Fasciola hepatica: a study of the shedding of cercariae from Lymnaea truncatula raised under constant conditions of temperature and photoperiod.

Investigations on the shedding of cercariae of Fasciola hepatica were carried out in Lymnaea truncatula in order to verify the existence of a low-frequency periodicity in the numerical distribution of metacercariae per snail and per day when the snails are raised under controlled conditions. Preadult L. truncatula were thus collected in the field, individually exposed to two miracidia, and subsequently raised until their death under constant temperature (20 degrees C) and photoperiod (12 h/12 h diurnal rhythm). The 102 snails shedding parasites produced 24,325 metacercariae of which 5% were floating cysts. The daily production of cercariae was maximal during the first 30 days of the patent period, subsequently decreased until day 114, and ceased on day 124. No infradian-type rhythm was noted in the daily distribution of mean values. The snails shed their cercariae in one to 14 waves; 20.6% and 15.7% of the snails produced their parasites in four and five periods respectively. The authors suggest that the seven-day periodicity found by Audousset et al. (1989) in the daily distribution of cercariae produced by three colonies of L. truncatula raised in seminatural conditions must be attributed only to the influence of environmental factors.