Isthmiophora melis: Experimental reinfection of Lymnaea stagnalis by implantation of miracidia after implantation of rediae

During the natural development in snails of the intestinal fluke Isthmiophora melis there are two generations of macropharyngeate rediae followed only by generations of micropharyngeate rediae and cercariae. Macropharyngeate rediae do not appear in snails infected by implantation of micropharyngeate rediae. Snails harboring micropharyngeate rediae only were reinfected by implantation of miracidia of the same trematode strain. Development of macropharyngeate rediae after reinfection in part of the snails is proof of reinfection success, but the mean infection success of miracidia is three to five times lower when the miracidia are introduced in a snail already harboring that parasite. There is evidence that this reduction of the infection success is not due to a lack of attractiveness nor to a surface barrier of the snail but is caused by an alteration of internal conditions, possibly by a stimulated defence mechanism.