can slash malaria infection rates by half, if they are properly installed and maintained.
The chemical Deltamethrin is recommended by the WHO for the treatment of bed nets.
But now, 37% of Senegalese malaria mosquitoes are believed to be resistant to it.
Joseph Keating of Tulane University in New Orleans said if the trend was confirmed in larger studies, it would have important implications for future prevention and control approaches to the deadly disease.
However, he called for an extension of the study by another couple of years to see whether the trend continued.
For the purposes of the study, the Senegalese researchers followed a small village, tracking the incidence of malaria both before and after the deployment of the nets in 2008.
Malaria infections began to fall within three weeks of the introduction of the nets, to a level that was reported as 13 times lower than before the nets came.
At the same time, the researchers were also collecting samples of the Anopheles gabiae mosquito, which is responsible for the transmission of malaria to humans in African countries.
They found that resistance among the mosquitoes to one type of pesticide rose from just 8% in 2007 to 37% in 2010.
As resistance rose among the mosquito population, so too did the rates of malaria infections in the village in the study.
More alarmingly, older children and adults, who might previously had had some more immunity than younger children, were seeing higher rates of infection than before the nets were introduced.
This suggests that the insecticide-soaked bed nets reduces immunity among older children and adults.
Overall infection rates rebounded sharply as a result.
The researchers, led by Jean-Francois Trape of Dakar's Institut de Recherche pour le Developpement, say they fear that their findings could be true elsewhere in Africa, not just Senegal.
Many parts of Africa could be seeing similar levels of insecticide resistance, which would impede any attempts to lower malaria morbidity in African countries.
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