Pyrethroid-resistant malaria vector Anopheles gambiae restored susceptibility after pre-exposure to piperonyl-butoxide: results from country-wide insecticide resistance monitoring in Tanzania, 2023

Abstract

Background: Effective vector control interventions, notably insecticide-treated nets (ITNs) and indoor residual spraying (IRS) are indispensable for malaria control in Tanzania and elsewhere. However, the emergence of widespread insecticide resistance threatens the efficacy of these interventions. Monitoring of insecticide resistance is, therefore, critical for the selection and assessment of the programmatic impact of insecticide-based interventions.

Methods: The study was conducted country-wide across 22 sentinel districts of Tanzania between May and July 2023 using standard World Health Organization susceptibility test with 1×, 5×, and 10× of deltamethrin, permethrin, and alpha-cypermethrin and discriminating concentrations of 0.25% pirimiphos-methyl. Synergist assays were conducted to explore the underlying mechanisms of the observed phenotypic pyrethroid-resistant mosquitoes. Three- to five-day-old wild adult females in the first filiar generation of Anopheles gambiae sensu lato (s.l.) were used for the susceptibility bioassays.

Results: Anopheles gambiae s.l. were resistant to all pyrethroids at the discriminating dose in most sentinel districts except in Rorya, which remains fully susceptible, and Ushetu, which remains susceptible to deltamethrin but not permethrin. In 5 sites (Bukombe, Ukerewe, Kilwa, Kibondo, and Kakonko), the An. gambiae s.l. species exhibited strong resistance to pyrethroids surviving the 10 X concentrations (mortality rate < 98%). However, they remained fully susceptible to pirimiphos-methyl in almost all the sites except in Kibondo and Shinyanga. Likewise, there was full restoration to susceptibility to pyrethroid following pre-exposure of An. gambiae s.l. to piperonyl-butoxide (PBO) in 13 out of 16 sites. The 3 sites that exhibited partial restoration include Kakonko, Tandahimba, and Newala.

Conclusion: The evidence of widespread pyrethroid resistance of the major malaria vector justifies the decision made by the Tanzania National Malaria Control Programme to transition to PBO-based ITNs. Without this switch, the gains achieved in malaria control could be compromised. Equally important, the lack of full restoration to susceptibility observed in three sentinel districts upon pre-exposure to PBO merits close monitoring, as there could be other underlying resistance mechanisms besides oxidase metabolic resistance.

Keywords: An. gambiae; Anopheles; Kdr; Insecticide resistance; PBO; Pyrethroid; Susceptibility; Tanzania.

Fig. 1

Map of Tanzania mainland showing the spatial distribution of the selected 22 sentinel districts based on malaria endemicity for insecticide resistance monitoring in 2023

Fig. 2

Percentage mortality of wild female An. gambiae s.l. local populations to different concentrations of permethrin in sentinel districts of mainland Tanzania. The red line at 98% mortality indicates any mortality above that is susceptible and any mortality below that represent either suspect (90–97%) or confirmed resistance (90%)

Fig. 3

Percentage mortality of wild female An. gambiae s.l. local populations to different concentrations of deltamethrin in sentinel districts in mainland Tanzania. The red line at 98% mortality indicates any mortality above that is susceptible and any mortality below that represents either suspect (90–97%) or confirmed resistance (90%)

Fig. 4

Percentage mortality of wild female An. gambiae s.l. local populations to different concentrations of alphacypermethrin in sentinel districts of mainland Tanzania. The red line at 98% mortality indicates any mortality above that is susceptible and any mortality below that represents either suspect (90–97%) or confirmed resistance (90%)

Fig. 5

Comparison of mortality rates of An. gambiae s.l. exposed to deltamethrin (0.05%) (A) and permethrin (0.75%) (B), alone and in combination with PBO, per site. The red line at 98% mortality indicates any mortality above that is susceptible and any mortality below that represents either suspect (90–97%) or confirmed resistance (90%)

Fig. 6

Percentage distribution of Anopheles gambiae s.l. sibling species in 22 sentinel districts. Footnote: One An. quadrannulatus was identified in Bariadi, and two An. merus was identified in Kilwa