The differential dispersal of nulliparous and parous Simulium damnosum.

Dissections of Simulium damnosum in the Sudansavanna zone of Cameroon indicated that nulliparous flies dispersed mainly inland away from their riverine breeding sites. Parous flies also dispersed inland to the same distances as nulliparous flies, but mainly they stayed close to, or moved along, the banks of the breeding rivers. This tendency was most marked in the older parous flied. As a result of this differenital dispersal, the numbers of S. damnosum carrying infective larvae of Onchocerca volvulus were highest on the banks of breeding rivers. This has an important bearing on the epidemiology of onchocerciasis. Tests at one place in Guinea savanna indicated a similar dispersal pattern there. In the forest zone nulliparous flies tended to stay near the rivers; and the proportions of parous and of infective flies were higher inland than on the river bank.