Variability of Fasciola hepatica infection in Lymnaea ovata in relation tosnail population and snail age

Bimiracidial infections of Lymnaea ovata with Fasciola hepatica were perfor med under laboratory conditions to determine the susceptibility of snails f rom six French populations to trematode infection. In five populations of L . ovata the prevalence of infection in the 1-mm groups ranged between 2.7% and 43.7% at day 35 postexposure; it decreased in the 2-mm snails and was z ero in larger groups. In the snails from Thenay (periodically polluted broo k) the prevalence of F. hepatica infection decreased from the l-mna group t o the 8-mm group (from 23.9% to 1.0%) and was zero in the 10-mm L. ovata. T he total number of cercariae shed per snail was 18.3 in the 1-mm group, inc reasing to 117 in the 8-mm group. The latter findings could be interpreted as a consequence of periodic pollution in the brook of Thenay; pollution mi ght disrupt the defense system of L. ovata and facilitate the subsequent la rval development of F. hepatica.