Faecal egg aggregation in humans infected with Schistosoma japonicum in China

In this study we examine the variation in Schistosoma japonicum egg counts caused by differences in worm loads between individuals and the variability of egg counts within individuals with a given worm load. Six villages were selected from an area moderately endemic for Asian schistosomiasis in the Dongting Lake region (Hunan province), China. From a total population of 3451 individuals (53.8% male; x = 31 years), 163 subjects were identified as consistently stool egg-positive based on three successive positive Kato-Katz (KK) smears (41.67 mg/smear) obtained from one stool specimen. The distribution of eggs among individuals was found to be strongly aggregated (k = 0.27), but the distribution within the three smears was found to be only slightly aggregated (k = 2.59), indicating only minor clustering of eggs in stools. The relatively slight clustering of eggs within stool specimens suggests that a single KK smear may be quite adequate for detecting individuals moderately to heavily infected (> 100 eggs/g stool (epg)), as needed for a strategy of morbidity control. However, for estimating the true prevalence of infection in a community, or for obtaining an accurate estimate of egg excretion for research studies, multiple KK smears are warranted.