VARIABILITY OF FASCIOLA-HEPATICA L INFECT IONS BETWEEN POPULATIONS OFLYMNAEA-TRUNCATULA MULLER - INFLUENCE OF PREVIOUS NATURAL ENCOUNTER OF THE POPULATION WITH THE PARASITE

Seventeen populations of Lymnaea truncatula were selected according to their frequency of encounter with Fasciola hepatica in the field. Cha racteristics of Fasciola infection were studied in snails originating from these colonies after their individual exposure to a single miraci dium. At day 30, there was a gradual increase of mortality rate in the infected snails when the facility of encounter F. hepatica in the fie ld became less and less frequent ; in correlation, a decrease was note d in the percentage of infected snails, and in the total burden of red iae. In populations with a small encounter frequency, there was a dela y in the patent period and its duration was extended ; there was also an increase in the percentage of snails which died without shedding wh ereas the snails died after several waves in their cercarial productio n decreased in percentages ; lastly, the number of floating metacercar iae was greater in these colonies and the fixed cysts were less numero us. The chronology of cercarial sheddings, however, was the same in th e 17 populations, whatever the contact frequency with F. hepatica ; th e floating cysts were always present at the beginning of the shedding waves. This variability in results is explained by a certain unsuitabi lity between the parasite with the L. truncatula originating from popu lations whose natural encounter with F. hepatica was rare or exception al.