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The reality of global warming and its consequent climate change seems indisputable, even if governments continue to argue about their causes and possible remedies. It would now seem prudent for Journal readers (i.e., those among us who are most familiar with the "tropical" infections) to anticipate the impact that climate change will have on disease transmission. Of special concern will be the changes that occur in climates now classified as sub-tropical or temperate. In this issue of the Journal, Xiao-Nong Zhou and others1 use advanced and well-parameterized spatial analytic techniques to map out the probable impact of climate change on schistosomiasis transmission in China.
Of particular importance is that fact that Oncomelania hupensis, the snail that transmits schistosomiasis in China, is cold tolerant and can survive in areas experiencing mean temperatures down to 0°C during the winter months. In addition, snail reproduction rates are temperature dependent, as is the rate of Schistosoma japonicum sporocyst development in infected snails. As a result, it is possible to use temperature values to predict where snail populations will thrive and where S. japonicum transmission is possible. Based on laboratory models of snail and schistosome survival at different ambient temperatures, the increasing average temperatures associated with global warming are hypothesized to increase the ranges of O. hupensis habitat. At the same time, longer seasonal periods of mean temperatures > 15.4°C (the minimum temperature needed for parasite development) mean that the "accumulated degree-days" necessary for parasite development and transmission will occur in progressively more regions than before. Ultimately, as global warming progresses, more areas currently deemed as marginal for snail and schistosome habitat are expected to become recognized transmission zones.
Using spatial modeling techniques, Zhou and others have carefully melded their snail–parasite survival models with environmental data to provide evidence-based maps projecting areas of S. japonicum transmission in China in 2030 and 2050. Their climate projections are based on their own time-series analysis of temperatures reported from 193 weather stations across China, combined with weighted interpolation (Kriging) of this spatially referenced data, to provide predictions of transmission risk.2 As a validated baseline, they accurately model areas suitable for transmission in 2000 and project habitat extensions in the future based on expected regional warming trends. The Kriging interpolation approach is particularly suitable in projects such as this where the data support is strong (i.e., consistent multi-year and multi-location recordings). Kriging also provides us information on the margin of error in its predictions, indicating where better information inputs are still needed.
The study of Zhou and others predicts that the range of S. japonicum transmission will expand broadly, both northward and westward within China, adding an estimated 784,000 km2 of threatened territory by 2050. The authors conclude that despite China’s great successes in reducing schistosomiasis prevalence in many areas, a combination of human migration, civil engineering changes to the environment, and, finally, significant climate warming can easily overwhelm their current approaches to schistosomiasis control. This well-grounded analysis raises a significant alarm and should no doubt provide a valid focus for planning future surveillance and control.
* Address correspondence to Charles H. King, Case Western Reserve University Center for Global Health and Diseases, Cleveland, OH. E-mail: chk{at}case.edu 
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