The infectivity of Apatemon (Australapatemon) minor cercariae to 7 species of freshwater leech was examined under laboratory conditions. The leech species exposed to infection were; Erpobdella octoculata, Helobdella stagnalis, Glossiphonia heteroclita, Glossiphonia complanata, Hemiclepsis marginata, Piscicola geometra, and Theromyzon tessulatum. Five species were capable of acting as second intermediate hosts. Despite this broad specificity only Erpobdella octoculata and Helobdella stagnalis could be considered to show a high degree of compatibility with the parasite. In these 2 leech species more than 38% of the cercariae successfully penetrated and established metacercarial cyst infections. The leeches Piscicola geometra and Theromyzon tessulatum did not become infected. Close parallels were found between the species order of host utilization determined experimentally and that revealed by a range of field surveys in Britain, Eastern Europe and Russia. This suggests that the order of second intermediate host utilization in the natural environment has, at least in part, a physiological basis. The degree to which different leeches are utilized as hosts may relate to the opportunity which they afford for transmission of A. minor to wildfowl definitive hosts.

During co-infection of Biomphalaria glabrata with Schistosoma mansoni and Echinostoma liei, S. mansoni cercariae released before the complete resorption of S. mansoni sporocysts show a very strong decrease of their infectivity in mice. Under conditions of high interspecific competition (i.e. when the snails are infected by E. liei 8 days after infection by S. mansoni), the mean overall worm return is five times lower than that of the control experiment. A marked decrease of the infectivity of cercariae is also noted when snails, infected exclusively with either sporocysts of S. mansoni or rediae of E. liei, are associated in the same tank.