24 August 2011 | EN | FR
The scale-up of bednet distribution programmes has led to a very rapid spread of pyrethroid resistance
Flickr/World Bank Photo Collection
[ABUJA] Scientists have linked growing insecticide resistance with a resurgence of malaria in Senegal.
Researchers working in the village of Dielmo warned that new approaches may be needed to fight the malaria scourge on the continent.
They found that in the two years (August 2008–2010) following the distribution of bednets treated with deltamethrin, a long-lasting insecticide recommended by the WHO, malaria cases significantly decreased.
But, in the following four months, cases increased to higher levels than those before the bednets were introduced. This increase in morbidity was therefore likely to be due to increased resistance by malaria-carrying mosquitoes to pyrethroid insecticides, said the authors, whose findings were published online in The Lancet Infectious Diseases last week (18 August).
Thirty-seven per cent of Anopheles gambiae mosquitoes were resistant to deltamethrin in 2010, and the proportion of mosquitoes with the kdr mutation that confers resistance to pyrethroids insecticides in general increased from 8 per cent in 2007 to 48 per cent in 2010.
The authors said that the scale-up of bednet distribution programmes and indoor spraying campaigns has "led to a very rapid spread of pyrethroid resistance in the major malaria vectors".
The study found that malaria resurged, in particular, in older children and adults, who are increasingly susceptible to the disease.
This susceptibility cannot be explained by increasing pyrethroid resistance, the authors said. They suggest that "either a recent increase in exposure to malaria vectors in older age groups or a decrease in protective immunity" could be responsible.
Pierre Druilhe, an author of the research and a researcher at the Malaria Vaccine Development Laboratory at the Pasteur Institute in France, said that the resurgence in malaria morbidity had been foreseen for a long time and goes beyond the problem of insecticide resistance.
He said that in areas that have a high incidence of the disease, exposure to malaria-carrying mosquitoes induces a strong immune response that protects individuals. While insecticide bednets do cut the number of bites from vectors, this in turn reduces acquired immunity — leading to a new equilibrium where the chance of an infectious bite causing malaria increases.
In an accompanying commentary, Joseph Keating and Thomas Eisele warned of the danger of generalising the research to the rest of the continent, particularly because of the short period of the study.
But Druilhe said: "This type of situation [pyrethroid resistance] can happen in all high transmission areas, which corresponds to the vast majority of malaria endemic areas in Africa".
Jonny ( Weizmann Inst Sci | Israel )
30 August 2011
It is hard to believe that mosquitoes evolved resistance to deltamethrin unless:
1. It is also used heavily in agriculture in the same area;
2. Anopheles gambiae has only human hosts and everyone in the village has bednets with deltamethrin.
Otherwise there would not be sufficient selection pressure for resistance. This epidemiology is necessary to understand how this may have happened, and how it can be prevented. The alternative is to return to DDT - spraying only indoors with it, and not in surrounding areas so that there a large majority of untreated mosquitoes. It is only necessary to kill those biting people and carrying the parasite
Chr. Kooyman ( Eléphant Vert | Morocco )
31 August 2011
Though the speed in this case is surprising, development of resistance to the products used to treat bednets had been predicted by many, including myself. As long as simple methods are employed to combat malaria, the vector and/or the parasite will find ways around them. E.g., there are indications that mosquitoes start to bite earlier and more outside than inside. What we need are smart combinations of methods, including biological ones, like the use of fungal pathogens against the mosquitoes (Metarhizium, Beauveria). The only 'simple' method that may work, is a vaccine against the malaria parasite.
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