Urban larval mosquito surveillance in Antananarivo, Madagascar: spatiotemporal heterogeneity and associated vector dynamics

Mosquito-borne diseases, such as malaria, remain an important public health problem globally, including on the island of Madagascar. Malaria vector control strategies are typically aimed at adult Anopheles mosquitoes in predominantly rural settings. However, very little information is available on larval populations of Anopheles and other vector genera, such as Aedes and Culex, in urban areas, where urbanization creates a multitude of potential larval habitats. Here, the objectives are to identify different types of existing larval habitats and the diversity of vector species in the capital city of Antananarivo, and to study the temporal dynamics and spatial distribution of mosquito vectors of public health interest. From 2022-2023, longitudinal larval surveillance was carried out across all six districts of Antananarivo and consisted of prospecting 2,856 potential larval habitats, of which 653 (23%) were found to contain Aedes, Culex, or Anopheles larvae. Anopheles larvae were reared and morphologically identified using taxonomic keys, and molecular identification by PCR and Sanger sequencing was used for Anopheles sibling species determination. A total of 27,418 larval specimens were collected from eight species: Culex quinquefasciatus (53%), Aedes albopictus (26%), Cx. pipiens (14%), Anopheles gambiae s.l. (3%, all molecularly identified as An. arabiensis), Cx. univittatus (1%), Cx. antennatus (1%), Cx. poicilipes (1%), and An. coustani (1%). Geospatial analysis reveals that 55% of larvae came from artificial larval habitats within the city, which harbor lower species diversity than natural larval habitats. Twenty-one different types of larval habitats were observed, and Cx. quinquefasciatus, a vector of West Nile virus, was present in all of them. Aedes albopictus, vector of arboviral pathogens, prefers artificial habitats such as tires and flower pots, whereas the common rural malaria vector, An. arabiensis preferred natural sites like brick pits and rice fields. An. coustani, often considered a livestock-associated rural vector, was found to thrive in brick pits, wells, ponds, and in aquatic agriculture. Notably, of the 1,270 Anopheles larvae collected, none were found in an artificial container. Culex quinquefasciatus, Cx. pipiens, and Ae. albopictus reached peak abundance in March during the rainy season, while malaria vectors An. arabiensis and An. coustani showed unexpected abundance peaks during the dry periods, in September and April. Two districts (arrondissements 2 and 5) showed higher abundance and diversity of mosquito vectors than the other districts. The data presented here reveal the spatiotemporal dynamics of malaria and arboviral mosquito vectors in urban Madagascar and highlight the heterogeneity of habitats across the urban ecosystem, with important practical implications for the surveillance and control of mosquito-borne diseases.