Reduced long-lasting insecticidal net efficacy and pyrethroid insecticide resistance are associated with over-expression of <i>CYP6P4, CYP6P3</i> and <i>CYP6Z1</i> in populations of <i>Anopheles coluzzii</i> from South-East Côte d’Ivoire

<jats:title>Abstract</jats:title><jats:sec><jats:title>Background</jats:title><jats:p>Resistance to major public health insecticides in Côte d’Ivoire has intensified and now threatens the long-term effectiveness of malaria vector control interventions.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>This study evaluated the bioefficacy of conventional and next-generation long-lasting insecticidal nets (LLINs), determined resistance profiles, and characterized molecular and metabolic mechanisms in wild <jats:italic>Anopheles coluzzii</jats:italic> from South-East Côte d’Ivoire in 2019.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>Phenotypic resistance was intense: more than 25% of mosquitoes survived exposure to ten times the doses of pyrethroids required to kill susceptible populations. Similarly, 24-hour mortality to deltamethrin-only LLINs was very low and not significantly different to an untreated net. Sub-lethal pyrethroid exposure did not induce significant delayed vector mortality 72 hours later. In contrast, LLINs containing the synergist piperonyl butoxide (PBO), or new insecticides, clothianidin and chlorfenapyr, were highly toxic to <jats:italic>An. coluzzii</jats:italic>. Pyrethroid-susceptible <jats:italic>An. coluzzii</jats:italic> were significantly more likely to be infected with malaria, compared to those that survived insecticidal exposure. Pyrethroid resistance was associated with significant over-expression of <jats:italic>CYP6P4, CPY6Z1</jats:italic> and <jats:italic>CYP6P3</jats:italic>.</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>Study findings raise concerns regarding the operational failure of standard LLINs and support the urgent deployment of vector control interventions incorporating PBO, chlorfenapyr or clothianidin in areas of high resistance intensity in Côte d’Ivoire.</jats:p></jats:sec>