The potential number of redial generations in echinostomatids (trematoda)

Abstract

By carrying out ‘chain-transplantations’ of rediae of Isthmiophora melis from infected to uninfected snails ( Lymnaea stagnalis) it has been shown to date that these rediae have passed through a minimum of 42, but possibly 102, successive generations. It is suggested that a high mortality due to transplantation procedures is responsible for the fact that only 11·94 per cent of transplanted rediae survive and give rise to further generations of rediae. Since rediae of different generation numbers do not differ significantly with regard to (a) their infectivity to recipient snails; (b) the time from their transplantation to the emergence of cercariae; and (c) the infectivity of nmetacercariae derived from them it is argued that the multiplication potential of daughter rediae of I. melis is unlimited. Similar results have been obtained with the rediae of Echinoparyphium aconiatum. Preliminary data for the rediae of Echinostoma revolutum suggest similar trends to the other species, although rediae of increasing generation number show a progressive decrease in their infectivity to recipient snails. It is concluded that the limiting factors for redial multiplication in the intermediate host must be the size and life span of the intermediate host.