Agro-ecosystems impact malaria prevalence: large-scale irrigation drives vector population in western Ethiopia

Abstract

Background: Development strategies in Ethiopia have largely focused on the expansion of irrigated agriculture in the last decade to reduce poverty and promote economic growth. However, such irrigation schemes can worsen the socio-economic state by aggravating the problem of mosquito-borne diseases. In this study, the effect of agro-ecosystem practices on malaria prevalence and the risk of malaria transmission by the primary vector mosquito, Anopheles arabiensis, in Ethiopia were investigated.

Methods: In three villages in western Ethiopia practising large-scale sugarcane irrigation, traditional smallholder irrigation and non-irrigated farming, cross-sectional parasitological surveys were conducted during the short rains, after the long rains and during the dry season. Entomological surveys were undertaken monthly (February 2010-January 2011) in each village using light traps, pyrethrum spray collections and artificial pit shelters.

Results: Malaria prevalence and the risk of transmission by An. arabiensis assessed by the average human biting rate, mean sporozoite rate and estimated annual entomological inoculation rate were significantly higher in the irrigated sugarcane agro-ecosystem compared to the traditionally irrigated and non-irrigated agro-ecosystems. The average human biting rate was significantly elevated by two-fold, while the mean sporozoite rate was 2.5-fold higher, and the annual entomological inoculation rate was 4.6 to 5.7-fold higher in the irrigated sugarcane compared to the traditional and non-irrigated agro-ecosystems. Active irrigation clearly affected malaria prevalence by increasing the abundance of host seeking Anopheles mosquitoes year-round and thus increasing the risk of infective bites. The year-round presence of sporozoite-infected vectors due to irrigation practices was found to strengthen the coupling between rainfall and risk of malaria transmission, both on- and off-season.

Conclusion: This study demonstrates the negative impact of large-scale irrigation expansion on malaria transmission by increasing the abundance of mosquito vectors and indicates the need for effective vector monitoring and control strategies in the implementation of irrigation projects.

Figure 1

Map of the study area. Map of the location of the study sites in Ethiopia (A) in the East Wollaga district of Sibu Sire (B) consisting of three villages, Baka-Boro (C), Machara (D) and Wama Kusaye (E), which practice traditional irrigation, no irrigation and sugarcane plantation irrigation, respectively.

Figure 2

Malaria prevalence in three agro-ecosystems practising large-scale irrigation, traditional irrigation and no irrigation, respectively. Cross-sectional surveys were undertaken in the short rainy season (May 2010), after the long rainy season (October 2010), and during the dry season (February 2011). Different lower case letters indicate the significant difference (P <0.05).

Figure 3

Monthly caught Anopheles arabiensis in three agro-ecosystems practising large-scale irrigation, traditional irrigation and no irrigation, respectively. Number of female mosquitoes caught by (A) CDC light traps, (B) pyrethrum spray sheet collections, and (C) artificial pit traps. The seasons are indicated with bars below the graphs: dry (white), short rains (gray) and long rains (black).

Figure 4

Overall monthly entomological factors that affect the risk of malaria infection by Anopheles arabiensis in three agro-ecosystems practising large-scale irrigation, traditional irrigation and no irrigation, respectively. (A) Estimated human biting rate, (B) parous rate, (C) sporozoite rate, and (D) entomological inoculation rate. The seasons are indicated with bars below the graphs: dry (white), short rains (gray) and long rains (black).