Environmental effects on parasitic disease transmission exemplified by schistosomiasis in western China

doi:10.1073/pnas.0701878104

Environmental effects on parasitic disease transmission exemplified by schistosomiasis in western China

*College of Public Health, Ohio State University, Columbus, OH 43210;
^†School of Public Health, University of California, Berkeley, CA 94720;
Institutes of ^‡Parasitic Disease and
^§Public Health Information, Sichuan Center for Disease Control and Prevention, Chengdu, Sichuan 610041, China; and
^¶Department of Microbiology and Tropical Medicine, George Washington University, Washington, DC 20037

Communicated by Kirk R. Smith, University of California, Berkeley, CA, March 11, 2007 (received for review June 21, 2006)

Abstract

Environmental effects on the transmission of many parasitic diseases are well recognized, but the role of specific factors like climate and agricultural practices in modulating transmission is seldom characterized quantitatively. Based on studies of Schistosoma japonicum transmission in irrigated agricultural environments in western China, a mathematical model was used to quantify environmental impacts on transmission intensity. The model was calibrated by using field data from intervention studies in three villages and simulated to predict the effects of alternative control options. Both the results of these interventions and earlier epidemiological findings confirm the central role of environmental factors, particularly those relating to snail habitat and agricultural and sanitation practices. Moreover, the findings indicate the inadequacy of current niclosamide-praziquantel strategies alone to achieve sustainable interruption of transmission in some endemic areas. More generally, the analysis suggests a village-specific index of transmission potential and how this potential is modulated by time-varying factors, including climatological variables, seasonal water-contact patterns, and irrigation practices. These time-variable factors, a village's internal potential, and its connectedness to its neighbors provide a framework for evaluating the likelihood of sustained schistosomiasis transmission and suggest an approach to quantifying the role of environmental factors for other parasitic diseases.

Footnotes

^‖To whom correspondence should be addressed. E-mail: spear{at}berkeley.edu
Author contributions: S.L., E.Y.W.S., A.H., G.M.D., and R.C.S. designed research; S.L., E.Y.W.S., J.V.R., B.Z., C.Y., X.G., D.Q., and R.C.S. performed research; S.L., E.Y.W.S., J.V.R., B.Z., C.Y., A.H., and R.C.S. analyzed data; and S.L., E.Y.W.S., J.V.R., and R.C.S. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0701878104/DC1.
Freely available online through the PNAS open access option.