Showing papers by "Edmund Seto published in 2002"

Disease transmission models for public health decision making: toward an approach for designing intervention strategies for Schistosomiasis japonica.

Abstract: Mathematical models of disease transmission processes can serve as platforms for integration of diverse data, including site-specific information, for the purpose of designing strategies for minimizing transmission. A model describing the transmission of schistosomiasis is adapted to incorporate field data typically developed in disease control efforts in the mountainous regions of Sichuan Province in China, with the object of exploring the feasibility of model-based control strategies. The model is studied using computer simulation methods. Mechanistically based models of this sort typically have a large number of parameters that pose challenges in reducing parametric uncertainty to levels that will produce predictions sufficiently precise to discriminate among competing control options. We describe here an approach to parameter estimation that uses a recently developed statistical procedure called Bayesian melding to sequentially reduce parametric uncertainty as field data are accumulated over several seasons. Preliminary results of applying the approach to a historical data set in southwestern Sichuan are promising. Moreover, technologic advances using the global positioning system, remote sensing, and geographic information systems promise cost-effective improvements in the nature and quality of field data. This, in turn, suggests that the utility of the modeling approach will increase over time.

Urinary schistosomiasis in schoolchildren on Zanzibar Island (Unguja), Tanzania: a parasitological survey supplemented with questionnaires.

Abstract: The distribution of urinary schistosomiasis in schoolchildren on Zanzibar Island (Unguja) was surveyed in May 2001 to test a potential correlation with the distribution of snail species of the Bulinus africanus group and to record contemporary baseline epidemiological data. Quasi-random samples of 40 schoolchildren of mixed sexes were selected from each of 10 schools. Schistosoma haematobium infections were detected upon the basis of micro-haematuria with subsequent confirmation by microscopy examination for schistosome eggs. At the time of urine collection, each child was interviewed with a suite of 12 questions prepared as a standardized questionnaire. Total prevalence of urinary schistosomiasis (known locally as kichocho) was 12% although schistosome infections were absent in 5 schools. Schools located west of 39 °19′E and north of 6 °10′S harboured nearly all of the infections; the highest prevalence (55%) was found at Kinyasini where many B. globosus habitats occur nearby. The general level of understanding of kichocho was low (24%) and individual self-diagnosis was poor (sensitivity, 8·5%; specificity, 85%). Grouped freshwater-contact patterns of schoolchildren differed significantly between schools and correlated well with prevalence of infections within schools. Across the island the area of active transmission of S. haematobium to humans appears confined within the distribution of B. globosus. There was no epidemiological evidence to suggest any involvement of B. nasutus in local transmission, confirming previous laboratory findings. In areas where B. globosus occurs, targeted snail control should be considered, to reduce schistosome transmission.

The Use of Remote Sensing for Predictive Modeling of Schistosomiasis in China

Abstract: The development of predictive models of the spatial distribution of schistosomiasis are hampered by the existence of different regional subspecies of the Oncomelania hupensis snail that serve as intermediate hosts for the disease in China. The habitats associated with these different subspecies vary considerably, with mountainous habitats in the west and floodplain habitats in the east. Despite these challenges, con tin uing environmental change resulting from the construction of the Three Gorges Dam and global warming that threaten to increase snail habitat, as well as limited public health resources, require the ability to accurately map and prioritize areas at risk for schistosomiasis. This paper describes a series of ongoing studies that rely on remotely sensed data to predict schistosomiasis risk in two regions of China. The first study is a classification of Landsat TM image~y to identify snail habitats in mountainous regions of Sichuan Province. The accuracy of this classification was assessed in an independent field study, which revealed that seasonal flooding may have contributed to misclassification, and that the incorporation of soil maps may greatly improve classification accuracy A second study presents the use of Landsat TM and water level data to understand seasonal differences in Oncomelania hupensis habitat in the lower Yangtze River

New insights into the transmission biology of urinary schistosomiasis in Zanzibar.

Abstract: A better understanding of the transmission biology of urinary schistosomiasis in Zanzibar, Tanzania was only possible after the development of molecular DNA markers for identification of Bulinus africanus group snails, the potential intermediate hosts of Schistosoma haematobium. Hitherto, identification of natural populations of B. globosus and B. nasutus was problematic and the intermediate host status and distribution of either species remained speculative. By recourse to molecular markers, snail distribution maps could be drawn, revealing an allopatric distribution and, more importantly, leading to the discovery that B. nasutus played no role in transmission. Indeed, in Unguja the area of active transmission of S. haematobium to humans is confined within the distribution of B. globosus. This strong relationship may prove useful for predicting the distribution of urinary schistosomiasis within Zanzibar and, if snail schistosome compatibilities persist, in other areas nearby, e.g. coastal Tanzania and Kenya. The transmission biology of urinary schistosomiasis in Zanzibar is reviewed, the paper reports on ongoing malacological studies in Zanzibar and Kenya and finally closes by posing the question whether medical malacology forms an essential component associated with mass-scale chemotherapy control programmes.

Applying GIS and Remote Sensing to the Epidemiology of Schistosomiasis in Poyang Lake, Jiangxi Province, China

Abstract: Remote sensing, using Landsat TM imagery, has been used to classify snail habitat in the Poyang Lake marshlands, a vast area of high endemism for schistosomiasis in China (Jiangxi Province). Major findings of the study were: 1. RS images are useful for delineating snail habitat and differentiating snail habitat from total bovine grazing ranges. 2. RS enabled tracking yearly dynamic changes in lake area and snail habitat. 3. Dynamic environmental factors are responsible for the fact that some areas suitable for snails may not have snails one year, but have snails another year. 4. The critical factor for maintaining stable population structure is relative temporal stability in mean low water levels. 5. The TMRC snail survey method, employed twice a year, enables a robust statistical evaluation (especially analysis of variance) of changes in snail population density and patterns of infection over large areas. This snail collecting method involving repetitive random sampling and 4 m frames has resulted in reducing the adverse effects of the severe negative binomial distribution of the snails in the sampling data sets, thus enabling statistical analyses.