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ESTIMATING AND MODELING THE DYNAMICS OF THE INTENSITY OF INFECTION WITH SCHISTOSOMA JAPONICUM IN VILLAGERS OF LEYTE, PHILIPPINES. PART II: INTENSITY-SPECIFIC TRANSMISSION OF S. JAPONICUM. THE SCHISTOSOMIASIS TRANSMISSION AND ECOLOGY PROJECT

A dynamic model of Schistosoma japonicum transmission is presented that incorporates effects of infection intensity, age, and sex. We use four infection intensity classes to investigate the impact of ecologic changes and public health interventions on the burden of infection within communities. Age- and sex-specific infection data from three disease-endemic villages in the Philippines are used to estimate the parameters of the model. The model gives good qualitative agreement with observed fecal egg counts adjusted for the accuracy of the Kato-Katz examination. Our results suggest that differences in infection burden between villages are caused by differences in both the infection process and the recovery process in humans. We describe the potential impact of mass treatment of all humans on the numbers with high infection. Furthermore, we show that a sudden reduction in snail population size would affect high prevalence and low prevalence communities in different ways.

Received April 13, 2004. Accepted for publication December 4, 2004.

Financial support: This project was supported by the National Institutes of Health (NIH)/National Science Foundation Ecology of Infectious Diseases program (NIH grant R01 TW01582).

Authors’ addresses: Steven Riley, Department of Community Medicine, Faculty of Medicine Building, 5/F, William M. K. Mong Block, 21 Sassoon Road, Hong Kong, Telephone: +852-2819-9283, Fax: +852-2855-9528, E-mail: steven.riley{at}hku.hk. Hélène Carabin, Department of Biostatistics and Epidemiology, College of Public Health, University of Oklahoma Health Sciences Center, Room 303, 801 NE 13th Street, Oklahoma City, OK 73116. Telephone: 405-271-2229 extension 48083, Fax: 405-271-2068, E-mail: helene-carabin{at}ouhsc.edu. Clare Marshall, Department of Epidemiology and Public Health, Division of Primary Health Care and Public Health, Faculty of Medicine, Imperial College, St. Mary’s Campus, Norfolk Place, London, W2 1PG, United Kingdom, E-mail: clare.marshall{at}imperial.ac.uk. Remigio Olveda, Research Institute for Tropical Medicine, Department of Health Compound, FILIN-VEST Corporate City, Alabang, Muntinlupa City 1781, The Philippines, Telephone: +632-809-7599, Fax: +632-842-2245, E-mail: Email: r.olveda{at}ritm.gov.ph. A. Lee Willingham, International Livestock Research Institute, PO Box 30709, 00100 Nairobi, Kenya, Telephone: 254-20-422-3069 or 650-833-6660 extension 4955, Fax: 254-20-422-3001 or 650-833-6661, E-mail: a.willingham{at}cgiar.org. Stephen T. McGarvey, Institute of International Health, Brown University, 171 Meeting Street, Box G-B495, Providence, RI 02912, Telephone: 401-863-1354, Fax: 401-863-1243, E-mail: Stephen_McGarvey{at}Brown.edu.

Reprint requests: Hélène Carabin, Department of Biostatistics and Epidemiology, College of Public Health, University of Oklahoma Health Sciences Center, Room 303, 801 NE 13th Street, Oklahoma City, OK 73116.