The relationship between the size of Lymnaea truncatula naturally infected with Fasciola hepatica and the intensity and maturity of the redial infection.

Of the 2,652 Lymnaea truncatula collected from sites in Cumbria and Gwynedd during 1973-75, when the prevalence of fascioliasis in the primary host was low and declining, only 123 were infected with Fasciola hepatica. Dissection of these snails revealed that the proportion infected, the mean redial burden and the proportion of mature rediae in each snail increased with shell length. The results are compared with a similar data set acquired when the prevalence of infection in both the primary and intermediate hosts was uncharacteristically high. Although the results are qualitatively similar, there are important quantitative differences. The mean redial burden of infected snails at times of high disease prevalence was generally twice as high as that reported in the present study. It is suggested that differences in habitat microclimate could not account entirely for the observed differences in redial burden and the role of multiple miracidial infection is discussed.