Fasciola gigantica: larval productivity of sheep-derived miracidia in Radix natalensis and Galba truncatula.

Experimental infections of Egyptian Radix natalensis and French Galba truncatula with miracidia of Fasciola gigantica of sheep were carried out to determine the larval productivity of this parasite. Rediae and cercariae were thus counted in snails dissected at regular intervals from day 21 to day 49 postexposure (p.e.) at 24 degrees C, while cercarial shedding was studied in other two groups of snails after day 30 p.e. At day 49, the total number of free rediae and that of cercariae-containing rediae in R. natalensis (shell height, 10.0 mm) were 71.5 and 44, respectively, whereas mean values in G. truncatula (shell height, 5.7 mm) were 57.3 and 33 rediae, respectively. The life span of cercaria-shedding snails, the prepatent period, the patent period, and the total number of cercariae shed showed insignificant differences between both snail species. Compared to the data already obtained with a cattle isolate of parasite, the number of live rediae was significantly greater in G. truncatula and significantly lower in R. natalensis when exposed to sheep-originating miracidia. In cercariae, the differences between cattle- and sheep-derived infections were insignificant, whatever snail species. The results may be explained by the existence of an interpopulation of snail infection with F. gigantica, probably due to variations in frequency of natural encounters between this snail population and the parasite isolate. However, the better production of rediae and cercariae in G. truncatula might be due to the origin of snails used for this study because allopatric snails produced more larvae than sympatric congeners when they are subjected to experimental infections.