Escalation of pyrethroid resistance in the malaria vector Anopheles funestus induces a loss of efficacy of PBO-based insecticide-treated nets in Mozambique

Background

Insecticide resistance poses a serious threat to insecticide-based interventions in Africa. There is a fear that resistance escalation could jeopardize malaria control efforts. Monitoring cases of aggravation of resistance intensity and its impact on the efficacy of control tools is crucial to predict consequences of resistance.

Methods

The resistance levels of an Anopheles funestus population from Palmeira in southern Mozambique was characterised and its impact on the efficacy of various insecticide-treated nets established.

Results

A dramatic loss of efficacy of all long lasting insecticidal nets (LLINs) including PBO-based nets (Olyset Plus) was observed. This An. funestus population consistently (2016, 2017 and 2018) exhibited high degree of pyrethroid resistance. Molecular analyses revealed that this resistance escalation was associated with a massive over-expression of duplicated cytochrome P450 genes, CYP6P9a/b and also the fixation of the resistance CYP6P9a_R allele in this population in 2016 (100%) in contrast to 2002 (5%). However, the low recovery of susceptibility after PBO synergist assay suggests that other resistance mechanisms could be involved.

Conclusions

The loss of efficacy of pyrethroid-based LLINs with and without PBO is a concern for the effectiveness of insecticide-based intervention and action should be taken to prevent the spread of such super-resistance.