Pyrethroid resistance in the major malaria vector Anopheles arabiensis in Nouakchott, Mauritania

Mauritania is one of the African countries with ongoing malaria transmission where data on insecticide resistance of local malaria vectors are limited despite an increasing use of long-lasting insecticide-treated nets (LLINs) as the main intervention for vector control. This study presents an evaluation of the level of insecticide resistance of Anopheles arabiensis in Nouakchott.

Anopheles gambiae (s.l.) larvae were collected in breeding sites during the rainy season (August-September) in 2015 and 2016 from two selected sites in Nouakchott and reared until emergence. Adult anopheline mosquitoes were tested against malathion (5%), bendiocarb (0.1%), permethrin (0.75%) and deltamethrin (0.05%) using standard World Health Organization (WHO) insecticide-impregnated papers. PCR assays were used for the identification of An. gambiae (s.l.) sibling species as well as knockdown resistance (kdr).

The mean knockdown times 50% (KDT₅₀) and 95% (KDT₉₅) were 66 ± 17 and 244 ± 13 min, respectively, for permethrin in 2015. The KDT₅₀ and the KDT₉₅ were 39 ± 13 and 119 ± 13 min, respectively, for deltamethrin. The KDT₅₀ and the KDT₉₅ doubled for both molecules in 2016. The mortality rates 24 h post-exposure revealed that An. arabiensis populations in Nouakchott were fully susceptible to bendiocarb and malathion in 2015 as well as in 2016, while they were resistant to permethrin (51.9% mortality in 2015 and 24.1% mortality in 2016) and to deltamethrin (83.7% mortality in 2015 and 39.1% mortality in 2016). The molecular identification showed that Anopheles arabiensis was the only malaria vector species collected in Nouakchott in 2015 and 2016. Both the West and East African kdr mutant alleles were found in An. arabiensis mosquitoes surviving exposure to pyrethroid insecticide, with a high rate of homozygous resistant genotypes (54.3% for the West African kdr mutation and 21.4% for the East African kdr mutation) and a significant departure from Hardy-Weinberg proportions (χ² = 134, df = 3, P < 0.001).

The study showed high levels of pyrethroid resistance in An. arabiensis populations in Nouakchott and presence of both West and East African kdr alleles in the resistant phenotype. These results highlight a need for routine monitoring of susceptibility of malaria vector populations to insecticides used in public health programs.