Researchers working in the village of Dielmowarned that new approaches may be needed to fight the malaria scourge on the continent.
|The scale-up of bednet distribution programmes has led to a very rapid spread of pyrethroid resistance – Flickr/World Bank Photo Collection|
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”.
Source – SciDev.Net – Emeka Johnkingsley – 24 Aug 2011