Fasciola hepatica: the development of tissue lesions and their frequencies in naturally-infected Galba truncatula.

A total of 530 Galba truncatula were collected from the oasis of Tozeur (south-western Tunisia) between February and July 1997 to detect snails with active or abortive infections with Fasciola hepatica, and determine the frequencies of their tissue lesions. Histological examination of 290 snails with active infections showed the presence of a multifocal or generalized necrosis in six viscera (albumen gland, digestive gland, gonad, intestine, kidney, and oviduct) and the absence of epithelial reconstitution. Tunnel-shaped lacunae and fibro-amoebocytic granulomas were found in the foot of all snails. Hypertrophied areas of the subepithelial muscular layer were noted in 16.7-71.4% of mantles. In the 240 snails with abortive infections, necrosis only developed in the albumen gland and the digestive gland from snails sacrificed in May and July. Contrary to this, epithelial reconstitution was observed in the digestive gland, gonad, and the albumen gland. In the mantle, the hypertrophy of the subepithelial muscular layer increased in frequency from February to July. Comparison of these frequencies using ANOVA demonstrated that the type of Fasciola infection (active, or abortive) had a significant effect on these percentages. In contrast, the type of the tissue and the period of snail collection had no significant effects. These results might be explained by the succession of two snail generations over time (overwintering snails, and spring-born generation), and probably by the fact that snail infections would be plurimiracidial and/or repeated over time in the oasis of Tozeur.