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Background  Long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS) have greatly reduced malaria transmission in sub-Saharan Africa, but are threatened by insecticide resistance. In south-eastern Tanzania, pyrethroid-resistant  Anopheles funestus  are now implicated in > 80% of malaria infections, even in villages where the species occurs at lower densities than the other vector,  Anopheles arabiensis . This study compared the insecticide resistance phenotypes between the two malaria vectors in an area where pyrethroid-LLINs are widely used .Methods  The study used the World Health Organization (WHO) assays with 1×, 5× and 10× insecticide doses to assess levels of resistance, followed by synergist bioassays to understand possible mechanisms of the observed resistance phenotypes. The tests involved adult mosquitoes collected from three villages across two districts in south-eastern Tanzania and included four insecticide classes.    Findings  At baseline doses (1×), both species were resistant to the two candidate pyrethroids (permethrin and deltamethrin), but susceptible to the organophosphate (pirimiphos-methyl).  Anopheles funestus , but not  An. arabiensis  was also resistant to the carbamate (bendiocarb). Both species were resistant to DDT in all villages except in one village where  An. arabiensis  was susceptible.  Anopheles funestus  showed strong resistance to pyrethroids, surviving the 5× and 10× doses, while  An. arabiensis  reverted to susceptibility at the 5× dose. Pre-exposure to the synergist, piperonyl butoxide (PBO), enhanced the potency of the pyrethroids against both species and resulted in full susceptibility of  An. arabiensis  (> 98% mortality). However, for  An. funestus  from two villages, permethrin-associated mortalities after pre-exposure to PBO only exceeded 90% but not 98%.    Conclusions  In south-eastern Tanzania, where  An. funestus  dominates malaria transmission, the species also has much stronger resistance to pyrethroids than its counterpart,  An. arabiensis,  and can survive more classes of insecticides. The pyrethroid resistance in both species appears to be mostly metabolic and may be partially addressed using synergists, e.g. PBO. These findings may explain the continued persistence and dominance of  An. funestus  despite widespread use of pyrethroid-treated LLINs, and inform new intervention choices for such settings. In short and medium-term, these may include PBO-based LLINs or improved IRS with compounds to which the vectors are still susceptible.

malaria journal

Comparative assessment of insecticide resistance phenotypes in two major malaria vectors, Anopheles funestus and Anopheles arabiensis in south-eastern Tanzania