Transmission dynamics of miracidia of Schistosoma haematobium in the Volta Lake.

Schistosoma haematobium miracidia were detected in sentinel snails placed in 16 human water contact sites in the Volta Lake, each month from March 1973 to November 1977. Results showed that rates of infection were seasonal, and that infected snails were more often found in water contact sites sheltered by emergent plant growth than in exposed open beach sites with no emergent vegetation. Sentinel snail infection rates were correlated with natural snail infection rates and with epidemiological levels of schistosomiasis in village inhabitants. After two years of chemotherapy and mollusciciding, levels of disease and sentinel snail infection rates dropped in two-thirds of the villages. In the remaining villages, however, the sentinel snail infection rates were not correlated with the fall in epidemiological level, because of ecological changes in the water contact sites.It is concluded that, unless control measures are continued, the constant changes in the lake shore environment will lead to a rapid re-establishment of previous levels of disease transmission.