Abstract:
Malaria remains a major health concern in sub-Saharan Africa. Recent decline in malaria cases and deaths was largely driven by vector control through Long Lasting Insecticidal Nets (LLINs) distribution. However, since 2016, this decline has stalled, coinciding with growing reports of resistance to pyrethroids—the only class of insecticides used in LLINs. In Gabon, there is limited LLIN deployment and data on transmission are scarce especially in the Moyen Ogooué Province. In addition, the contribution of non-falciparum species to the overall burden of malaria is often overlooked. There are also knowledge gaps on the vector competency of Anopheles mosquitoes to Plasmodium species such as P. malariae, the second most common plasmodial species in Gabon. Therefore, there is a need to address gaps in entomological data on malaria transmission in the Moyen Ogooué Province of Gabon by investigating malaria vector bionomics and developing protocols to investigate the mosquito-stages of P. malariae.
In the first chapter of this thesis, I assessed malaria transmission and the distribution of
insecticide resistance genes in Anopheles populations in three rural areas. Anopheles gambiae sensu stricto (s.s.) was the principal vector with other Anopheles spp. acting as secondary vectors. All An. gambiae s.s. except for one were either heterozygous and/or homozygous for the L1014F and L1014S resistance mutations indicating potentially high levels of resistance to pyrethroids. Malaria transmission was seasonal and heterogenous in the three study areas, with perennial transmission in Zilé and seasonal transmission in Bindo and Nombakélé. Although mosquitoes in the three areas were exophagic, the fact that the peak biting times in the three study areas was at times most people are sleeping argue for a positive effect of LLINs. The entomological inoculation rates (EIRs) recorded, have been previously associated with a high prevalence of P. falciparum in humans. Although most mosquitoes were infected with P. falciparum, one third of the mosquitoes were infected with other human-infecting species pointing to their important contribution to malaria. These results call for the deployment of tailored vector control measures adapted to the local setting.
In the second chapter, I established a protocol for the experimental infection of An. gambiae s.s. mosquitoes using P. malariae field isolates. I successfully infected An. gambiae s.s. mosquitoes with higher infection rates and infection intensities obtained after serum replacement compared to whole blood for P. malariae mono-infected isolates and coinfected P. malariae–P. falciparum isolates. The establishment of this platform will allow for the assessment of transmission blocking interventions, the investigation of vector-pathogen interactions to identify new targets for the control of P. malariae and to fill the gaps in knowledge about its transmission.
In addition, I collected baseline data on the susceptibility of malaria vectors to pyrethroids and organochlorides. The principal malaria vector An. gambiae s.s. was resistant to both
deltamethrin, permethrin and DDT in both Lambaréné and Zilé, with a more pronounced
resistance to the two latter. The resistance to these three insecticides was accompanied with a loss of knockdown effect for both permethrin and DDT and fold increases in the knockdown time for deltamethrin in addition to all the mosquitoes carrying knockdown resistance mutations. The results from this study provide baseline data on the resistance of malaria vectors that will be important for the choice of LLINs to be deployed in our study areas.
Finally, I evaluated the susceptibility of mosquitoes to pyrethroids, organophosphates and
carbamates, to assess the level of resistance and the effect of piperonyl butoxide (PBO). The mosquitoes were resistant to permethrin, deltamethrin, alphacypermethrin, bendiocarb and susceptible to malathion. The resistance was high for permethrin and at least moderate for deltamethrin. Preexposure to PBO restored the susceptibility of mosquitoes to deltamethrin while it increased the mortality to permethrin by 4-fold highlighting the implication of cytochrome P450 enzymes in the resistance phenotype to pyrethroids. All the mosquitoes were either homozygous or heterozygous for Vgsc-L1014F and -L1014S knockdown resistance mutations, however, despite resistance to bendiocarb no mosquito was carrying the G119S Ace- 1 resistance allele. The current results demonstrate the suitability of the PBO-Deltamethrin for the impregnation of LLINs to be distributed in Lambaréné.
This thesis provides a detailed picture of the high level of malaria transmission and calls for the deployment of tailored interventions to significantly reduce the level of transmission. In addition, the collection of baseline susceptibility data and the identification of the role of
cytochrome P450 enzymes as the main driver of resistance allow us to suggest the use of the PBO-Deltamethrin combination for LLINs. This work calls for the investigation of additional resistance mechanisms for improved resistance management. The establishment of the experimental infection of P. malariae has filled a gap that had restricted research on this neglected plasmodial species that is increasingly reported as a major contributor to the overall burden of malaria.
A. gambiae s.s.