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. Method Parasitological, entomological and socio-economic studies were conducted in a village with and a village without irrigation. Blood smear samples were taken from individuals during the dry and wet seasons of 2005/2006. Socio-economic data were collected from household heads and key agricultural and health informants through interviews and questionnaires. Larval and adult mosquitoes were sampled during the dry and short wet seasons of 2006. Female anopheline mosquitoes were tested by enzyme-linked immunosorbent assay for blood meal sources and sporozoite infections. Results Malaria prevalence was higher in the irrigated village (19%, P < 0.05) than the non-irrigated village (16%). In the irrigated village, malaria prevalence was higher in the dry season than in the wet season while the reverse occurred in the non-irrigated village. Households with access to irrigation had larger farm land sizes and higher incomes, but also higher prevalence of malaria. Larval and adult abundance of the malaria vectors, Anopheles arabiensis and Anopheles pharoensis, was higher in the irrigated than in the non-irrigated village throughout the study period. Furthermore, the abundance of An. pharoensis was significantly higher than that of An. arabiensis during the dry irrigated period of the year. Canal leakage pools, irrigated fields and irrigation canals were the major breeding habitats of the two vector mosquitoes. Plasmodium falciparum sporozoite infection rates of 1.18% and 0.66% were determined for An. arabiensis and An. pharoensis in the irrigated village. Peak biting activities of the vectors occurred before 22:00 h, which is a source of concern that the effectiveness of ITNs may be compromised as the mosquitoes feed on blood before people go to bed. Conclusion Irrigation schemes along the Ethiopian Rift Valley may intensify malaria by increasing the level of prevalence during the dry season. To reduce the intensity of malaria transmission in the small-scale irrigation schemes currently in operation in Ethiopia, year-round source reduction by using proper irrigation water management, coupled with health education, needs to be incorporated into the existing malaria control strategies. Objectif: Evaluer l’impact d’un régime d’irrigation à petite échelle dans la région de Ziway, une zone semi-aride dans la vallée du Rift dans le centre de l’Ethiopie, sur la transmission de la malaria. Méthode: Des études parasitologiques, entomologiques et socio-économiques ont été conduites dans un village avec et un village sans irrigation. Des frottis d’échantillons de sang ont été obtenus des personnes pendant les saisons sèches et humides de 2005/2006. Les données socio-économiques ont été recueillies auprès des chefs de ménage et des informateurs clés agricoles et de la santé au moyen d’interviews et de questionnaires. Des larves et des moustiques adultes ont étééchantillonnés au cours de la saison sèche et de la courte saison humide de 2006. Les moustiques anophèles femelles ont été testés par ELISA pour les sources d’origine du sang et pour les infections à sporozoaires. Résultats: La prévalence de la malaria était plus élevée dans le village irrigué (19,2%; P < 0,05) que dans le village non-irrigué (16,0%). Dans le village irrigué, la prévalence de la malaria était plus élevée pendant la saison sèche que pendant la saison des pluies alors que l’inverse était observé dans le village non-irrigué. Les ménages ayant accès à l’irrigation avaient des terrains agricoles de plus grandes tailles et des revenus plus élevés, mais aussi une plus forte prévalence de malaria. Une abondance de larves et d’adultes des vecteurs de la malaria, An. arabiensis et An. pharoensisétait plus élevée dans le village irrigué que dans le non-irrigué tout au long de la période étudiée. En outre, l’abondance d’An. pharoensisétait significativement plus élevée que celle d’An. arabiensis durant la période sèche irriguée de l’année. Les canaux de fuites des bassins, les champs irrigués et les canaux d’irrigation étaient les principaux habitats de reproduction des deux moustiques vecteurs. Des taux d’infection à sporozoïtes de P. falciparum de 1,18% et 0,66% ont été déterminés pour An. arabiensis et An. pharoensis dans le village irrigué. Le pic des activités de piqûre des vecteurs a eu lieu avant 22h00, ce qui est une source de préoccupation que l’efficacité des moustiquaires à insecticide pourrait être compromise car les moustiques se nourrissent de sang avant l’heure de coucher des gens. Conclusion: Les systèmes d’irrigation dans la vallée du Rift éthiopien pourraient intensifier la malaria en augmentant le niveau de prévalence au cours de la saison sèche. Afin de réduire l’intensité de la transmission de la malaria dans les petits systèmes d’irrigation à petite échelle actuellement en opération en Éthiopie, une réduction des sources tout au long de l’année par l’utilisation d’une gestion appropriée des eaux d’irrigation, associée à l’éducation sanitaire, doit être intégrée dans les stratégies existantes de lutte contre la malaria. Objetivo: Evaluar el impacto sobre la transmisión de malaria de un esquema de irrigación a pequeña escala en el área de Ziway, un área semi-árida del Gran Valle del Rift de Etiopía Central. Método: Se realizaron estudios parasitológicos, entomológicos y socio-económicos en un poblado con y un poblado sin irrigación. Durante las estaciones seca y lluviosa del 2005/2006 se tomaron muestras de sangre para láminas de los individuos del estudio. Mediante entrevistas y cuestionarios a los cabeza de familia e informantes clave agricultores y sanitarios, se recolectaron los datos socio-económicos. Se muestrearon mosquitos adultos y larvas durante las estaciones seca y lluviosa del 2006. Los mosquitos anofelinos hembra fueron testados mediante un ensayo ELISA para fuentes de comida e infecciones por esporozoitos. Resultados: La prevalencia de malaria era más alta en el poblado con irrigación (19.2%, P < 0.05) que en el no irrigado (16.0%). En el poblado con irrigación la prevalencia de malaria era más alta en la estación seca que en la lluviosa, mientras que lo contrario ocurría en el poblado no irrigado. Los hogares con acceso a irrigación tenían terrenos cultivables más grandes y mayores ingresos, pero también una mayor prevalencia de malaria. La abundancia de larvas y adultos de los vectores de malaria, An. arabiensis y An. pharoensis fue mayor en el poblado irrigado que en el no irrigado durante todo el periodo de estudio. Más aún, la abundancia de An. pharoensis era significativamente mayor que la de An. arabiensis durante el periodo seco e irrigado del año. Las piscinas de drenaje de los canales, los campos irrigados y los canales de irrigación fueron los principales hábitats de reproducción de los dos mosquitos vectores. Las tasas de infección de esporozoitos de P. falciparum de 1.18 y 0.66% se determinaron para An. arabiensis y An. pharoensis en el poblado irrigado. Los picos de actividad de mordedura de los vectores ocurría antes de las 22:00 horas, siendo una fuente de preocupación ya que la efectividad de las mosquiteras puede estar comprometida puesto que los mosquitos se alimentan de sangre antes de que las personas se vayan a la cama. Conclusión: Los esquemas de irrigación a lo largo del Valle de Rift Etiope podrían intensificar la malaria al aumentar los niveles de prevalencia durante la estación seca. Con el fin de reducir la intensidad de la transmisión de malaria en los esquemas de irrigación a pequeña escala en operación en Etiopía, se requiere incorporar a las estrategias de control de la malaria ya existentes, una reducción de las fuentes durante todo el año manejando adecuadamente la irrigación apropiada, junto con una educación sanitaria. Development of irrigation schemes is widely recognized as a key for promoting economic growth, ensuring food security and alleviating poverty in most developing countries (Lipton et al. 2003). However, past experience shows that inadequate consideration of public health aspects can seriously undermine the sustainability of such schemes (Hunter 1993; McCartney et al. 2007). Key among the potential negative impacts is the link between irrigation and malaria – a disease that affects between 300 and 500 million people each year globally and claims the lives of 1.5–2.5 million people annually (Keiser et al. 2005; WHO 2008). Especially in areas where malaria is unstable, irrigation may alter the malaria transmission pattern from seasonal to annual (Marramaa et al. 2004;Sissoko et al. 2004). However, the relationship between irrigation and malaria is not straightforward (Lindsay et al. 1991; Boudin et al. 1992; Henry et al. 2003). In some cases, the most anthropophilic malaria vector Anopheles funestus is replaced by Anopheles arabiensis that has lower vectorial capacity and thrives better in irrigated fields (Ijumba Abose et al. 1998a,b; Ye-ebiyo et al. 2000). Abene-Girmamo, the ‘irrigated village’, is situated at an altitude of 1647 m. The village was inhabited by 934 people in 2005, mainly dependent on irrigated subsistence farming during the dry season. Most families owned livestock (mainly bovine, ovine and equine), with a human to cattle ratio of 1:0.4. Irrigating farmers often stayed outdoors late, working on their field. In the rainy season, farmers practice rain-fed agriculture. Woshgulla, the ‘non-irrigated village’, is an agricultural village without irrigation, situated at an altitude of 1654 m, with a population size of 741 in 2005. The village is located 8 km away from the irrigation scheme in Abene-Girmamo. The inhabitants were dependent on subsistent rain-fed agriculture during the wet seasons and on livestock (mainly bovines and some equines) rearing. The mean human to cattle ratio in the village was 1:0.6. In both study villages, livestock spent the night either indoors in the same homesteads as the owners or outdoors in open cattle enclosures. Borrow pits for making mud bricks were commonly found at the backyards of houses. Each village had a water-harvesting pool (approximately 2 by 2 m wide and 6 m deep) covered with corrugated iron. Four different types of houses were distinguished in the area based on combinations of walls made of mud bricks or of wood plastered with mud, and roofs made of corrugated iron sheets or grass tops. The water source for irrigation in Abene-Girmamo is Lake Ziway, located 5–6 km away from the scheme. Water is pumped from the lake through three long underground pipes to the unlined elevated main irrigation canals feed that the agricultural and were the main irrigated throughout most of the year. conducted in the same study area found prevalence during the main transmission period (Abose et al. with malaria data from the Ziway an size was determined as 500 people village by using people were for the in the irrigated village and in the non-irrigated village. In the study villages, blood samples were taken by health from a of during the long rainy season and the dry season The with and blood were and a was by the of the for Addis and individuals were 2004). the socio-economic a household was as at of the was found for malaria. socio-economic was using a in the same for the after the household heads had a The on of and farm annual of agricultural or livestock and on et al. were sampled from the study for 8 each in and in the dry season and in April and May in the short rainy season of 2006. each potential habitats were using a et al. In the non-irrigated village, and no adult mosquitoes were found during the dry season, are not for et al. found peak and activities of An. An. pharoensis and An. during the period of the night 22:00 with the activities of the people in the study biting was in (Abose et al. et al. The biting of An. arabiensis be a of of as has also for northern et al. biting activities of the malaria vector are to compromise the of as most occurred before the go to higher with larger farm size in the irrigated village not lower malaria prevalence in protective such as are of The for An. arabiensis and An. pharoensis in the study the of in malaria transmission in Ziway has in of the country & et al. their The P. falciparum sporozoite of An. arabiensis in the study is to that from et al. et al. et al. but lower than the sporozoite in the area In the P. falciparum sporozoite of An. pharoensis in the dry season the vectorial role of this in malaria transmission in the irrigated of Ziway Anopheles arabiensis was with P. falciparum both in the dry and in the short rainy that this plays a role in malaria transmission throughout the year. be the of P. falciparum infection in vector mosquitoes in the dry season and also for An. pharoensis that an role in P. In development of irrigation schemes is of to increase and food security in Ethiopia, the of irrigation schemes in semi-arid areas Ziway in malaria transmission during the dry season, mosquito abundance is The also a for health that in protective and in the than to on that are no with mosquitoes biting water and for source reduction through to reduce and malaria transmission. study was by the through a on of Irrigation Development on and the and by the through the on Malaria and and Addis are to the and for to the and and for their in the and in the also the of the inhabitants of Abene-Girmamo and during the