LYMNAEA-TRUNCATULA, INTERMEDIATE HOST OF SOME PLAGIORCHIIDAE AND NOTOCOTYLIDAE SPECIES IN LEON, NW SPAIN

To study the distribution of Lymnaea truncatula in the Porma river bas in (Leon, NW Spain) and its helminth fauna, malacological samplings we re carried out at 66 points in the basin and the presence of these mol luscs was detected in 31. To trace the dynamics of this mollusc popula tion and the prevalence and intensity of its infection by trematodes, malacological samplings were made at fortnightly intervals over two ye ars at five locations, situated in the upper and middle regions of the river Porma basin. To confirm the identity of the larval stages found in the molluscs, second intermediate and/or definitive hosts, dependi ng on the trematode species, were experimentally infected to complete the life cycles. Two different species of Plagiorchiidae and one of No tocotylidae were identified. The infection prevalence of Plagiorchis e legans for the 6291 specimens of L. truncatula examined was 2.8% and i nfection was observed in snails collected in 7 of the 31 sample locali ties. This parasite was found in all months of the year, with the high est prevalence observed in July and October. When the corrected freque ncy values were considered, a slightly positive relationship was obser ved between the infection prevalence and the latter months. The highes t percentages of snails harbouring immature sporocysts were detected i n March and June-July, while the highest percentages with sporocysts c ontaining mature cercariae were observed in spring and at the end of s ummer-autumn, and cercarial shedding in the latter. Of the 6291 L. tru ncatula examined 0.3% were infected by sporocysts of Opisthioglyphe ra nae. The infection was detected in only one locality, in four months o f the year, with the highest prevalence in May. Notocotylus neyrai was found in 2.6% of the 6291 mollusc specimens checked and was collected from 12 of the 31 localities. The highest prevalence was observed in October, April and July and the corrected frequency values reveal a po sitive relationship between the infection prevalence and those three m onths. The highest percentage of molluscs harbouring rediae of N. neyr ai with germinal mass was found in April and September-October. On the other hand, rediae with mature cercariae were present in almost every month, but we only observed cercarial shedding in October and March. The L. truncatula infection prevalence by P. elegans, 0. ranae and N. neyrai increased with the size of the molluscs. Double infection was o nly observed in four snails: one harboured 0. ranae and N. neyrai and three F. hepatica and P. elegans.