INTRASPECIMEN FECAL EGG COUNT VARIATION IN SCHISTOSOMA-MANSONI INFECTION

To determine the degree of intraspecimen fecal egg count variation in Schistosoma mansoni infection and its impact on commonly used parasito logic parameters obtained by single egg counts, 10 25-mg Kato-Katz sli des were prepared from each of three stool specimens collected on diff erent days in a study group of 20 infected people. Individual fecal eg g counts in these series of examinations varied considerably and this had profound consequences for the reliability of both qualitative and quantitative diagnosis. In light infections, S. mansoni eggs in stools appeared to be homogeneously mixed. However, this distribution became heterogeneous as the intensity of infection increased, indicating clu stering of eggs in stool. The cumulative egg counts in the 10 slides o f the same 20 people examined in this study were compared with those i n 14 slides prepared from seven stool samples collected on different d ays. This revealed significantly different mean egg counts for six peo ple, even after such exhaustive series of examinations. Intraspecimen variation also biased considerably some operational parameters used to determine the infection status at the group level, particularly when these were determined by the examination of single 25-mg slides. The e xamination of duplicate or multiple slides improved the intraspecimen estimates of these parameters but did not overcome day-to-day variatio n. The examination of fewer samples taken on different days proved to be more adequate than examining more slides from one stool specimen fo r the determination of precise estimates of the real infection status.