Mosquito larvae occurrence and habitat characterization on urban wastewater irrigated vegetable farms in the Kumasi metropolis, Ghana

The risk of mosquito borne disease transmission is dependent on water-driven ecological regimes, such as the presence and persistence of favourable breeding habitats for the development of immature mosquitoes, which definitely influences vector competence. Wastewater irrigated farming systems provide food for more than 10% of the world’s population, but also tend to create hotspots for the breeding of mosquito vectors, which could lead to malaria transmission. This study investigated the occurrences and habitat characterization of mosquito larvae on urban wastewater irrigated vegetable farms in the Kumasi metropolis. Mosquito larvae were collected weekly from different sources of irrigation water and other pools from vegetable farms in the city of Kumasi in both the wet and dry seasons. At the same time, triplicate water samples were collected for microbiological and physicochemical analysis using standard methods. The collected larvae were microscopically identified using morphological keys. Overall, 9,823 mosquito larvae were collected during the eight months study period and species identified were Anopheles, Culex and Aedes. Of these, Culex species were most abundant (57.3%), followed by Anopheles species (30.5%) and the Aedes species were the least abundant (12%). A total of 139 breeding habitats, composed of ponds, streams and furrows (including foot prints and storm drains), were observed throughout the study. Furrows were the most frequent (55.4%), followed by ponds (41%) and the streams were the least frequent (3.6%). Furrows had the highest larval densities (56.41%), followed by ponds (39.5%) and the streams had the least (4%). Furrows and streams had high species evenness, 0.614 and 0.639 respectively, whereas ponds had low species evenness (0.395). Total coliforms, phosphorus, temperature, electrical conductivity (EC) and pH were the environmental determinants that were positively associated with increasing abundance of the identified larval mosquitoes. Nitrite (p=0.291) and dissolved oxygen (p=0.001) had negative association with larval abundance; and their concentrations, nitrite (0-1.4 mg/L) and DO (0.01-12.25 mg/L), favoured larval persistence and survival. The Culex species were dominant in ponds which was rich in nitrite, and had low mean total coliform concentrations (30.33×107 MPN) and high mean E. coli concentrations (3.74×107 MPN). The Anopheles and Aedes species, on the other hand, occurred largely in furrows that had moderate mean concentrations of total coliforms (48.27×107 MPN) and E. coli (3.22×107 MPN). The mosquito larvae were abundant in the wet season than the dry season, however, the difference was significant for Culex (p=0.000), but not for Anopheles (p=0.874) and Aedes (p=0.093). There were no significant differences in the occurrence and abundance of the breeding habitats in the rainy and dry seasons (p=0.089). The high abundance of larval mosquitoes and the favourable levels of microbial and physicochemical parameters suggest that the epidemiological importance of urban irrigated vegetable farms may be the provision of increased numbers of favourable larval habitats, as well as oviposition sites for the adult female mosquitoes. There is therefore a probable high risk of malaria transmission in the city of Kumasi due to the significant contribution of larval mosquitoes to the production of competent adult mosquito populations.

Source: KNUST