Characterization of invasive Anopheles stephensi populations in Djibouti: understanding population dynamics, biotypes and insecticide resistance to guide new vector management methods

Abstract

Anopheles (An.) stephensi (Liston, 1901) was first detected on the African continent in 2012 in Djibouti. Its arrival is implicated in sharp increases in malaria cases in Djibouti: from 2012 to 2021 there was a > 2500-fold rise in case numbers, reaching > 70,000 cases in 2021. Unlike other malaria vectors , An. stephensi is well-adapted to colonising urban environments. Existing control tools provide inadequate protection, and the mosquito is resistant to commonly used insecticides. An. stephensi has now spread to other countries in Eastern and Western Africa, posing a new urban malaria threat across the continent. To support the development of future An. stephensi management strategies, and to inform deployments of a new genetically engineered self-limiting An. stephensi, we conducted studies to characterise population dynamics and insecticide resistance profiles of An. stephensi in Djibouti. From December 2021 to January 2023, monitoring of adult An. stephensi was conducted using odour-baited BG-Pro traps in urban and rural/semi-rural communities in Djibouti to characterise the vector’s distribution and seasonal dynamics. Data were also collected on the biotypes using OBP1 sequencing and the prevalence of known insecticide resistance-associated markers using PCR-based methods. An. stephensi abundance increased from January to April 2022, after which numbers declined during the hot summer season, followed by a further population increase from August to October 2022, before falling to low levels until the end of the monitoring period. All three biotypes of An. stephensi were detected, with the majority being type and intermediate , both of which are known to be competent malaria vectors. The target site mutation L1014F, associated with pyrethroid insecticide resistance, was found in 60% of individuals, with 17% of samples homozygous for the allele. This study provides new insights into the distribution and population dynamics of An. stephensi in Djibouti, confirming year-round presence of the vector in both urban and rural/semi-rural communities. Genetic evidence suggests that the populations show high levels of pyrethroid insecticide resistance, rendering existing vector control tools less effective and reinforcing the urgency for new vector management solutions. This research provides further insight into the biology of An. stephensi in Djibouti, and informed the design of pilot studies for self-limiting An. stephensi .