VARIABILITY OF FASCIOLA INFECTIONS IN LYMNAEA-TRUNCATULA AS A FUNCTION OF SNAIL GENERATION AND SNAIL ACTIVITY

Field investigations were carried out over a 4-year period in three fa rms of the Haute-Vienne department (France) in order to determine the prevalence of Fasciola hepatica infection in the annual generations of Lymnaea truncatula. Infection rates found in March were significantly lower than those recorded in July or in September (0.8% to 2.2% compa red to 5.7% to 13.5% and 4.4% to 9.3%, respectively). They were always lower in overwintering snails than in other snail generations (summer generation in 1989, 1990 and 1991; spring and summer generations in 1 992). Experimental infections of L. truncatula by F. hepatica were per formed to evaluate the characteristics of Fasciola infection with refe rence to snail generation and snail activity. Survival of summer gener ation snails at day 30 postexposure was significantly lower in snails collected in September than in those collected in May and June. In the winter generation of L. truncatula, snail survival was significantly higher in snails collected from December to March than in those from N ovember. The prevalence of infection was significantly lower in snails collected in September (summer generation) than in those collected fr om December to March (winter generation). The duration of the prepaten t period was significantly delayed in snails collected from January to March (a mean of 54.2 to 58.2 days instead of 43.1 to 49 days in the other groups), whereas that of the patent period was significantly sho rter (5 to 9.3 days instead of 18.3 to 40.3 days, respectively). In sn ails collected from January to March, the number of metacercariae was significantly lower than that found from other snail groups (a mean of 21.5 to 42 metacercariae instead of 72.8 to 151.4 cysts, respectively ). Fasciola infections of L. truncatula originating from spring and su mmer generations were more efficient than those from the winter genera tion of snails.