Effects of pyrethroid exposure and insecticide resistance on the sporogonic development of Plasmodium falciparum in Anopheles gambiae s.l.

Pyrethroid resistance has spread in the Anopheles gambiae s.l. populations in most African countries, often at high frequency. As pyrethroids are still used in all insecticide treated nets, this poses a potentially major threat to the effectiveness of vector control strategies. However, even though insecticide resistance is widespread, malaria control has not yet failed outright, but detecting the effects of resistance on control measures and measuring how much the effectiveness of control has changed is problematic. A few laboratory-based studies carried out over two decades between the 1980s and early 2000s tried to test whether insecticide exposure affects parasite development, with little follow up. The aim of this project was therefore to investigate the possibility that pyrethroid exposure of An. gambiae s.l. might impair the sporogonic development of Plasmodium falciparum in field conditions, and to explore if insecticide resistance further affects sporogony. The effects of sub-lethal doses of deltamethrin on sporogony in wild pyrethroid resistant An. gambiae s.s. in Uganda were studied, showing that exposure of kdr resistant mosquitoes to sub-lethal doses of pyrethroids significantly reduces both parasite prevalence and intensity of infection. Mean ambient temperature during the incubation period, and temperature range during the first 24 hours and on day 4 post-infectious feed also had a highly significant effect on risk of infection, where increases in mean temperature and temperature range were associated with lower infection. Furthermore, deltamethrin significantly impaired survival of kdr homozygous mosquitoes, while mean temperature and relative humidity also had a significant effect on mosquito mortality. Deltamethrin exposure significantly impaired both ookinete conversion and motility of P. berghei at doses that malaria parasites are likely to encounter when mosquitoes are exposed to insecticides in field conditions, while high performance liquid chromatographyphotodiode array assay (HPLC-PDA) analysis showed that each mosquito picks up to approximately 10ng of deltamethrin following exposure to a long-lasting insecticidal net (LLIN) (PermaNet 2.0). Potential interference of kdr resistance with the development of P. falciparum within the vector was also investigated. The effects of kdr genotype on Plasmodium infection rates in An. gambiae s.s. and An. arabiensis were explored in mid-western Uganda, together with 4 variations in phenotypic and genetic resistance against commonly used insecticides. Bioassay mortality was only weakly associated with kdr genotype in both sibling species, implying that other metabolic resistance mechanisms play a significant role in the study area. Oocyst prevalence rates and infection intensity were not significantly different between kdr genotypes, nor did they vary between the two species, while sporozoite rates in An. gambiae s.s. were not significantly different between kdr genotypes. These results imply that even if resistant mosquitoes survive insecticide exposure, their vector competence is impaired as parasite development is affected, suggesting that pyrethroid-based interventions could still have a role in malaria control at least until alternative insecticides are available.