EPIDEMIOLOGIC IDENTIFICATION OF CHINESE INDIVIDUALS PUTATIVELY SUSCEPTIBLE OR INSUSCEPTIBLE TO SCHISTOSOMA-JAPONICUM - A PRELUDE TO IMMUNOGENETIC STUDY OF HUMAN RESISTANCE TO ASIAN SCHISTOSOMIASIS

An epidemiological method, field-tested in Hunan, China, to identify r esidents potentially susceptible or insusceptible to endemic schistoso miasis japonica is described, as a prelude to selection of subjects fo r immunogenetic studies. After an initial cross-sectional survey on tw o islands (Qingshan and Niangashan-population 2990) in 1995-1996, an i nformative cohort (N = 249) was selected for treatment and 9-month fol low-up to measure exposure and re-infection. Both the population preva lence (15.8%) and the geometric mean intensity of infection (26.2 eggs /g faeces) indicated that the islands were moderately endemic for schi stosomiasis. Exposure measurements revealed a strong, positive, linear association (r = 0.70) between daily activity diaries and direct wate r-contact observation. Individuals identified as steel-positive for sc histosomiasis had significantly more water contact than those who were egg-negative (P = 0.03). Almost all (93%) of the cohort had ultrasono graphic evidence of periportal fibrosis before treatment but in only 1 .2% was this fibrosis scored >1 in terms of the stages identified by t he World Health Organization. At the follow-up it was possible to clas sify the 249 subjects into three, distinct, exposure-infection epidemi ological groups. The first group (N = 20) was susceptible to re-infect ion and constituted 8% of the cohort. The second group (N = 61) was ap parently insusceptible to re-infection despite the continuing high lev els of exposure and included 24% of the cohort. The other 68% of the c ohort (N = 168) remained uninfected but were at most only moderately e xposed, or had a status indeterminate due to non-compliance. This epid emiological identification of susceptibles and insusceptibles for schi stosomiasis japonica links held and ongoing laboratory studies aimed a t characterising the genetic and immunological factors associated with resistance to re-infection and/or disease.