London: Insecticide-treated bed nets have so far been the cheapest and most effective weapon in the war against malaria, but a new study has claimed that mosquitoes can rapidly develop resistance to the nets.
The study by researchers from the Institut de Recherche pour le Development in Senegal also suggested that the nets reduced the immunity of older children and adults to malaria infection.
However, other experts said the study was too small to draw conclusions about the long-term effectiveness of nets, the BBC reported.
In the latest study, the researchers looked at one small village in Senegal and tracked the incidence of malaria both before and after the introduction of nets in 2008.
Within three weeks of their introduction the scientists found that the number of malaria attacks started to fall -- incidence of the disease was found to be 13 times lower than before the nets were used.
The researchers also collected specimens of Anopheles gambiae, the mosquito species responsible for transmitting malaria to humans in Africa.
Between 2007 and 2010 the proportion of the insects with a genetic resistance to one type of pesticide rose from eight percent to 48 percent.
By 2010 the proportion of mosquitoes resistant to Deltamethrin, the chemical recommended by the World Health Organisation for bed nets, was found to be 37 percent.
In the last four months of the study, the researchersfound that the incidence of malaria attacks returned to high levels. Among older children and adults the rate was even higher than before the introduction of the nets.
The researchers argue that the initial effectiveness of the bed nets reduced the amount of immunity that people acquire through exposure to mosquito bites. Combined with a resurgence in resistant insects, there was a rapid rebound in infection rates.
The scientists, who were led by Dr Jean-Francois Trape, are worried that their study has implications beyond Senegal.
"These findings are a great concern since they support the idea that insecticide resistance might not permit a sbstantial decrease in malaria morbidity in many parts of Africa," they said.
But other experts in this field say that it is impossible to draw wider conclusions.
In a commentary, Dr Joseph Keating from Tulane University in the US, acknowledged 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 said 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 generalising these data to the larger continent of Africa as a whole; there is plenty of variation between communities and within communities."
Dr Keating also said that 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.