Schistosomiasis control in China : strategy of control and rapid assessment of schistosomiasis risk by remote sensing (RS)and geographic information system (GIS)

Human schistosomiasis remains one of the most prevalent parasitic infections in the tropics and subtropics. The disease currently is endemic in 76 countries and territories and continues to be a major public health concern, especially in the developing world. It is estimated that 650 million people are at risk of infection. Among the 200 million people actually infected, 120 million are symptomatic and 20 million suffer severe disease. Although morbidity control – in line with recommendations put forth by the World Health Organization – has been carried out in China for more than 20 years, it is estimated that 90 million people still live in areas where they are at risk of infection, and 820,000 people are infected with the parasite, i.e. Schistosoma japonicum. The estimated area of intermediate host snail habitats comprise 3,436 km2, concentrated in the 5 lake regions along the Yangtze River that include the provinces of Anhui, Jiangsu, Jiangxi, Hubei and Hunan. The marshlands of the Poyang Lake region represent some of the strongholds for the transmission of S. japonicum. In these settings, for example, the percentages of acute cases and intermediate host snail habitats represent 79.5% and 96.4%, respectively. With the World Bank Loan Project (WBLP) to control schistosomiasis in China, the overall prevalence of S. japonicum was significantly reduced, but in highly endemic areas the re-infection rates are high. In the first part of the present thesis, I summarize the 50-year history of China’s experience and expertise in schistosomiasis control. Particular emphasis is placed on morbidity control and achievements made by the WBLP carried out between 1992 and 2001. Reviewing this body of literature reveals that morbidity control of schistosomiasis in China has been successful, and hence this strategy will continue to form the backbone of protecting people’s health. However, total expenditures have been considerable, and with the termination of the WBLP there is concern that schistosomiasis might re-emerge. In the second part of this thesis, I describe the successful development of a novel compound model to identify the habitats of Oncomelania hupensis, the intermediate host snail of S. japonicum, and hence the identification of high-risk areas of disease transmission. There are three findings that warrant particular notion. First, visual land use classification on multi-temporal Landsat images was performed for preliminary prediction of O. hupensis habitats. Second, extraction of the normalized difference vegetation index and the tasseled cap transformation greenness index were used for improved snail habitat prediction. Third, buffer zones with distances of 600 and 1,200 m were made around the predicted snail habitats to differentiate between high (>15%), moderate (3-15%) and low risk of S. japonicum infection prevalence (< 3%). Preliminary validation of the compound model against ground-based snail surveys in the Poyang Lake region revealed that the model had an excellent predictive ability. The model therefore holds promise for rapid and inexpensive identification of high-risk areas, and can guide subsequent control interventions, such as whether mass or selective chemotherapy should be employed. The model can also be used for diseases surveillance in general and the monitoring of ecological transformations on the transmission dynamics of S. japonicum, for example in the Three Gorges Dam area.