IMMUNO-EPIDEMIOLOGY OF SCHISTOSOMA-MANSONI INFECTIONS IN A RECENTLY EXPOSED COMMUNITY IN SENEGAL

Schistosoma mansoni was introduced in the Senegal basin around 1988, due to man-made ecological changes. Since 1991, we investigate a recent but very intense focus, Ndombo, a village near the city of Richard Toll where the outbreak was first described. Four cohorts, each a ramdom sample (+/-400 subjects each) from this community, were examined and followed up after treatment, starting at 8 month intervals over a 2-year period. Each cohort is examined parasitologically (Kato-Katz), clinically, serologically (circulating antigen and antibody profiles); treated with praziquantel 40 mg/kg; followed up 6-10 weeks, one and two years after treatment; and monitored for water contact patterns and local snail densities. In the first cohort, the prevalence was 91%, with a mean egg count of 663 epg. Prevalences are near 100% in all age groups, but egg counts decline strongly in adults. Antigen detection in serum and urine confirmed that the egg counts genuinely reflect variations of worm burdens, not e.g of worm fecundity. This is surprising, as in this focus acquired immunity in adults should not have yet developed according to current hypothesis. The antigen detection assays (CAA/CCA) showed high sensitivity and quantitative power and promising perspectives as a research tool and possibly as a method for noninvasive diagnosis and screening in urine. Epidemiological in subsequent cohorts were high!v similar although seasonal variations were observed possibly due to transmission fluctuations. Anti-AWA and anti-SEA IgE levels increased,vith age, while IgG4 peaked in the age-group 10-19 years and correlated well with egg counts. The levels of IgE and IgG4 increased strongly between cohorts, indicating a dynamic immunological situation, but no immediate impact on infection levels. Morbidity was little specific: abdominal discomfort was reported by 61%, diarrhoea by 33% of the subjects; mild hepatomegaly was found in 16%, splenomegaly in 0.5%. No relation to egg counts was observed for any symptom. This mild morbidity may be due to the recent nature of the focus. In the first cohort, the percentage of people with negative egg counts ten week after treatment was only 18%, though egg counts declined strongly. Antigen detection confirmed these results. Praziquantel treatement provoked transient but impressive side effects (colics, vomiting, urticaria, oedema), the occurrence of which correlated with intensity of infection. Cure rates in subsequent cohorts were followed up shorter after treatment but remained low. Reinfection nevertheless appears limited. This lower drug efficacy may be due to very rapid reinfection and/or to the lack of immunity in the population, but ii also reduced susceptibility of the local parasite strain must be considered and studied.