PARAMPHISTOMUM DAUBNEYI AND FASCIOLA-HEPATICA - THE REDIAL BURDEN ANDCERCARIAL SHEDDING IN LYMNAEA-TRUNCATULA SUBJECTED TO SUCCESSIVE UNIMIRACIDIAL CROSS-EXPOSURES

The development of redial burden and cercarial shedding were studied i n two groups of Lymnaea truncatula subjected to successive cross-expos ures to one miracidium of Paramphistomum daubneyi and one of Fasciola hepatica per snail, or vice versa. The results were compared with thos e obtained in controls subjected to two unimiracidial exposures to the same trematode species. The infection rate was 61% in the group cross -exposed to P. daubneyi/F. hepatica and 37% in that cross-exposed to F . hepatica/P. daubneyi; it was 37% in the control group exposed to F. hepatica and 21% in that exposed to P. daubneyi. Snails harboring larv al forms of both trematodes were few in number in cross-exposed groups and the redial burden was low, with one trematode dominating over the other. Free cercariae of F. hepatica and those of P. daubneyi were si gnificantly more numerous at day 35 in the group cross-exposed to P. d aubneyi/F. hepatica than in the controls or the other cross-exposed gr oup. Mixed cercarial sheddings were obtained from 40% of snails with e mission in the group cross-exposed to P. daubneyi/F. hepatica and from 21% of those in the F. hepatica/P. daubneyi group. The numbers of P. daubneyi metacercariae were significantly greater in the group cross-e xposed to P. daubneyi/F. hepatica than in the other cross-exposed grou p, whereas no significant difference in mean numbers was noted for the F. hepatica cysts. Repartition of metacercariae over the patent perio d was clearly more irregular for P. daubneyi than for the other tremat ode.