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Following the first recorded introduction of the Old World screwworm fly (OWS),Chrysomya bezzianaVilleneuve (Diptera: Calliphoridae), into the Mesopotamia valley in Iraq in September 1996, cases of livestock myiasis caused by OWS developed a distinctly seasonal pattern. The annual cycle of clinical OWS cases is explained here on the basis of environmental variables that affect the different life-cycle stages ofC. bezziana. This analysis suggests that low temperatures restricted pupal development during the winter, whereas the dispersal of adult flies was constrained by hot/dry summer conditions. A restricted number of OWS foci persisted throughout the year. In these foci, pupal development was fastest during the autumn months. In autumn, rapid multiplication, lasting several OWS generations, allowed subsequent adult fly dispersal across the valley floor during the winter. Hence, the monthly incidence of clinical OWS cases in livestock peaked during December–January and was lowest during July–August. In addition to temperature and humidity, vegetation cover played a role in OWS distribution. Hence the majority of OWS cases were clustered in the medium density type of vegetation[normalized difference vegetation index (NDVI) values of 0.2–0.4] along the main watercourses in the marshy Mesopotamia valley. Although sheep were the host most commonly infested byC. bezziana, local sheep density was not found to be a major factor in disease spread. Satellite imagery and the application of Geographical Information System (GIS) tools were found to be valuable in understanding the distribution of OWS in relation to vegetation and watercourses. The presence of screwworm in Iraq, at the perimeter of the intercontinental OWS distribution, may give rise to major seasonal flare-ups. [ABSTRACT FROM AUTHOR]