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Am. J. Trop. Med. Hyg., 33(5), 1984, pp. 882-890
Copyright © 1984 by The American Society of Tropical Medicine and Hygiene

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Seroepidemiology of Schistosomiasis Japonica by Elisa in the Philippines

III. Selective Mass Chemotherapy with Praziquantel in a Control Program

Mariano G. Yogore, Jr.*, Robert M. Lewert* AND Bayani L. Blas{dagger}
* The University of Chicago, Department of Microbiology, 920 East 58th Street, Chicago, Illinois 60637
{dagger} The Philippine Ministry of Health, Schistosomiasis Research Control Project, Palo, Leyte, the Philippines

After a pre-control (pre-treatment) prevalence rate of schistosomiasis japonica was established by stool examination and by ELISA in about 400 school children in school year 1979, annual prevalence rates were measured in the following 3 assessment years. From 54–80% of children found infected primarily by stool examination were treated with praziquantel 3–9 months before the assessment examinations. Both stool examination and ELISA showed statistically significant reductions of prevalence from the 1st to the 3rd assessment years (average reduction of 50% measured by stool examination and average reduction of 19% measured by ELISA). These reductions were expected as a result of the biologic activity of the effective schistosomicide; they are therefore not acceptable evidence of reduced transmission levels. By decreasing human sources of snail infection, an effective schistosomicide may have the epidemiologic impact of reducing transmission; assessment prevalence rates can be relevant indices of transmission levels only if the biologic effect of the schistosomicide is dissociated from its epidemiologic impact in a control program in which selective mass chemotherapy plays a dominant role. This dissociation was accomplished in this study by measurements in an essentially untreated group of "new" pupils, i.e., those who entered the school at the beginning of each assessment year. Overall prevalence rates among the "new" pupils did not show a significant reduction until the 3rd assessment year as measured by stool examination (reduction by 36% of pre-control level); as measured by ELISA, a significant reduction (by 15–17%) was detected in the 2nd and 3rd assessment years. The youngest children, and girls more than boys, were the most sensitive indicators of change. These reductions are acceptable evidence of reduced transmission level in the locality with the reduction determined by ELISA close to reality. We believe that the results of a 1979 study in Luzon have been misinterpreted. That study has convinced health authorities in the Philippines that selective mass chemotherapy with praziquantel reduced prevalence and therefore transmission by 60–70%. The unsuitability of the assessment population in that study has drawn an overly optimistie picture in which effectiveness of the control procedure relative to transmission has been exaggerated and the "response time" shortened.

Accepted for publication March 23, 1984.






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Copyright © 1984 by the American Society of Tropical Medicine and Hygiene.