The transmission of onchocerciasis at a forest village in Sierra Leone. II. Man-fly contact, human activity and exposure to transmission.

In the first of these papers we reported on the biting-densities of Simulium damnosum s.l., vector of onchocerciasis, at sites in and near to a village in the forest area of Sierra Leone that was well separated from any large Simulium breeding river. It was found that biting-densities and transmission levels were higher in open farmland than at riverside sites. In this paper we examine the relative time spent by the villagers at the same five sites in and near to the village of Baoma Lungibu, and compare the time spent and activity, with known Simulium biting-densities and transmission indices. It is concluded that, in this situation, the highest risk of infection with onchocerciasis was to persons of either sex in the 20-39 age group who were engaged in farming, or travelling through open farmland, during the months of June to August at the beginning of the wet season. Activities in and close to the village, even by the riverside, where shade cover was heavy, presented little risk of infection.