VARIABILITY OF FASCIOLA-HEPATICA L INFECTIONS BETWEEN POPULATIONS OF LYMNAEA-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. Characteristics of Fasciola infection were studied in snails originating from these colonies after their individual exposure to a single miracidium. At day 30, there was a gradual increase of mortality rate in the infected snails when the facility of encounter F. hepatica in the field became less and less frequent ; in correlation, a decrease was noted in the percentage of infected snails, and in the total burden of rediae. In populations with a small encounter frequency, there was a delay in the patent period and its duration was extended ; there was also an increase in the percentage of snails which died without shedding whereas the snails died after several waves in their cercarial production decreased in percentages ; lastly, the number of floating metacercariae was greater in these colonies and the fixed cysts were less numerous. The chronology of cercarial sheddings, however, was the same in the 17 populations, whatever the contact frequency with F. hepatica ; the floating cysts were always present at the beginning of the shedding waves. This variability in results is explained by a certain unsuitability between the parasite with the L. truncatula originating from populations whose natural encounter with F. hepatica was rare or exceptional.