Delayed reinfection of Schistosoma mansoni in the Blue Nile Valley of western Ethiopia 10 years after mass chemotherapy

In the Blue Nile Valley of western Ethiopia a successful control programme against Schistosomiasis mansoni starting from 1985 was in 1989 interrupted by local guerrilla warfare. The control was based on human mass chemotherapy campaigns during the rainy season of 1985 and 1986 and a limited annual, focal molluscicidal activity where re-infection was demonstrated. In 1995 the area was revisited and selected schools in previously hyperinfected villages were examined for reinfection. The results were compared to re-calculated figures for the 5–19 year age group from previous pre-, per- and post-control surveys in the same localities. In 1995 prevalence rates in the 5–19 year age group had risen to 68% (Salba-Korka) and 63% (Sirba), which are very close to the 1985–1986 pre-control prevalences. Infection intensities, however, had not risen to the same degree. In Salba-Korka the 1986 pre-control geometric mean S. mansoni eggs per gram of faeces in the 5–19 year age group was 73, after mass treatment until 1989 it was close to 0, whereas in 1995 it was still only 27. In Sirba the result was similar. This might indicate a slower transmission rate after the control period than previously. The reported seasonality of transmission and the local use of the molluscicidal bark of the ‘Bitza’ tree might possibly have had some delaying effect.