A pilot control trial of schistosomiasis in central Liberia by mass chemotherapy of target populations, combined with focal application of molluscicide.

In an area of high transmission of Schistosoma haematobium and S. mansoni in Central Liberia, populations of five villages and intermediate host snails were surveyed for two years. In three of these villages focal application of molluscicide (niclosamide) in the main transmission sites was combined with mass chemotherapy of a target population representing 76 to 90% of the contamination index. In the two other villages, which served as control, the prevalence indexes remained stable or increased a little during the period of this study. A three dose metrifonate mass treatment was applied in one village with only S. haematobium infections. The compliance was very poor for the second and third dose but the quantity of eggs eliminated by the whole population present before and after mass treatment was reduced by 50%. No snails were found after molluscicide applications but as the incidence remained unchanged it is suspected that inhabitants have been reinfected by going to their fields. Concurrent metrifonate and niridazole mass treatment in one dose was applied in another village with only S. haematobium infections. Molluscicide applications reduced the snail population by 80% but did not affect the transmission. Prevalence indexes were almost the same before and after this intervention. In the last village, praziquantel (40 mg/kg in 1 dose) was used because both S. haematobium and S. mansoni infections were present. Molluscicide applications reduced the snail population by 99% and 87% for Bulinus globosus and Biomphalaria pfeifferi, respectively. This intervention stopped the transmission of S. haematobium for at least one year and reduced the prevalence from 21% to 4.6%. On the contrary for S. mansoni infections, the incidence remained very high (50%) and the prevalence was unchanged after one year follow-up. This could be explained by lower efficacy of praziquantel against S. mansoni (cure rate: 53%) and of molluscicide application against B. pfeifferi, which are highly susceptible to S. mansoni (infection rate 44%). The importance of migration in these villages is emphasized. Prevalence indexes were largely influenced by the arrival of newcomers who played a more important role in the maintenance of transmission after target mass chemotherapy than the infected persons excluded from this treatment. The costs per capita protected were 3.33 US $ for metrifonate, 1.53 for metrifonate and niridazole combined and 1.67 US$ for praziquantel. These figures do not include costs for parasitological examinations.(ABSTRACT TRUNCATED AT 400 WORDS)