Statistical methods analyzing changes in dispersion patterns of parasites among hosts were applied to Schistosoma haematobium egg excretion data from a five-year study of a cohort of 1,400 boys in 3 villages in Upper Egypt. Despite significant changes in mean density of the parasites in the first 4 years, the degree of aggregation of S. haematobium among the cohort did not change markedly in analysis of egg count data obtained semiannually. Two other related measures of dispersion, mean crowding and patchiness, were also obtained and compared to prevalence and mean intensity, the components determining mean density of schistosomes among hosts. Anti-schistosomal chemotherapy with metrifonate in the cohort did not succeed in appreciably reducing the mean density of S. haematobium over a one-year follow-up. The dispersion pattern of S. haematobium among the study group was not markedly altered. Prevalence and mean intensity were useful indicators of changes in the degree of aggregation of S. haematobium among hosts. The inability of chemotherapy to provide a persistent decline in the mean density parameter is reflected in the rise of mean intensity and prevalence of S. haematobium 1 year post-therapy. It is suggested that analysis of quantifiable changes in dispersion patterns of S. haematobium among their hosts can be a useful aid in planning and assessment of control strategies.
Current address: Department of Parasitology, The Hebrew University-Hadassah Medical School, P.O. Box 1172, Jerusalem 91010, Israel.
Address reprint requests to: Dr. G. I. Higashi, Department of Epidemiology, School of Public Health, 109 Observatory Street, Ann Arbor, Michigan 48109.