Studies on the Spatial Ecology of Malaria and the Impact of Mass Trapping of Anopheles Mosquitoes on Malaria Transmission in Southern Ethiopia
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Date
2020-07-06
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Addis Ababa University
Abstract
The sustainability of current indoor vector control methods is challenged by the emergence
of insecticide resistance in malaria mosquito populations and the behavioural shift of
vectors, resulting in increased outdoor biting activities. As a result, there is a dire need for
novel vector control tools, which complement the existing strategies, particularly targeting
the outdoor-active mosquitoes. Effective control of mosquitoes outdoors can be achieved
through in-depth understandings of their spatial and behavioural ecology.
The effect of landscape elements on the resting site selection of the outdoor
Anopheles mosquito population was assessed. Fine-scale characterization of landscape
factors within 10 m radius from resting clay pots was conducted and their association with
the number of resting anophelines was determined. Canopy cover, distance from the
nearest focal house, and land cover type significantly influenced the aggregation of resting
mosquitoes. Canopy cover was the strongest predictor for both the number and presence of
Anopheles mosquitoes in the clay pots. Female Anopheles were most frequently found
resting in the pots placed within the banana plantations, and at sampling points that were
≥75 m from the focal house.
To identify the factors underlying hotspots for higher vector densities and malaria
incidence, and associated landscape features, monthly entomological monitoring, and fourstage
repeated seasonal malaria prevalence surveys were conducted in two rural villages in
southern Ethiopia. Moreover, characterization of the landscape features in and around
every household of the study villages was conducted. Spatial analyses using Getis-Ord Gi*
statistics were used to identify hotspots for malaria incidence, as well as malaria vector
density and associated sporozoite prevalence. The result from the regression models
revealed that household occupancy, location and housing conditions were the main
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predictors of vector density, entomological inoculation rate, and malaria incidence. The
spatial analyses revealed that statistically significant hotspots for malaria vector densities
and Plasmodium-infected individuals were identified at village edges.
The impact of mass trapping of Anopheles mosquitoes using odour-baited traps was
assessed in a controlled before-and-after study design in two rural villages of Southern
Ethiopia. Baseline monthly entomological and seasonal cross-sectional malaria prevalence
surveys were conducted in the two villages for a year. Then, mass trapping was
implemented in one of the villages immediately before the beginning of the rainy season,
while the monthly entomological monitoring and the seasonal malaria prevalence surveys
continued in both villages for nine months, throughout the long and short rainy seasons.
The impact of the mass trapping was then assessed by computing the relative reduction of
entomological indices and malaria incidence in the intervention village in a seasonal
comparison with the control village. The mass trapping resulted in a significant reduction
in the population of the primary malaria vector in the area, An. arabiensis and the
associated entomological indices (the human biting-, sporozoite-, and entomological
inoculation rates) in the intervention compared to the control village. This resulted in a
relative reduction of malaria incidence by 61 %, 44 %, and 49 % in the long rain, short
rain, and dry seasons, respectively.
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Keywords
Exophilic, Hotspot, Malaria Incidence, Mass Trapping, Relative Reduction, Anopheles