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Three experiments on the infection of Lymnaea fuscus with Fasciola hepatica were carried out to determine if successful infections and maturation of the parasite were dependent on the size of snails at miracidial exposure. The first experiment was performed using 1-4-mm-high snails from 2 populations of L. fuscus and 1 population of Lymnaea palustris. In these snails each subjected to a single bimiracidial exposure, the prevalence of F. hepatica infection at day 35 postexposure ranged from 20.3% to 46.2% in snails measuring 1 mm in height at exposure; it was lower in the 2-mm snails and was 0 in higher size classes. The second experiment was performed by subjecting 1- and 4-mm L. fuscus to 1, 2, and 3 bimiracidial exposures. The prevalence of F. hepatica infection at day 35 postexposure was maximum in the 1-mm snails exposed once to miracidia and decreased with increasing number of exposures. The results were negative in 4-mm snails. Cercarial shedding of F. hepatica was studied in the third experiment using 1- and 2-mm L. fuscus each subjected to a single bimiracidial exposure. The total number of cercariae released from these snails was less than 50. From these results, it can be concluded that L. fuscus showed a partial resistance to F. hepatica infection due to snail age.

Preadult Lymnaea glabra measuring 4-6 mm in height were each exposed to 1 Paramphistomum daubneyi miracidium before being exposed to 1 miracidium of Fasciola hepatica. Total prevalence of infection in the snail groups from 3 different populations ranged from 33% to 39%. In each group, snails harboring larval forms of P. daubneyi, F. hepatica, or both, were noted. If the results from the 3 snail populations are pooled, the total prevalence of snail infection was 13.6% in snails harboring only F. hepatica larvae, 13% in those harboring only P. daubneyi larvae, and 10% in those with both trematodes. Cercarial shedding was obtained from snails harboring F. hepatica larval forms; in the case of snails infected with both trematodes, the mean number of metacercariae ranged from 13 to 21 for F. hepatica, and from 8 to 14 for P. daubneyi. No infected snails were found in snail groups exposed only to 1 trematode miracidia.