Larval habitat diversity, physicochemical characteristics and their effect on the larval density of malaria vectors in the city of Accra, Ghana

Background: Malaria is more prevalent in rural areas than the urban environment partly due to less availability of Anopheles breeding habitats of natural origin in urban settings. However, recent studies have reported an increase in malaria cases in urban areas, possibly due to an increase in urban irrigated vegetable farming, open sewers, and discarded containers creating mosquito breeding habitats. This study investigated the diversity and distribution of mosquito larval habitats and the impact of physicochemical characteristics on the presence and density of Anopheles gambiae sensu lato (s.l.) larvae in Accra, Ghana. 

Methods: Larval surveys and collections were conducted at 15 locations in Accra, during both the dry season (February to March) and rainy season (June to July) of 2022, using the WHO standard dipping method. These sites were divided into five categories: irrigated urban farming (IUF); Lower Socio-economic Status (LS); Middle Socio-economic Status (MS); High Socio-economic Status (HS); and, Peri-urban (PU) areas. Physicochemical parameters were measured and species identification was performed using morphological and molecular methods. 

Results: A total of 727 potential mosquito habitats were identified, with 65.34% (475/727) positive for Anopheles larvae. Drainage ditches were the most common habitat type (48.21%; 229/475). The highest abundance of An. gambiae s.l. was found in IUF sites (27.24%; 6244/22,919), especially during the rainy season (77.01%; 17,650/22,919; R² = 3.46, p = 0.000). Polluted habitats, including household effluents, had higher ammonium levels (3.4 mg/L NH₄–N) compared to unpolluted ones (1.3 mg/L NH₄–N). Other distinguishing parameters included dissolved oxygen (34% vs 52.9%), conductivity (5106 µS/cm vs 2049 µS/cm), and total dissolved solids (3181 mg/L vs 1255 mg/L). The predominant malaria vector was Anopheles coluzzii (54.4%; 368/677). Additionally, the presence of invasive Anopheles stephensi was detected in this study. 

Conclusion: Malaria vectors were seen breeding in a variety of habitats in Accra, with irrigated urban farming sites having the highest Anopheles larval abundance. Anopheles coluzzii was the dominant malaria vector. Continuous vector monitoring and control in urban settings is recommended.