The ethiopian
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INTRODUCTIONGenerally speaking, three types of transmission ecosystems are recognized in Ethiopia. These comprise small streams, irrigation schemes, and lakes. The small streams are, by far , the most important because the majority of the endemic areas belong to this category at the moment (1,2). With the increasing need to meet the demands of a growing population, the necessity of developing irrigation schemes has long been recognized. As a result irrigation schemes have spread at a relatively fast rate. This same economic endeavour results in the creation of new habitats for the snail host of schistosome and increased human water contact. The irrigation ecology gives rise to incidences of schistosomiasis by creating factors which did not exist prior to the introduction of such ventures. Already there are adequate evidence that the disease has spread as a result of irrigation (3,15-18). This makes it axiomatic to make a sound diagnosis of the ecology and pattern of schistosoma transmission to form a basis for the development of a rational control strategy suited to a particular habitat. In realization of this, the Institute of Pathobiology of the Addis Ababa University, started a longitudinal study of schistosome transmission in three endemic localities, representing the three ecological settings. They comprise: 1. the town of Jigga in Gojjam Administrative region, northwestern Ethiopia, a highland stream, 2. Metahara sugar estate, central Ethiopia, an irrigation ecology and, 3.Lake Zway, central Ethiopia, a lake situation (Fig. 1).
small scale irrigation purposes. Intestinal schistosomiasis is long known to be endemic in the town (3). ____________________________________________ 1 Institute of Pathobiology and Dept. of Biology, 2Institute of Pathology and Dept. of Community Health; Addis Ababa University, P.O.Box 1176, Addis Ababa. Metahara Sugar Estate: an irrigation ecology. The site is situated 190km southeast of Addis Ababa and lies within the Upper Valley of the Awash River Basin of Ethiopia (Fig. 1). Metahara is located at an altitude of 1000m. The water for irrigation is taken from the Awash River (Fig. 1). It is then led into the fields through earthen canals by gravity feed. The network consists of 248.4 km of irrigation canals and 27.08 km of open main drains. The irrigation canals and drains in turn end up in numerous field ditches and smaller drains with velocities of less than 0:12 m/sec, thereby forming semi-stagnant water bodies. In addition to these, there are 18 reservoirs with a total surface area of 4.8 ha. Health service is rendered by a polyclinic located at the headquarter and satellite clinics in the villages. Schistosomiasis mansoni has now emerged as a serious health problem in this area (1). Zway: a lake ecology. The lake is located 160 kIn south of Addis Ababa at about 1850 m above sea level. It is the shallowest, with a maximum depth of 4m, among the Rift valley lakes and covers an area of about 14 sq.km. (Fig. 1). There are 5 islands in the lake whose inhabitants depend mainly on fishing or subsistence farming. The estimated total population of the islands is close to 1500. Zway town, with 7 ,000 inhabitants, is situated on the western shore of the lake. There are a number of other villages located on the shores of the lake with population sizes ranging from 500 to 2000. There are intense and varying human water contact activities including recreational activities, fishing, irrigation, laundry , water fetching etc. Intestinal schistosomiasis is long known to be endemic in the area (1,3,15,18). METHODS1.Site Selection: After a reconnaissance tour by a group of experts in 1983, three sites were selected for the purpose of studying the transmission characteristics of Schistosoma mansoni in streams, irrigation schemes and lake ecological settings in Ethiopia. Selection of snail monitoring stations and parasitological examination were conducted in each area at about the same time. Results of snail studies and other concurrent studies such as socio economy, water contact behaviour and morbidity will be presented elsewhere. 2.Population Census and Parasitological Examination: In each study site all houses were mapped and numbered. The inhabitants were registered by household, age, sex, occupation, religion, education, place of birth and duration of stay in the area. Systematic random samples of households were then taken and, wherever possible, all the 
Table 1: Population age structure, prevalence (%), intensity of infection and relative index of potential contamination (RIPC) in the three ecological types.
inhabitants of the selected households were parasitologically examined for S. mansoni infection. The population examined comprised 20% in Jigga (stream ecology), 8% in Metahara (irrigation scheme), and 20-50% in and around Lake Zway. In all cases, double slides were prepared from specimens of each individual using the Kato's thick smear techniques (19) and average egg counts were recorded for positives. 3 .Statistical analysis: Age-specific prevalence rates, geometric mean egg counts per gram of faeces (EPG) and relative index of potential contamination (RIPC) (see Table I) were calculated for each ecological setting and their variations tested for significance using z-test.
people aged 15 to 34 years in Metahara sugar plantation is perhaps related to concentration of work force in search of job opportunities in the plantation. The age specific population examined, percent positive, intensity of infection in terms of geometric mean egg count per gram of faeces and the relative index of potential contamination versus the base population are shown in Table 1 and Fig.2. Comparing the three ecological types, the lake ecology shows the highest human prevalence (58.1% ) and intensity of infection (450 EPG). The stream ecology was second in human prevalence (41 %) but lightest in terms of intensity (172 EPG) whereas the irrigation scheme showed lowest human prevalence (20.0% ) but moderate intensity (218 EPG).
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