The effects of insecticide-treated bed net introduction on anopheline and culicine mosquito behavior and insecticide resistance in southern Zambia.

摘要

Defeating malaria, a disease that affects 225 million people and kills 781,000 T1° people yearly, is a major public health challenge. In the Southern Province of Zambia, there has historically been hyperendemic transmission of Plasmodium falciparum malaria, vectored by Anopheles arabiensis mosquitoes. As part of the country's malaria control scale-up, the Zambian government has initiated free distribution of insecticide- treated bed nets (ITNs), which has provided ∼75% ITN coverage for the Macha area of Southern Zambia. The entomological effects of the ITN roll-out are unknown. Therefore, the aims of this study were: 1) to investigate the extent of multiple blood feeding by An. arabiensis prior to the ITN introduction, 2) to determine the effects of high ITN coverage on multiple blood feeding and heterogeneity in host choice, 3) to determine the efficacy of ITNs in Macha against An. arabiensis, as well as the species' insecticide-susceptibility status, 4) to determine the insecticide susceptibility status of Culex quinquefasciatus, a sympatric viral and filarial vector species, and 5) to construct a phylogeny of Anopheles species in Southern Africa. To determine multiple feeding rates and household heterogeneity in biting, the blood meals of An. arabiensis mosquitoes were genotyped at 4 human microsatellite loci to identify the unique human hosts on which the mosquitoes had fed. Pre-ITN introduction, 18.9% of An. arabiensis blood meals were taken from more than one human host, but the multiple blood feeding frequency was reduced by more than half to 9.1% post-ITN introduction. Within-house heterogeneity in mosquito host choice increased post-ITNs, most likely due to ITNs protecting most people from biting, but leaving a minority unprotected. CDC bottle bioassays and ITN susceptibility bioassays were used to assess the insecticide susceptibility status of the local An. arabiensis and Cx. quinquefasciatus mosquito populations. Although the An. arabiensis field population is susceptible to DDT, pyrethroids, and malathion, the Cx. quinquefasciatus population is highly resistant to all insecticides tested, due to the presence of the kdr-west allele and upregulated glutathione-s-transferases and esterases. Importantly, the presence of ITNs appears to have selected for the kdr-west allele in Cx. quinquefasciatus over a short time-span. In conjunction with these experiments, long-lasting insecticide-treated bed nets (LLINs) that had been used by Macha villagers for two years were collected for evaluation. There was high variability in the residual deltamethrin concentration and quality of the nets. It is likely that many of these nets are ineffectual for protecting their owners against mosquito bites, and may contribute to selection for insecticide resistance. Finally, a phylogeny of Southern African Anopheles mosquitoes was constructed using sequences from the mitochondrial cytochrome oxidase subunit I (COI) and the nuclear internal transcribed spacer 2 (ITS2) genes. This phylogeny helps to elucidate the relationships between anthropophilic, vector species, and more zoophilie secondary vector species that may become increasingly important as control measures such as ITNs become more widespread. The studies described in this dissertation extend our knowledge about the effects of ITNs on vector species in Zambia and provide a starting point for further entomological research in the context of changing malaria control measures in Africa.