The Next Malaria Threat: Insecticide Resistance and Strategies for Adapting

World Malaria Day is an appropriate time to reflect on the achievements of the past few years and the challenges we face. Vector control has undoubtedly played a primary role in the successes of recent years, with more than 150 million long-lasting insecticidal nets (LLIN) distributed in sub-Saharan Africa in 2015 contributing to 55 percent of people sleeping under a net, compared to less than 2 percent in 2000.

Indoor residual spraying (IRS) of insecticides has also been strengthened through the President’s Malaria Initiative (PMI), with 50 million people protected by IRS in 2014. The upscaling of vector control, together with improved diagnosis and treatment, has led to a substantial reduction in malaria cases and deaths in sub-Saharan Africa. The decrease in deaths is particularly impressive, with a 48 percent reduction, from 764,000 in 2000 to 395,000 in 2015. Still, clearly there is much work still to be done.

Despite the undoubted progress made in the past decade, insecticide-resistant mosquitoes pose a major threat. The scaling up of insecticide-based vector control as well as agricultural pesticide use has increased resistance to the pyrethroid class of insecticides. Pyrethroid insecticides have been the chemical of choice for both LLIN and IRS – between 2000-2009 pyrethroids accounted for 81 percent of insecticide use for malaria control – but resistance now is widespread in sub-Saharan Africa. In recent years, entomology monitoring in control programs has been of somewhat marginal importance, with the main focus being on implementation.

Insecticide Resistance Highlights Need for Entomological Monitoring

The spread of insecticide resistance has highlighted the importance of entomological monitoring so that key decisions to change insecticides can be made before control failure occurs. In addition, increased use of insecticides for malaria control has led to important changes in mosquito species composition, biting and resting behavior, often with small-scale variation by district.

The PMI Africa Indoor Residual Spraying (AIRS) Project, led by Abt Associates, has addressed this call to arms by increasing the geographical coverage, scope and frequency of entomological monitoring. The project is conducting new tests designed by Centers for Disease Control and Prevention to determine insecticide resistance intensity all AIRS IRS countries to better detect changes in resistance that may compromise the effectiveness of LLINs and IRS. The results have been alarming in many locations, with malaria vectors surviving more than 10 times the dose of pyrethroid that is normally lethal. The intensification of pyrethroid resistance is a particular threat to LLIN effectiveness, as there are currently no alternative chemicals recommended by the World Health Organization for net use.

Indoor Residual Spraying offers more options for resistance management, with four classes of insecticide recommended by the World Health Organization (WHO). In 2013, a new long-lasting formulation of the organophosphate pirimiphos-methyl CS was developed and soon became the insecticide of choice for IRS. The change to pirimiphos-methyl has resulted in impressive reductions in malaria cases. In Manicaland, Zimbabwe, PMI AIRS replaced pyrethroid spraying with pirimiphos-methyl CS in 2014, which resulted in a 75 percent reduction in malaria cases during the peak transmission season. This highlights the importance of vector control with insecticides to which mosquitoes are fully susceptible. Resistance management in the form of insecticide rotations have been successfully implemented in agriculture, yet in disease control we are lagging behind. This is partly due to the lack of insecticide options.

Thankfully, the future is bright as two new IRS formulations with new modes of action are at an advanced stage of WHO testing. We must maximize this opportunity through the development and implementation of insecticide rotations for IRS coupled with intensified entomological monitoring to aid decision making. 
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