Epidemiology, immunology and chemotherapy of Schistosoma mansoni infections in a recently exposed community in Senegal.

Schistosoma mansoni was introduced in the Richard Toll area (Senegal) around 1988, probably due to man-made ecological changes in the Senegal river basin. Since 1991, we investigate the community of Ndombo, close to Richard Toll. Four random population samples of approximately 400 subjects are surveyed, starting at 8 months intervals. Each cohort is examined parasitologically (Kato-Katz), clinically, serologically (circulating antigen and antibody profiles); treated with praziquantel 40 mg/kg; and followed up 6-12 weeks, 1 and 2 years after treatment. Water contact patterns and snail densities are longitudinally surveyed. In the first cohort, prevalence of infection was 91%, with 41% excreting over 1000 eggs per gram (epg); the mean egg count was 646 epg, individual counts up to 24,000 epg. Prevalences remained almost 100%, but egg counts declined strongly in adults, in spite of continued exposure and the supposed lack of acquired immunity. Antigen detection in serum and urine confirmed that the egg counts genuinely reflect variations of worm burdens. Serum circulating anodic antigen (CAA) provided intriguing epidemiological information on worm burdens, while circulating cathodic antigen (CCA) showed promise for non-invasive diagnosis and screening. So far, similar epidemiological results were found in subsequent cohorts, although some variations were observed, possibly due to seasonal transmission fluctuations. IgE levels increased with age, while IgG4 peaked in the age-group 10-19 years. IgE and IgG4-levels against adult worm antigen (AWA) and soluble egg antigen (SEA) increased between cohort 1 and cohort 3 in almost all age-groups. In all 3 cohorts examined so far a strong correlation between IgG4 and pre-treatment egg-load was observed. Further follow-up and analysis, and comparison with chronically infected populations will provide insight in the development of acquired immunity. Abdominal discomfort was reported by 61% and diarrhoea by 33% of the subjects in the first cohort; mild hepatomegaly was found in 16%, splenomegaly in 0.5%. There was no correlation between frequency of symptoms and egg counts. This low morbidity, in spite of intense infections, was confirmed by ultrasound, and may be due to the recent nature of the focus. In the first cohort, 82% of treated subjects still excreted eggs 12 weeks after treatment, though egg counts declined strongly. Antigen detection confirmed these results. Parasitological negativation rates in subsequent cohorts, followed up sooner after treatment, improved but remained remarkably low. The low drug efficacy may be due to very rapid reinfection (though further reinfection after one year was limited), and/or to the lack of immunity in the population. Reduced susceptibility of the local schistosome strain can not be excluded, however. Praziquantel treatment provoked impressive but transient side effects (colics, vomiting, urticaria, oedema), the frequency of which correlated with intensity of infection.