Variations in fecal Schistosoma japonicum egg counts.

Abstract

Abstract. Variations in fecal Schistosoma japonicum egg counts were studied in ZhuXi administrative village, JiangXi Province, China. Population stool examinations were collected with duplicate, standard, 41.5-mg Kato-Katz thick smears on seven consecutive days for 570 individuals from two natural (individual) villages: village I with high endemicity and village II with low endemicity. The proportion of individuals with at least one positive count increased from 42.4% after a single measurement to 68.3% after seven measurements in village I (n ϭ 356), and from 17.0% to 36.0% in village II (n ϭ 214), respectively. This demonstrates a very high variation in repeated S. japonicum egg counts and a considerable lack of sensitivity of the Kato-Katz technique; light and moderate infections are especially missed with a single or a few measurements. The observed day-to-day variation in individual egg counts is highly aggregated (variance higher than the mean) and suggestive of a negative binomial distribution. For five individuals on three days, repeated sampling from different locations of a stool specimen shows a clear trend with egg counts decreasing from the beginning of the stool to the end and from the outside layer to the center. Ten multiple samples from a particular subsection (10-30 g) of a stool specimen for 44 positive individuals still showed aggregation in egg counts, particularly for high intensities of infection. This means that the aggregation in repeated daily S. japonicum egg counts cannot be explained alone by a specific day-to-day component and variation in the concentration of eggs at different locations in the stool. There also exists clustering of eggs within parts of the stool.Most control programs and epidemiologic studies on Schistosoma japonicum and S. mansoni infection are based on the detection of parasite eggs using the modified KatoKatz method. 1-4 Generally, levels of egg excretion are used as an index of the intensity of infection. Individual diagnosis of infection is commonly based on one stool examination. However, for S. mansoni it has been shown that a single stool examination considerably underestimates the prevalence; many light infections are especially missed. 5 Fecal egg counts as a quantitative measure of infection on S. mansoni in epidemiologic studies are the result of inter-and intraindividual variations: the former reflecting egg counts varying from individual to individual caused by differences in worm loads, and the latter reflecting egg count fluctuation for an individual with a given worm load from time to time and/or within a fecal specimen. [5][6][7][8][9] Based on a stochastic model that takes into account this inter-and intra-individual variation in egg counts, projections can be made of how many individuals are truly infected with S. mansoni in a population. 10 We expect similar mechanisms to exist for S. japonicum infection, but thus far detailed studies have not been undertaken. 11 Imperfect diagnosis of S. japonicum by routine stool examination has however been ...

Prevalence, intensity and associated morbidity of Schistosoma japonicum infection in the Dongting Lake region, China

“…Third, day-to-day and intra-specimen variations in egg output can underestimate prevalence in the case of limited stool sampling and diagnostic effort. 25 Although we examined three Kato-Katz thick smears, all were derived from a single stool specimen, producing a test-sensitivity ranging from 20-70%. 26 In previous PESs the miracidial hatching test was used to examine all individuals, while a Kato-Katz thick smear examination -three smears derived from a single stool specimen -provided intensity data on individuals who tested positive by miradicial hatching.…”

Advances in the Diagnosis of Human Schistosomiasis

“…Also, there is the possibility that eggs clumping together in stool samples can lead to a significant variability in egg counts, resulting in underreporting of infection prevalence and/or infection intensity. Accordingly, the precise disease burden in a community can be grossly underestimated with the KK procedure, and this can also lead to misinterpretation in the assessment of therapeutic outcomes (59,(62)(63)(64). Furthermore, these limitations can also lead to inaccuracies in determining the efficacy of other diagnostic techniques, as the KK test is still regarded as the gold standard or reference diagnostic method for schistosomiasis (7,46).…”

Field Evaluation of a PCR Test for Schistosoma japonicum Egg Detection in Low-Prevalence Regions of China

“…When the results of the first miracidium hatching test, based on a single stool sample, were compared with PCR, the number of samples that were PCR negative but hatching test positive decreased to four, suggesting some individuals may have been infected with S. japonicum, however eggs may not be clinically detectable in every stool sample. Variability in egg shedding, caused by day-to-day variability in stool output, clustering of eggs within stool, and inter-individual variance, [32][33][34] as well as stool sample size could explain why overall, more samples were found to be hatching test positive than PCR positive.…”