Anopheles species and malaria transmission risk in a highland area, south-central Ethiopia

Abstract

Malaria is a growing public health problem in Butajira area, a highland in south-central Ethiopia. However, the occurrence of vectors and the entomological aspects of the disease remain poorly described. This thesis describes abundance, host feeding preference, resting behaviour and entomological inoculation rates (EIRs) of Anopheles mosquitoes in low- (Hobe), mid- (Dirama) and high- (Wurib) altitude villages of the area. Housing conditions and the exposure of households to the bite of Anopheles arabiensis are also described. A larval survey and collection were undertaken in the villages following standard entomological methods to describe breeding habitats and their dynamics. Habitats were characterized and late larval instars were identified to species. Adult mosquitoes were sampled from indoors and outdoors and identified to species, and their host preferences and sporozoite infection rates were determined. From larval and adult collections 10 and nine Anopheles species, respectively, were identified. During the dry seasons, the streams serve as the main breeding habitats of Anopheles mosquitoes, including An. arabiensis. The occurrence of immature An. arabiensis was correlated positively with habitat temperature (r = 0.33, p < 0.05) and negatively with habitat depth (r = -0.56; p < 0.05). Adult An. arabiensis fed on human and cattle with a similar preference. From CDC light trap catches, the annual P. falciparum EIR for An. arabiensis was 3.7 in the first year (July 2008 -June 2009) in the low-altitude village, while in the same village, the annual P. falciparum EIR was zero in the second year (July 2009 - June 2010). The annual P. vivax EIR for An. arabiensis was 33 in the first year and 14.5 in the second. Sporozoite-positive An. arabiensis and An. pharoensis were caught inside houses closer to streams. Moreover, houses located in the low-altitude village, and in mid-altitude houses with open eaves, were associated with a high density of indoor-resting An. arabiensis. The density of An. arabiensis larvae and the densities of adult An. arabiensis and An. pharoensis, including sporozoite-positive ones, decreased with an increasing altitude starting from the low-altitude village, whereas densities of the other anophelines increased with an increase in altitude.