Routine Kato–Katz technique underestimates the prevalence of Schistosoma japonicum : A case study in an endemic area of the People's Republic of China

Abstract There is an evidence that the Kato–Katz technique lacks sensitivity and may hence be an unsuitable method for the assessment of the ‘real infection status’ in community with low-intensity infections. In this study, six Kato–Katz thick smears (examination of two stool samples with three thick smears each) were used as the diagnostic ‘gold’ standard for estimating the prevalence of Schistosoma japonicum infection and the results were compared with results based on fewer Kato–Katz thick smear readings. A total of 1055 individuals in 2005 and 725 in 2006 from an endemic village were recruited for the study. The observed prevalence increased gradually with the number of Kato–Katz thick smears examined, and hence the rate of underestimation decreased accordingly. The prevalence based on single Kato–Katz thick smear readings was significantly lower than that obtained using five or six thick smears. The rate of underestimation based on using two and three Kato–Katz thick smears, a typical diagnostic effort in the national schistosomiasis control programme, was about 36.0% (28.4–48.9%) and 25.0% (15.9–40.7%). The number of Kato–Katz thick smears required to secure detection of a S. japonicum infection varies with the infection intensity level. Indeed, examination of a single thick smear was sufficient when the geometric mean of the fecal content of eggs per gram (EPG) was 250 or higher in infected individuals, while six Kato–Katz thick smears were required when the EPG score was lower than 10. In conclusion, our results confirm that the prevalence of S. japonicum infection in a community is generally considerably “underestimated”. Moreover, our findings provide a benchmark for the proper application of the Kato–Katz technique and the rational evaluation of the epidemic situation, as well as a scientific basis for constructing a mathematic diagnostic model.