Biological forms of Anopheles stephensi in the city of Djibouti, Horn of Africa

BACKGROUND: Anopheles stephensi is an invasive malaria vector found in South Asia and the Middle East. It was first described in Djibouti in 2012. It has since been linked to an increase in malaria cases and its spread to several African countries in the region. Given that the three biological forms of An. stephensi - Type, Intermediate, and Mysorensis - have different vectorial capacities and habitats, it is essential to know which forms are present in urban areas in Africa and which are responsible for malaria transmission. METHODS: In April 2023, in the city of Djibouti, a total of 600 eggs of An. stephensi were collected from six sites and analyzed using a scanning electron microscope to identify the biological forms based on the number of ridges. Sequence polymorphism in the intron I region of the odor-binding protein 1 (Obp1) gene was also analyzed in 153 specimens of An. stephensi. The distribution of biological forms of An. stephensi across the six sites was also analyzed in light of the incidence rate of malaria in the corresponding areas. RESULTS: At all sites, the number of ridges observed on the eggs corresponds with all three biological forms of An. stephensi living sympatrically in Djibouti. In peri-urban and urban areas, even in areas with high malaria incidence, the majority of eggs had ridge counts consistent with the Mysorensis form, which is described as being associated with rural areas and low vector capacity. In addition, there was no correlation between morphological analysis and molecular analysis of Obp1. CONCLUSIONS: All biological forms of An. stephensi are present in the city of Djibouti, where their role in malaria transmission requires further study in order to improve malaria control policies. Furthermore, Obp1 is not a suitable molecular tool for distinguishing the three biological forms of An. stephensi.