Observations on the field transmission dynamics of Schistosoma mansoni and S. mattheei in southern Natal, South Africa.

Cercarial transmission of Schistosoma mansoni and S. mattheei was monitored in two small rivers near Durban, South Africa. The seasonal patterns recorded corresponded to those already documented for these parasites. In the case of S. mansoni, however, this was interrupted at the height of the transmission season. The reason for this was believed to be very low oxygen concentrations in the snail habitat due to unusually extensive growth of the plant Ludwigia stolonifera over the water. The failure of the spring rains, which would normally have flushed the system is seen as contributing to this phenomenon. Infection rates in the snail intermediate hosts were low (less than 10%). A preponderance of male worms of both schistosome species was noted.