Do local environmental changes resulting from the construction of microdams lead to increased malaria transmission in Tigray, Ethiopia?

Abstract

To investigate the impact of microdams on local microclimate and malaria transmission and in order to develop appropriate methods for the control of this disease, a longitudinal microclimatic, entomological, and clinical study was conducted. The study took place in two topographically similar villages: Deba, near a microdam and Maisheru, 3-4km away, in Tigray, northern Ethiopia, where an extensive dam construction is in progress in epidemic prone areas. The weather was recorded in both villages, on the dam embankment and at different mosquito habitats. A community-led larval intervention study followed the impact assessment phase in the dam village. There was a 3.1 times greater prevalence of an enlarged spleen and 2-5.5 times higher incidence of falciparum malaria in children under 10 years of age in the dam village compared to the remote village, indicating a greater level of exposure experienced in the dam village. Anopheles arabiensis, which was highly anthropophilic, partially exophilic, and showed early biting peak (19:00-20:00hr), was 6.5-8 times more abundant in the dam village compared to the remote village and this was even higher during the dry season (xl6) when a second peak in density was observed in the former. This corresponded to when the fields were irrigated indicating that transmission of malaria was no longer restricted to the rainy season in the dam village compared to the remote village. Despite DDT spraying and prompt treatment of clinical cases with Fansidar at a community level, the findings demonstrated that dams have the potential to increase transmission of malaria parasites. The dam not only created abundant mosquito breeding sites throughout the year, but also resulted in significant microclimate changes; notably elevated minimum atmospheric temperature, the influence weakening with distance from the dam. The dam site was 1.5 ºC warmer than the dam village and that of the dam village was 1.3 ºC higher than the remote village and was especially pronounced (>3.5 ºC) in the cool/dry season. Mean air temperature recorded under vegetation was 1.7 ºC lower than outside and the dam provided numerous cool and humid microhabitats elsewhere by raising the water table, increasing water seepage, saturating the soil and promoting abundant growth of vegetation. Indoor temperature was relatively stable with higher minima and lower maxima than outdoors and on the average 2.8-3.4 ºC warmer than outdoors. The warming effect of the dam, together with profuse breeding sites, suitable indoor and outdoor resting microhabitats may have contributed to the extended and dramatic increased risk of malaria observed in near dam communities in the Ethiopian highlands in Tigray. Most importantly the larval intervention study, which involved minimal community participation, showed a49% relative reduction in An. arabiensis abundance in the dam village compared with the pre-intervention period. Thus, if dams could be constructed further away from settlements and if serious and sustainable community-led environmental management measures could be introduced as a package, it would be possible to mitigate the impact of microdams to an appreciable extent and calls for an integrated approach to malaria control, in fringe areas of malaria, such as the Tigray highlands, particularly nearmicrodams. Key words: Dams, microclimate, mosquito ecology, Ethiopia, Tigray, malaria, Anopheles arabiensis, highland fringes.