Visceral pathology and size of the host snail: comparative studies inLymnaea glabra infected byFasciola hepatica

Abstract

Some tissue lesions have been described in adult Lymnaea truncatula infected by Fasciola hepatica (Rondelaud and Barthe 1983). This pathology also occurs, with some differences, in the juveniles of two other lymnaeid species (L. glabra, L. palustris) exposed to fasciolid miracidia in the 2 h following snail hatching (Bouix-Busson et al. 1985 a, b; Sindou et al. 1986). However, the influence of snail size on the frequency of tissue lesions has not yet been tested: is there a relationship between the shell height of the snail at miracidial exposure and the frequency of certain tissue lesions? The aim of the present study was to assess the validity of this hypothesis by an experiment with L. glabra. Newborn (height, 0.5 mm) and older snails (height, 1, 2, and 5-6 ram) were collected in a snailbreeding set under laboratory conditions. In all, 160 molluscs (40 for each snail size) served as controls. The other snails were individually exposed for 4 h to a single miracidium of F. hepatica; they were reared at 20~ in standard containers until Reprint requests to: P. Sindou day 30 or 45 postexposure. The surviving snails were killed by dipping them in the Bouin fixative and immediately breaking the shell; 5-gm serial sections were made and stained in Harris' hematoxylin-modified Gabe's trichrome. Methods for breeding snails and exposing them to miracidia have previously been described (Bouix-Busson et al. 1985a). The characteristics of controls and of snails exposed to miracidia are indicated in Table 1. We distinguished snails with living parthenitae (evolutive infection) from those in which the migrations of the sporocyst could be recognized by the presence of tunnel-shaped cavities after the parasite's death (abortive infection) (Bouix-Busson et al. 1985b). Our observations concerned only the digestive gland. The structure of this organ was normal in all controls on days 30 and 45 postexposure (data not shown). The gland of infected snails showed normal development but also the existence of three types of lesion: (a) a multifocal epithelial necrosis, (b) a generalized epithelial necrosis, and (c) an epiTable 1. Distribution of surviving and infected snails in the different groups on the two dates of sacrifice