Mosquitoes become resistant to bed-net insecticide

Malaria-carrying mosquitoes can quickly develop resistance to insecticide-treated bed nets, researchers in Senegal have found.

Mass distribution of insecticide-treated bed nets has become part of malaria control programs in Africa, with studies in Kenya, Senegal and Gambia suggesting the policy helps reduce deaths and illnesses from malaria.

Now researchers studying villagers in central Senegal say insecticide resistance challenges the approach.

They compared malaria illnesses and mosquito populations 1½ years before the bed nets were introduced and 2½ years after.

During the first two years after the bed nets were introduced, there was a big drop in malaria attacks.

But 27 months after the insecticide-treated bed nets were rolled out, malaria attacks increased to even higher levels than before the introduction of the bed nets in adults and older children, researchers say in Thursday's online issue of the journal The Lancet Infectious Diseases.

Dr. Jean-François Trape from the Institut de Recherche pour le Développement in Dakar and his co-authors said that between 2007 and 2010, the proportion of the insects showing resistance to one type of pesticide rose from eight per cent to 48 per cent.

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

"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," the study's authors concluded. 

"Strategies to address the problem of insecticide resistance and to mitigate its effects must be urgently defined and implemented."

In a journal commentary accompanying the paper, Dr. Joseph Keating from Tulane University in New Orleans said the authors should be commended for trying to assess changes in malaria after the introduction of insecticide-treated bed nets.

"However, caution is needed when generalizing these results to populations across Africa," Keating said.

The before-and-after design of the study cannot show a cause-and-effect relationship, Keating said.

The findings reinforced the importance of monitoring for insecticide resistance and malaria changes in Africa, the commentary concluded.