Natural infections of Omphiscola glabra (Lymnaeidae) with Fasciola hepatica in central France.

As larval forms of Fasciola hepatica have periodically been detected in Omphiscola glabraafter their collection from watercress beds or from meadows since 1995, field investigations in 37 populations of O. glabra were carried from 1996 to 2002. This was done in order to determine the changes in prevalences and intensities of these natural infections with F. hepatica in relation to the type of snail habitat and the year of snail collection. Snails infected with F. hepatica were found in all samples made in swampy meadows and roadside ditches in all years. In fenced pools and walled gardens, snail infections were only found from 1998 and 1999 onwards, respectively. In the four types of habitats, the prevalences of F. hepatica infections increased slightly over time (0.8-2.1% for snails sampled in swampy meadows, for example) but this increase varied with the habitat. The mean shell heights of infected snails (6.2-7.8 mm) were similar whatever the type of habitat. The numbers of cercariae-containing rediae counted in snails sampled in swampy meadows, roadside ditches, and fenced pools significantly increased over time. Significant numerical variation between these redial burdens was also observed in relation to snail habitat. As the larval development of F. hepatica is facilitated by the presence of another trematode larval form ( Paramphistomum daubneyi), the finding of some naturally infected O. glabra in watering places known to have no contact with domestic or wild large mammals might be explained by the development of P. daubneyi in small mammals such as lagomorphs. However, a progressive adaptation of F. hepatica miracidia to O. glabra over time, which would permit the infection of snails at sizes larger than 2 mm, could not be excluded.