From Low Transmission to the Elimination of Schistosomiasis in Irrigated Agricultural Regions of China
As the increased efforts devoted to schistosomiasis control in China have borne fruit, new challenges have emerged. In particular, we are reaching limits of detection of current diagnostic techniques used for human infection surveillance in these communities. There is an urgent need for new, sensitive tools to identify low-level infections in mammalian or snail hosts, or the presence of S. japonicum in environmental media. Environmental monitoring is particularly important to define the scale of the transmission process in the low transmission environment. Where villages are connected to neighbors via hydrological connections and by mobile human and animal hosts, a very modest external input may be adequate to re-initiate transmission. There is mounting indirect evidence that these connections are important, particularly in the low transmission environment. Hence our current objectives are:
We use tracer techniques and rudimentary flow monitoring interpreted via Geographic Infrormation Systems (GIS) -based hydrological modeling techniques to characterize inter-village hydrological transport between sites in our study area. We use remotely sensed digital elevation models to map and characterize hydrological pathways.
Landscape genetics is an emerging field integrating landscape ecology, population genetics, and spatial statistics to better understand how physical, biological, and chemical variation in the landscape shape genetic diversity and structure. We are currently applying landscape genetic tools to model the influence of landscape variables such as riparian habitat quality, topography, and hydrology on snail migration and parasite diffusion.
To estimate gene flow in both snail intermediate host and different parasite life-stage populations, we are using established panels of molecular markers (AFLP and msats, respectively).
Geographic Information Systems (GIS) and Remote Sensing (RS)
Remote sensing and GIS technology promise to aid the identification of new areas of potential snail habitat and sites with high potential for disease transmission, based on image analysis and specification of land-cover features associated with agriculture and human habitation. GIS analyses will also aid in understanding hydrological connectivity and land use changes that may be associated with social connectivity.
Water contact questionnaires, self-reported contact diaries, and direct observation are all typical methods for assessing water contact behaviors that may be associated with schistosomiasis transmission. However, such methods are problematic due to poor recall for surveys and diaries, and altered behavior for observational studies. Ultimately, we would like to know where, when and the intensity of water contact behaviors and their relationship to parasites in the environment. Global positioning system (GPS) receivers may be used for personal time-activity monitoring to assess these relationships.
Field epidemiologic studies have played an important role in all our work, whether it be parameterization and calibration of our mathematical model, understanding the spatial and temporal determinants of infection risk and re-emergence, identifying effective strategies for sustainable local control, and elucidation of the roles that parasite diffusion and social and hydrological factors play in transmission. Much of our work has environmental epidemiology, emphasizing the role that environmental factors play in allowing for and mediating transmission.