Evaluating the entomological effects of adjunctive ivermectin mass drug administration for malaria control on the Bijagós Archipelago, Guinea Bissau

Malaria continues to be a major threat to life for billions of people throughout the world. Regardless of widespread bednet distributions and high reported usage on the Bijagós Archipelago, Guinea Bissau, malaria persists, reaching a qPCR prevalence of 17.5% during peak transmission. Should more control tools not be developed to target residual transmission in this setting, disease elimination is unlikely. The Bijagós Archipelago is geographically isolated, with limited population movement between islands. It provides an ideal location to trial interventions. The primary aims of this PhD were to characterise the malaria vector population on the Bijagós and evaluate the impact of ivermectin in addition to dihydroartemisinin piperaquine mass drug administration (MDA) for malaria control on vectors using a cluster-randomised placebo-controlled trial design (alias: MATAMAL). In addition, this PhD described the built environment throughout the Archipelago and identified risk factors associated with mosquito house entry. A baseline survey was performed in 2019 and showed that mosquitoes within the Anopheles gambiae complex were present in both indoor and outdoor CDC miniature light traps throughout the Archipelago. Results from the baseline survey were used to randomise clusters into either the ivermectin or ivermectin-placebo arm. Monthly rounds of adjunctive ivermectin MDA at a dose of 300 μg/kg/day was distributed for three consecutive days in the months of July, August and September in 2021 and 2022. Cross-sectional surveys following completion of MDA and during a peak-transmission survey in November of 2021 and 2022 were conducted. From post-MDA collections, vector age structure was assessed using a newly validated technique of dry-preserving and rehydrating specimens prior to parity assessment. From all mosquito collections, vector density, species identification, sporozoite rate and entomological inoculation rate was investigated. The impact of ivermectin on the entomological outcomes was assessed using unadjusted and adjusted t-tests on cluster-level summaries. Adjustments were made for clustering and environmental variables. No significant impact from the intervention was seen in any entomological outcome at any time point, suggesting that ivermectin MDA does not significantly affect vector populations in this setting. Further investigation is needed into human and vector behaviour to better contextualise the results from the trial. Households selected for entomological surveillance were asked to take part in a survey collecting variables on household demographics, bednet practices and the built environment. After adjusting for clustering, univariable and multivariable analyses were performed using a negative binomial regression to identify any risk factors associated with high Anopheles house entry. These results may feed into future studies in order to design houses that are more resilient to mosquito entry, thereby building disease out of the Bijagós Archipelago.