A GEOGRAPHIC INFORMATION-SYSTEM FORECAST MODEL FOR STRATEGIC CONTROL OF FASCIOLIASIS IN ETHIOPIA

A geographic information system (GIS) forecast model based on moisture and thermal regime was developed to assess the risk of Fasciola hepat ica, a temperate species, and its tropical counterpart, Fasciola gigan tica, in Ethiopia. Agroecological map zones and corresponding environm ental features that control the distribution and abundance of the dise ase and its snail intermediate hosts were imported from the Food and A griculture Organization (FAO) Crop Production System Zones (CPSZ) data base on east Africa and used to construct a GIS using ATLAS GIS 3.0 so ftware. Base temperatures of 10 degrees C and 16 degrees C were used f or F. hepatica and F: gigantica, respectively, to calculate growing de gree days in a previously developed climate forecast system that was m odified to allow use of monthly climate data values. The model was val idated by comparison of risk indices and environmental features to ava ilable survey data on fasciolosis, Monthly Fasciola risk indices of fo ur climatic regions in Ethiopia were used to project infection transmi ssion patterns under varying climatic conditions and strategic chemoth erapeutic fasciolosis control schemes. Varying degrees of F. hepatica risk occurred in most parts of the country and distinct regional F: he patica transmission patterns could be identified, Ln the humid west, c ercariae-shedding was predicted to occur from May to October, In the s outh it occurred from April to May and September to October, depending on the annual abundance of rain, In the north-central and central reg ions, risk was highest during heavy summer rains and pasture contamina tion with metacercariae was predicted to occur during August-September , except in wet years, when it may start as early as July and extend u p to October, At cooler sites above altitude of 2800 m, completion of an infection cycle may require more than a year. Fasciola gigantica ri sk was present in the western, southern and north-central regions of t he country at altitudes of 1440-2560 m. However, a transmission cycle could be completed in a single year only at elevations below 1700 m. T he greatest risk of F. gigantica infection was in the humid western re gion. Regional strategic chemotherapy schemes of two or three treatmen ts per year were developed. Results suggest that the model can be extr apolated to ail CPSZ in the country and adapted for use in control of other vector-borne diseases of economic and public health importance. (C) 1998 Elsevier Science B.V.