THE DEVELOPMENT OF FASCIOLA-HEPATICA PARTHENITAE IN LYMNAEA-TRUNCATULA BY MODIFICATION OF MUELLERIUS-CAPILLARIS INFECTION

Adult and juvenile Lymnaea truncatula (height 4 and 1 mm, respectively) were subjected to a dual infection with either Fasciola hepatica and Muellerius capillaris, or with M. capillaris and F. hepatica at 4-h, 7-, 14- and 30-day intervals between the two exposures. The results were compared with those of snails which had been infected with a single parasite species. In dually infected snails, there was a high mortality at the second exposure to both parasite species, and it was strongly increased by each reinfection. The interexposure time had a significant influence on these mortality rates. Histological examination demonstrated a decreased number of rediae (50% or greater) in dually infected snails beginning with the 4-h interval in adults and the 7-day interval in juveniles. The exposure order and the time interval between exposures had a significant influence on the number of rediae: the number of parasites per snail decreased as the time interval lengthened. Delayed maturity occurred in live free rediae in snails infected by F. hepatica and M. capillaris at the 14- and 30-day intervals. In those infected by M. capillaris and F. hepatica, time to reach maturity was not modified compared to that in snails infected with a single species. Protostrongylid larvae were clearly less numerous in dually infected adult snails than in corresponding mono-infected snails. Third-stage larvae were found in the 14- and 30-day groups, regardless of exposure order and were absent in mono-infected snails.