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Mosquitoes can rapidly develop resistance to bed nets treated with insecticide, a study from Senegal says.

In recent years the nets have become a leading method of preventing malaria, especially in Africa.

, the researchers also suggest the nets reduced the immunity of older children and adults to malaria infection.

But other experts say the study was too small to draw conclusions about the long-term effectiveness of nets.

In the war against malaria, the cheapest and most effective weapon to date has been the long-lasting insecticide-treated bed net.

Over the last few years the nets have been widely distributed in Africa and elsewhere - the World Health Organization says that when properly deployed they can cut malaria rates by half.

In Senegal, around six million nets have been distributed over the last five years. In this study researchers looked at one small village in the country 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 8% to 48%.

By 2010 the proportion of mosquitoes resistant to Deltamethrin, the chemical recommended by the World Health Organization for bed nets, was 37%.

In the last four months of the study the researchers found 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 were led by Dr Jean-Francois Trape from the Institut de Recherche pour le Developpement in Dakar. The authors 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 substantial decrease in malaria morbidity in many parts of Africa," they write.

But other experts in this field say that it is impossible to draw wider conclusions.

True trend?

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 generalising 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."