Habte Tekie

The impact of a small-scale irrigation scheme on malaria transmission in Ziway area, Central Ethiopia

Objective  To assess the impact of a small‐scale irrigation scheme in Ziway area, a semi‐arid area in the Central Ethiopian Rift Valley, on malaria transmission.

Increased malaria transmission around irrigation schemes in Ethiopia and the potential of canal water management for malaria vector control

BackgroundIrrigation schemes have been blamed for the increase in malaria in many parts of sub-Saharan Africa. However, proper water management could help mitigate malaria around irrigation schemes in this region. This study investigates the link between irrigation and malaria in Central Ethiopia.MethodsLarval and adult mosquitoes were collected fortnightly between November 2009 and October 2010 from two irrigated and two non-irrigated (control) villages in the Ziway area, Central Ethiopia. Daily canal water releases were recorded during the study period and bi-weekly correlation analysis was done to determine relationships between canal water releases and larval/adult vector densities. Blood meal sources (bovine vs human) and malaria sporozoite infection were tested using enzyme-linked immunosorbent assay (ELISA). Monthly malaria data were also collected from central health centre of the study villages.ResultsMonthly malaria incidence was over six-fold higher in the irrigated villages than the non-irrigated villages. The number of anopheline breeding habitats was 3.6 times higher in the irrigated villages than the non-irrigated villages and the most common Anopheles mosquito breeding habitats were waterlogged field puddles, leakage pools from irrigation canals and poorly functioning irrigation canals. Larval and adult anopheline densities were seven- and nine-fold higher in the irrigated villages than in the non-irrigated villages, respectively, during the study period. Anopheles arabiensis was the predominant species in the study area. Plasmodium falciparum sporozoite rates of An. arabiensis and Anopheles pharoensis were significantly higher in the irrigated villages than the non-irrigated villages. The annual entomological inoculation rate (EIR) calculated for the irrigated and non-irrigated villages were 34.8 and 0.25 P. falciparum infective bites per person per year, respectively. A strong positive correlation was found between bi-weekly anopheline larval density and canal water releases. Similarly, there was a strong positive correlation between bi-weekly vector density and canal water releases lagged by two weeks. Furthermore, monthly malaria incidence was strongly correlated with monthly vector density lagged by a month in the irrigated villages.ConclusionThe present study revealed that the irrigation schemes resulted in intensified malaria transmission due to poor canal water management. Proper canal water management could reduce vector abundance and malaria transmission in the irrigated villages.

Fresh, dried or smoked? repellent properties of volatiles emitted from ethnomedicinal plant leaves against malaria and yellow fever vectors in Ethiopia

BackgroundIn the search for plant-based mosquito repellents, volatile emanations were investigated from five plant species, Corymbia citriodora, Ocimum suave, Ocimum lamiifolium, Olea europaea and Ostostegia integrifolia, traditionally used in Ethiopia as protection against mosquitoes.MethodsThe behaviour of two mosquitoes, the malaria vector Anopheles arabiensis and the arbovirus vector Aedes aegypti, was assessed towards volatiles collected from the headspace of fresh and dried leaves, and the smoke from burning the dried leaves in a two-choice landing bioassay and in the background of human odour.ResultsVolatile extracts from the smoke of burning dried leaves were found to be more repellent than those from fresh leaves, which in turn were more repellent to mosquitoes than volatiles from dried leaves. Of all smoke and fresh volatile extracts, those from Co. citriodora (52-76%) and Oc. suave (58-68%) were found to be the most repellent, Os. integrifolia (29-56%) to be intermediate while Ol. europaea (23-40%) and Os. integrifolia (19-37%) were the least repellent. One volatile present in each of the fresh leaf extracts of Co. citriodora, Oc. suave and Os. integrifolia was ß-ocimene. The levels of ß-ocimene reflected the mosquito repellent activity of these three fresh leaf extracts. Female host-seeking mosquitoes responded dose-dependently to ß-ocimene, both physiologically and behaviourally, with a maximal behavioural repulsion at 14% ß-ocimene. ß-ocimene (14%) repels mosquitoes in our 6-minute landing assays comparable to the synthetic insect repellent N,N-diethyl-m-toluamide (10% DEET).ConclusionsVolatiles in the smoke of burning as well as fresh leaves of Co. citriodora and Oc. suave have significant repellent properties against host seeking An. arabiensis and Ae. aegypti mosquitoes. ß-ocimene, present in the fresh leaf headspace of Co. citriodora, Oc. suave and Os. integrifolia, is a significantly effective volatile mosquito repellent in the laboratory. In addition to its repellent properties, ß-ocimene has long approved safe for use in food and cosmetics, making this volatile an intriguing compound to pursue in further tests in the laboratory and field to validate its mosquito repellent activity and potential for use in a commercial product. Also, the landing bioassay with humanised membranes is a potentially useful repellent screening technique that does not require the exposure of humans to the vectors, however further tests in parallel with conventional techniques are advised.

Breeding Sites of Aedes aegypti: Potential Dengue Vectors in Dire Dawa, East Ethiopia.

Background and Objectives. Entomological survey was carried out from May-June to September-October 2014 to investigate the presence of dengue vectors in discarded tires and artificial water containers in houses and peridomestic areas. Methods. A cross-sectional immature stage survey was done indoors and outdoors in 301 houses. Mosquito larval sampling was conducted using pipette or dipper depending on container types. Larvae were identified morphologically and larval indices were also calculated. Results. A total of 750 containers were inspected, and of these 405 were positive for mosquito larvae. A total of 1,873 larvae were collected and morphologically identified as Aedes aegypti (n = 1580: 84.4%) and Culex (n = 293: 15.6%). The larval indices, house index, container index, and breteau index, varied from 33.3 to 86.2, from 23.2 to 73.9, and from 56.5 to 188.9, respectively. Conclusion. Aedes aegypti is breeding in a wide range of artificial containers. To control these mosquitoes, the integration of different methods should be taken into consideration.

Population dynamics and habitat preferences of Phlebotomus orientalis in extra-domestic habitats of Kafta Humera lowlands – kala azar endemic areas in Northwest Ethiopia

BackgroundKafta Humera lowlands are endemic for kala-azar (visceral leishmaniasis). These lowlands are characterized by black clay soil which is used for growing sesame, sorghum and cotton for commercial purposes.The aim of this study was to determine seasonal dynamics and habitat preferences of Phlebotomus orientalis, the vector of kala-azar, in extra-domestic habitats of Kafta Humera lowlands.MethodsCDC-light Trap [CDC-LT] and Sticky paper Trap [ST] were used to collect sand flies from different habitats before species identification by their morphological characteristics using appropriate keys. Data summarized and analyzed included: species, sex, density, habitats, type of trap used and date (month).ResultsA total of 389,207 sand flies using CDC-LT (n = 955) and ST (n = 5551) were collected from May 17, 2011 to June 6, 2012. The highest Mean Monthly Density (MMD) of P. orientalis trapped by CDC-LT was found in thickets of Acacia seyal in March (64.11 ± 75.87). The corresponding highest MMD of P. orientalis trapped by STs was found in April (58.69 ± 85.20) in agricultural field. No P. orientalis were caught in September using CDC traps and July-October using sticky traps. The overall MMD of P. orientalis trapped by CDC-LT was 15. 78 ± 28.93 (n = 320) in agricultural field, 19.37 ± 36.42 (n = 255) in thickets of A. seyal, and 3.81 ± 6.45 (n = 380) in dense mixed forest. Similar habitats in different localities did not show statistically significant difference for the MMD of P. orientalis trapped by CDC-LT (p = 0.117) and ST (p = 0.134).ConclusionAgricultural fields and thickets of A. seyal habitats, which exhibit extensive soil cracks and fissures, as opposed to dense mixed forests, serve as preferred breeding sites for P. orientalis.

Rice volatiles lure gravid malaria mosquitoes, Anopheles arabiensis

Mosquito oviposition site selection is essential for vector population dynamics and malaria epidemiology. Irrigated rice cultivations provide ideal larval habitats for malaria mosquitoes, which has resulted in increased prevalence of the malaria vector, Anopheles arabiensis, in sub-Saharan Africa. The nature and origin of the cues regulating this behaviour are only now being elucidated. We show that gravid Anopheles arabiensis are attracted and oviposit in response to the odour present in the air surrounding rice. Furthermore, we identify a synthetic rice odour blend, using electrophysiological and chemical analyses, which elicits attraction and oviposition in laboratory assays, as well as attraction of free-flying gravid mosquitoes under semi-field conditions. This research highlights the intimate link between malaria vectors and agriculture. The identified volatile cues provide important substrates for the development of novel and cost-effective control measures that target female malaria mosquitoes, irrespective of indoor or outdoor feeding and resting patterns.

Chicken volatiles repel host-seeking malaria mosquitoes

BackgroundAnopheles arabiensis is a dominant vector of malaria in sub-Saharan Africa, which feeds indoors and outdoors on human and other vertebrate hosts, making it a difficult species to control with existing control methods. Novel methods that reduce human-vector interactions are, therefore, required to improve the impact of vector control programmes. Investigating the mechanisms underlying the host discrimination process in An. arabiensis could provide valuable knowledge leading to the development of novel control technologies. In this study, a host census and blood meal analysis were conducted to determine the host selection behaviour of An. arabiensis. Since mosquitoes select and discriminate among hosts primarily using olfaction, the volatile headspace of the preferred non-human host and non-host species, were collected. Using combined gas chromatography and electroantennographic detection analysis followed by combined gas chromatography and mass spectrometry, the bioactive compounds in the headspace collections were identified. The efficiency of the identified non-host compounds to repel host-seeking malaria mosquitoes was tested under field conditions.ResultsThe host census and blood meal analyses demonstrated that An. arabiensis strongly prefers human blood when host seeking indoors, while it randomly feeds on cattle, goats and sheep when found outdoors. However, An. arabiensis avoids chickens despite their relatively high abundance, indicating that chickens are a non-host species for this vector. Eleven bioactive compounds were found in the headspace of the non-host species. Six of these were species-specific, out of which four were identified using combined gas chromatography and mass spectrometry. When tested in the field, the chicken-specific compounds, isobutyl butyrate, naphthalene, hexadecane and trans-limonene oxide, and the generic host compounds, limonene, cis-limonene oxide and β-myrcene, significantly reduced trap catches within the house compared to a negative control. A significant reduction in trap catch was also observed when suspending a caged chicken next to the trap.ConclusionsNon-host volatiles repel host-seeking An. arabiensis and thus play a significant role in host discrimination. As such, this study demonstrates that non-host volatiles can provide protection to humans at risk of mosquito-vectored diseases in combination with established control programmes.

The Influence of Dams on Malaria Transmission in Sub-Saharan Africa

The construction of dams in sub-Saharan Africa is pivotal for food security and alleviating poverty in the region. However, the unintended adverse public health implications of extending the spatial distribution of water infrastructure are poorly documented and may minimize the intended benefits of securing water supplies. This paper reviews existing studies on the influence of dams on the spatial distribution of malaria parasites and vectors in sub-Saharan Africa. Common themes emerging from the literature were that dams intensified malaria transmission in semi-arid and highland areas with unstable malaria transmission but had little or no impact in areas with perennial transmission. Differences in the impacts of dams resulted from the types and characteristics of malaria vectors and their breeding habitats in different settings of sub-Saharan Africa. A higher abundance of a less anthropophilic Anopheles arabiensis than a highly efficient vector A. gambiae explains why dams did not increase malaria in stable areas. In unstable areas where transmission is limited by availability of water bodies for vector breeding, dams generally increase malaria by providing breeding habitats for prominent malaria vector species. Integrated vector control measures that include reservoir management, coupled with conventional malaria control strategies, could optimize a reduction of the risk of malaria transmission around dams in the region.

Species composition of phlebotomine sand flies and bionomics of Phlebotomus orientalis (Diptera: Psychodidae) in an endemic focus of visceral leishmaniasis in Tahtay Adiyabo district, Northern Ethiopia

BackgroundVisceral leishmaniasis (VL) is a neglected tropical disease, which is strongly associated with poverty. VL caused by Leishmania donovani and transmitted by Phlebotomus orientalis is endemic in various remote areas of north and north-west Ethiopia. The present study was designed to determine the sand fly fauna and bionomics of P. orientalis in the VL endemic focus of Tahtay Adiyabo district.MethodsSand flies were collected using CDC light traps (n = 602), sticky traps (n = 9,350) and indoor pyrethrum spray catches (n = 578 house visits) from indoor, peri-domestic and agricultural field habitats between May 2011 to April 2012. All sand fly specimens collected were identified to species level and counted.ResultsIn total, 100,772 sand fly specimens, belonging to 25 sand fly species (nine Phlebotomus and sixteen Sergentomyia) were collected and identified. S. africana and P. orientalis made up 59.1% and 23.5% of the collected sand flies, respectively. As it could be determined from the proportion of collections from outdoor (peri-domestic and agricultural fields) and indoor locations, P. orientalis appears to exhibit increased exophilic behavior. The outdoor to indoor index was 79:1 on m2 of sticky traps. Mean density of P. orientalis caught was significantly higher on horizontally placed sticky traps (mean = 60 ± 14.56/m2/night) than vertically deployed sticky traps (12 ± 3.57/m2/night). The highest abundance of P. orientalis occurred between March and April. Through July to September, there was a sharp decline in abundance of P. orientalis population. Regarding climatic variables, P. orientalis density in light traps and on sticky traps showed a significant positive and negative association with temperature and relative humidity, respectively. However, non-significant negative correlation was observed with rainfall pattern.ConclusionsOverall, P. orientalis was found to be the most abundant Phlebotomus species, showing marked seasonal abundance that mainly peaks during the dry season (March to April). Likewise, the people in the area usually sleep in compounds during these months that potentially expose them to a high risk of peri-domestic VL transmission.

Host choice of Phlebotomus orientalis (Diptera: Psychodidae) in animal baited experiments: a field study in Tahtay Adiyabo district, northern Ethiopia

BackgroundHost choice and feeding success of sand fly vectors of visceral leishmaniasis (VL) are important factors in understanding the epidemiology and for developing efficient control strategies. The aim of the present study was to determine the host preference of Phlebotomus orientalis in the VL focus of Tahtay Adiyabo district, northern Ethiopia.MethodsTwo separate experiments were conducted testing attraction of P. orientalis to humans, domestic animals, and small wild animals. The host choice of P. orientalis and other sand fly species was assessed using tent traps baited with seven different animals (human, cow, sheep, goat, donkey, dog and chicken) and a blank control. Baited traps were rotated every night in a Latin square design for two consecutive full rounds totaling 16 trap-nights. The second set of experiments tested attraction to small wild animals including; ground squirrel (Xerus rutilus), hare (Lepus sp.), gerbil (Tatera robusta) and spiny mouse (Acomys cahirinus). Animals were caged in standard rodent traps or cylindrical wire-mesh cages. The bait animals were placed in agricultural field and the attracted sand flies were collected using unlit CDC traps for 10 trapping nights. Sand fly specimens collected from each of the experiments were identified to species level and counted.ResultsSignificant difference (P < 0.05) was observed in the attraction and feeding rate of P. orientalis to different baits. In the first experiment, cow-baited tent traps attracted the highest mean number of P. orientalis (mean = 510 flies). The engorgement rate of P. orientalis on donkey was the highest followed by cow, and much lower on goat, sheep, dog and chicken. In the case of smaller wild animals, more numbers of P. orientalis females were attracted to squirrels followed by hares, gerbils and the spiny rat. However, the engorgement rates for P. orientalis in the smaller animals were very low (1.08%) compared with larger domestic animals (30.53%).ConclusionThe tendency of female P. orientalis to engorge in large numbers on certain species of domestic as well as wild animals strongly indicated that the species is primarily zoophilic in its host preference with feeding habits that may vary depending on the availability of hosts.