Do bednets including piperonyl butoxide offer additional protection against populations of Anopheles gambiae s.l. that are highly resistant to pyrethroids? An experimental hut evaluation in Burkina Fasov

Malaria control is dependent on the use of Long Lasting Insecticidal Nets (LLINs) containing pyrethroids. A new generation of LLINs containing both pyrethroids and the synergist piperonyl butoxide (PBO LLINs) have been developed in response to increasing pyrethroid resistance in African malaria vectors but questions remain about the performance of these nets in areas where pyrethroid resistance is at very high levels. This study was conducted in two settings in Southwest Burkina Faso, Vallée du Kou 5 and Tengrela where Anopheles gambiae s.l. mortality rates in WHO discriminating dose assays were < 14 % for permethrin < 33 % for deltamethrin (< 33 %). When mosquitoes were pre-exposed to PBO in WHO tube assays mortality rates increased substantially but full susceptibility was not restored. Molecular characterisation revealed high levels of kdr alleles and elevated levels of P450s previously implicated in pyrethroid resistance. In cone bioassays and experimental huts the PBO LLINs outperformed the pyrethroid only equivalents from the same manufacturers. Blood feeding rates were 1.6 - 2.2 fold lower, and mortality rates were increased by 1.69 – 1.78 -fold in huts with PBO LLINs versus non PBO LLINs. This study indicates that PBO LLINs provide greater personal and community level protection than standard LLINs against highly pyrethroid resistant mosquito populations