The threat (or not) of insecticide resistance for malaria control

Malaria burdens have fallen dramatically this century, in large part because around a billion long-lasting insecticide-treated bed nets (LLINs) have been introduced into Africa (1, 2). Hanging over this success is a key question: What if the insecticides stop working? Pyrethroids are the only chemical class currently approved for bed net use and, predictably, pyrethroid resistance is emerging in Anopheles populations (1, 3). How much does this matter? Incredibly, we have little idea. The paper by Viana et al. (4) in PNAS explores one piece of the puzzle.In agriculture, the impact of insecticide resistance is relatively straightforward to anticipate: insects eat crops and insects that survive otherwise effective insecticide exposure continue to eat crops and reduce yield. But in public health, the situation is more nuanced. What matters is not simply whether mosquitoes survive insecticide exposure but, rather, whether insecticide resistance enhances the ability of mosquitoes to acquire and transmit pathogens (vectorial capacity). For example, most mosquitoes do not survive the 10–14 d it typically takes malaria parasites to become infectious. Insecticides work by reducing the number of survivors still further. Whether insecticide resistance negates this reduction depends critically on the lifespan of resistant mosquitoes. If resistance is incomplete—so the mosquitoes nonetheless die younger after repeated insecticide exposure—or if resistance itself is a life-shortening trait because it is metabolically costly, the impact of resistance on disease transmission might be negligible (5).Could it be that LLINs will continue to function in the face of increasing resistance? Although far from definitive, certain field data are consistent with this possibility. Genetic markers for known resistance alleles (such as knockdown resistance, KDR) and insecticidal bioassays (WHO standard cone and tube tests) point to the emergence of insecticide resistance in many malaria endemic areas, but as yet there is little … [↵][1]1To whom correspondence should be addressed. Email: a.read{at}psu.edu. [1]: #xref-corresp-1-1