Impact de la résistance aux insecticides sur la composition des protéines salivaires (sialome) et ses conséquences sur la capacité vectorielle chez Anopheles funestus Giles, 1900 (Diptera : Culicidae), vecteur majeur du paludisme au Cameroun

Abstract

In recent decades, malaria-related mortality has fallen considerably in sub-Saharan Africa. Unfortunately, efforts to eliminate malaria are jeopardised by insecticide resistance in the main vectors. Indeed, the widespread use of indoor residual spraying (IRS) or long-acting insecticide-treated nets (LLINs) is the only means of mass prevention against malaria. However, although insecticide resistance confers a selective advantage on mosquitoes under insecticide pressure, it can also result in genetic costs that impact on the life traits, reproductive capacity and physiology of mosquitoes. More recently, laboratory studies have revealed that insecticide resistance may also alter the composition of salivary proteins in Anopheles mosquitoes. These salivary proteins play a crucial role in the mosquito's success in taking a blood meal and in the transmission of the malaria parasite by counteracting the host's reaction. Thus, changes in salivary gland content caused by resistance to insecticides could have an impact on the mosquito's blood meal intake, as well as on its ability to transmit parasites to humans. However, despite the importance of the role played by salivary proteins and the fact that invasion of the salivary glands is an essential stage in the life cycle of Plasmodium in mosquitoes, the impact of insecticide resistance on the composition of salivary proteins (sialome) and its consequences for vectorial capacity remain largely unexplored in natural populations of Anopheles funestus. The study of the impact of pyrethroid resistance on the composition of salivary proteins in the An. funestus mosquito, based on transcriptomic analysis, revealed the existence of 13588 genes, of which 5233 could be annotated. Among these annotated genes, we observed that 998 genes were differentially expressed between ‘Resistant’ and ‘Susceptible’ mosquitoes, with 377 genes over-expressed and 621 genes under-expressed. Similarly, 979 genes were differentially expressed between ‘Control’ and ‘Susceptible’ strains, with 472 genes over-expressed and 507 genes under-expressed, while 1040 genes differed between ‘Resistant’ and ‘Control’ strains, with 651 genes over-expressed and 389 genes under-expressed. Analysis of the expression of genes encoding proteins involved in mosquito blood meal intake and/or sporozoite invasion into the salivary glands of both ‘Resistant’ and ‘Susceptible’ strains of An. funestus strains, both ‘Resistant’ and ‘Susceptible’, identified a total of 07 genes differentially expressed between ‘Control’ and ‘Susceptible’ strains (03 overexpressed and 04 underexpressed), 06 genes were differentially expressed between ‘Control’ and ‘Susceptible’ strains (03 underexpressed and 03 overexpressed) and finally 06 genes were differentially expressed between ‘Resistant’ and ‘Control’ strains (01 underexpressed and 05 overexpressed). Evaluation of the impact of pyrethroid resistance on the ability of the mosquito to take a blood meal showed no influence xxxiii of the mechanism due to the L119F-GSTe2 mutation. Similarly, for this mechanism, no influence of pyrethroid resistance in An. funestus was observed on the duration of probing and feeding. However, it was observed that resistance linked to the CYP6P9a/b mutation was significantly associated with a higher probability of taking the blood meal for the resistant mosquito than for the susceptible mosquito. Evaluation of the consequences of the mechanisms associated with pyrethroid resistance on the size of the blood meal taken in the An. funestus mosquito showed that the L119F-GSTe2 mutation would not impact the quantity of blood absorbed by a mosquito during a meal on humans, whereas overexpression of the CYP6P9a/b genes was observed to be associated with a larger blood meal in mosquitoes carrying the resistance allele than mosquitoes carrying the susceptible allele. Investigation into the possible consequences that modulation of salivary protein expression by pyrethroid resistance might have on the invasion of sporozoites into the salivary glands of An. funestus showed that mosquitoes heterozygous for L119F-RS appeared to be more infected than susceptible homozygotes for L119F-SS. However, this difference was not significant. However, the small sample size of mosquitoes carrying the homozygous resistant genotype and infected with Plasmodium falciparum did not allow us to draw any relevant and clear conclusion on how the modulation of salivary protein expression by insecticide resistance could influence the establishment of Plasmodium sporozoites in the salivary glands of the An. funestus mosquito. At the end of our work, the various results obtained, although they do not provide conclusive evidence as to the impact of the modulation of salivary proteins on the vectorial capacity of An. funestus vectorial capacity in the presence of insecticide resistance, indicate that the L119F GSTe2 and CYP6P9a/b mutations could increase the probability of a successful blood meal and the intensity of sporozoite invasion in the salivary glands, which could affect the capacity of this vector to transmit malaria parasites to humans. Consequently, the implementation of management strategies based on the use of new insecticides and LLINs impregnated with other insecticides could help to combat pyrethroid resistance in the field and maintain the effectiveness of malaria control measures.