(CN) — In the war to control malaria, disease-spreading mosquitoes are fighting back.
In the past two decades, scientists have treated bed nets — a mainstay of malaria prevention across Africa — with insecticides to thwart the spread of malaria, which kills 400,000 people each year, including more than 200,000 in sub-Saharan Africa.
But a study released Thursday found that resistance to five mainstream insecticides increased dramatically between 2005 and 2017. The study, published in the open-access journal PLOS Biology by Catherine Moyes and Penelope Hancock of the University of Oxford, U.K., mapped the patterns of insecticide resistance in Anopheles gambiae mosquitoes across Africa in an effort to better control the mosquitoes that transmit malaria.
In recent years, field studies have suggested an increase in insecticide resistance among mosquitoes, leading to a decrease in the effectiveness of mosquito-control program interventions, including bed nets.
Analyzing a published database of information on mosquitoes collected throughout mainland Sub-Saharan Africa between 2005 and 2017, researchers mapped when and where insecticide resistance had arisen in populations of Anopheles gambiae mosquitoes.
“Our maps show alarming increases in the prevalence of resistance to pyrethroids and DDT across sub-Saharan Africa from 2005 to 2017, with mean mortality following insecticide exposure declining from almost 100% to less than 30% in some areas,” the authors wrote.
In West Africa, resistance to pyrethroids — the only class of insecticides used in all treated bed nets — increased drastically over the timeframe. For instance, 15% of West Africa had mosquitoes with resistance to deltamethrin in 2005, but by 2017 this had risen to 98%.
Meanwhile in East Africa, resistance to pyrethroids increased to a lesser degree, expanding from 9% to 45% of the region. Similar increases were seen among populations of mosquitoes resistant to DDT, a chemical often used for indoor spraying to kill malaria-transmitting mosquitoes.
“The rapid spread of resistance across large parts of the sub-Saharan Africa signals an urgent need to quantify the efficacy of different resistance management strategies, and to understand the impact of resistance on malaria transmission and control,” the authors said. “Relationships between insecticide resistance and malaria prevalence are currently poorly understood, but there is evidence that resistance can reduce the efficacy of standard pyrethroid-treated [bed nets].”
Such resistance has serious consequences for malaria prevention. Insecticide-treated nets have achieved substantial reductions in malaria prevalence in Africa, but the number of insecticides currently available for use in bed nets is very limited.
Until recently, pyrethroids were the only class approved for use in bed nets, and next-generation nets still use pyrethroids in combination with other chemicals. A wider range of options is available for indoor residual spraying, but less expensive are still used in malaria-endemic sub-Saharan African countries.
Associations between agricultural pesticide use and insecticide resistance have also been found, and there is evidence that pesticide contamination of water may also influence the rate of resistance.
Hoping to better understand the historical spread of resistance and assist in designing insecticide resistance management strategies, researchers looked at 6,423 observations across 1,466 locations.
“Our capacity to understand and predict insecticide resistance can benefit from considering the variables that may influence selection for resistance,” the authors wrote.
These results, which highlight the urgency of identifying and implementing effective resistance-management strategies, show substantial variation in resistance trends between East and West Africa, as well as within these two regions.
Variations in resistance were also found among the sibling species within the A. gambiae complex mosquitoes. For example, the distribution of A. arabiensis extends further than other species in the complex, and this species is known to be more plastic in its feeding behavior, biting outdoors and feeding on cattle.