# Efficacy of long-lasting insecticidal nets before and after the selection of pyrethroid-resistant Anopheles gambiae s.l. and Anopheles funestus s.l. in northeastern Tanzania: A longitudinal meta-analysis of experimental hut trials at a single location

## TL;DR

This study shows that pyrethroid resistance in mosquitoes reduces the effectiveness of insecticide-treated nets for malaria control in Tanzania.

## Contribution

The study provides longitudinal evidence of LLIN efficacy decline due to pyrethroid resistance in wild mosquito populations.

## Key findings

- Mortality of susceptible mosquitoes on LLINs was significantly higher than resistant mosquitoes.
- Resistant mosquitoes were more likely to exit huts, reducing the nets' mass-killing effect.
- Personal protection indicators like blood-feeding rates were less affected by resistance.

## Abstract

The extent to which insecticide resistance is affecting malaria vector control in community and home is not fully understood. This study assessed the implications of insecticide resistance for entomological efficacy of long-lasting insecticidal nets (LLINs) against wild free-flying field Anopheles gambiae s.l. and Anopheles funestus s.l. in experimental hut trials (EHT) in northeastern Tanzania before and after the evolution of pyrethroid-resistance.

Evaluations of LLIN efficacy were conducted according to World Health Organization (WHO) guidelines in ten EHT commissioned by WHO between 2006 and 2017, before and after resistance development in Muheza. The entomological parameters taken into account were mortality, blood-feeding inhibition, induced exophily, personal protection, mass killing effect, and deterrence. WHO bioassays determined resistance levels in terms of susceptibility profile and intensity of resistance, while polymerase chain reaction (PCR) molecular diagnostics detected resistance alleles (L1014S and L1014F) and identified mosquitoes to species.

Anopheline mosquitoes were fully susceptible to pyrethroids until 2010, when they began to show resistance. The voltage gated sodium channel (VGSC) L1014S point mutation kdr was detected in An. gambiae s.l. at allelic frequency of 47%; no L1014F point mutation was detected. Synergist tests with piperonyl butoxide (PBO) restored efficacy only partially, indicating involvement of metabolic mechanisms. Meta-analysis of the ten EHT showed that mortality of susceptible An. gambiae s.l. was 6.7- and 5.2-fold greater on zero-times (z = 6.6, P = 0.001) and 20-times washed LLINs (z = 2.3, P= 0.023) than against resistant An. gambiae s.l. The mortality of unwashed and washed LLINs against susceptible An. funestus s.l. was 3.3-fold (z = 2.8, P = 0.004) and 2.6 (z = 2.9, P = 0.004) greater than against resistant An. funestus s.l. Resistant Anophelines were more likely to exit the huts as compared with susceptible Anophelines (z = 2.79, P = 0.005). The transition from susceptibility to resistance on changes to blood-feeding rates was nonsignificant for either species.

Reduced mortality induced by LLINs after selection of pyrethroid resistance indicates that resistance undermines household control of vector populations. Personal protection indicators such as proportions feeding on blood seemed less affected by the transition to resistance. Meta-analysis, comparing the same net brands before and after selection of resistance, revealed the factors (decreases in mortality, status in blood-feeding rates, and increases in pyrethroid-induced exiting rates) most affected by resistance.

The online version contains supplementary material available at 10.1186/s13071-026-07294-4.