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

塞内加尔的一项新研究显示，蚊子可对带有杀虫剂的蚊帐迅速产生抵抗力。

In recent years the nets have become a leading method of preventing malaria¹, especially in Africa.

The researchers also suggest the nets reduced the immunity² of older children and adults to malaria（疟疾） 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 deployed³ 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 specimens⁴ 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 genetic⁵ resistance to one type of pesticide⁶ rose from 8% to 48%.

By 2010 the proportion of mosquitoes resistant⁷ 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 resurgence⁸（复活，再现） in resistant insects, there was a rapid rebound⁹ in infection rates.

The scientists were led by Dr Jean-Francois Trape from the Institut de Recherche¹⁰ 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 morbidity¹¹ in many parts of Africa," they write.

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