Cleaner cooking fuels are increasingly promoted to reduce morbidity and mortality from household air pollution in low and middle income countries such as Rwanda. However, smoke is a traditional insect repellent and it is unknown whether the replacement of smoky biomass fuels with cleaner fuels could alter vector behavior and exposure to malaria or other vector-borne diseases (VBDs). Additionally, Rwanda experienced a 20-fold increase in reported malaria cases in the last decade. The Ministry of Health hypothesized that environmental changes such as increased temperatures as well as insecticide resistance could have driven these trends, but no scientific studies have assessed their effects on vector ecology and the root causes of the malaria resurgence. An improved understanding of the impacts of these multi-scale environmental changes is necessary to mitigate potential risks and tailor appropriate malaria control interventions.
In Aim 1 we employed a semi-field, Latin square design to experimentally investigate the effects of traditional and cleaner fuels on Anopheles mosquito behavior in rural Rwanda. Aim 2 consisted of a randomized controlled trial to assess the effects of liquified petroleum gas (LPG) adoption on vector density among houses that traditionally cook with biomass fuels. Finally, Aim 3 used a retrospective observational design to examine the effects of insecticide resistance, vector control, and regional warming on Anopheles bionomics and malaria incidence in eastern Rwanda.
In Aim 1, household entry and host-seeking by lab-reared Anopheles mosquitoes were higher in experimental huts that cooked with LPG compared to traditional biomass fuels, whereas mosquito mortality was lower. Lower PM2.5 and temperatures in LPG huts appeared to meditate this effect. However, in field conditions in Aim 2 we did not find a statistically significant difference in Anopheles or culicine density among intervention houses that received LPG stoves and fuel compared to control houses which cooked with biomass. In contrast, synanthropic fly density was reduced by 61% in intervention houses. Finally, in Aim 3 we found that insecticide resistance and regional warming were associated with the reemergence of An. gambiae after it was previously controlled in the early 2010s. The reemergence of this vector combined with a >2°C increase in regional temperatures drove a dramatic malaria resurgence in eastern Rwanda from 2010 to 2016, but An. gambiae and malaria transmission were controlled following implementation of non-pyrethroid indoor residual spraying (IRS) campaigns in the latter half of the decade.
Environmental changes at multiple scales can have important implications for vector ecology and transmission of malaria and other VBDs in Rwanda. Although we found experimental evidence that the adoption of cleaner fuels can affect Anopheles behavior, other environmental determinants were more important drivers of mosquito density in field conditions. Reductions in flies could constitute a health co-benefit of LPG adoption in this setting. At a larger scale, insecticide resistance and regional warming are major threats to vector control and malaria prevention in Rwanda. However, the success of non-pyrethroid IRS suggests that existing control measures can mitigate vector reemergence and climate-related malaria increases, providing insecticide resistance is managed.
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