Experimental infections of two susceptible French populations of Lymnaea truncatula (Courcelles and Saint Ours) with an allopatric (FeÁ s, Morocco) or a semi-sympatric (Limoges, France) isolate of Fasciola hepatica miracidia were performed to determine the effect of allopatric miracidia on redial and cercarial production. In both populations, cercarial release was signi®cantly greater in allopatric than in semi-sympatric snails. Compared to semi-sympatric snails, the examination of allopatric snails killed from day 14 to day 35 post-exposure demonstrated (1) a signi®cant decrease after day 28 in the number of daughter rediae (R2a group) exiting from the ®rst-appearing mother redia (R1a redia), and (2) the dierentiation of numerous daughter rediae (R2b group) in the body of second-appearing mother rediae (R1b group). These experiments demonstrated that the exposure of L. truncatula to an allopatric isolate of miracidia disturbed the usual developmental pattern of redial generations and caused the formation of numerous R2b rediae. The authors hypothesized that the increase in cercarial release noted in allopatric snails would be assured by the rediae from the R2b group after their emergence from the body of R1b mother rediae.
Field investigations were performed over a 2- or 3-year period at five sites in central France to determine the prevalence of Haplometra cylindracea infection in three species of adult Lymnaea, and to compare it with that of Fasciola hepatica infection recorded in the same snails. L. glabra, L. palustris, and L. truncatula harboured Haplometra larval forms but the prevalence of this infection varied according to the site and snail species studied. The prevalences in May ranged over 0-66.9% in L. glabra, 3-7.7% in L. palustris, and 0-31.5% in L. truncatula. The mean intensity of H. cylindracea infection in May did not show any significant variation, whatever the site and species studied. The prevalence of F. hepatica infection was significantly lower in snails collected in sites known for H. cylindracea infection than in those originating from nearby meadows (0-1% instead of 1-5%). H. cylindracea could develop in some Lymnaea species and its occurrence had an impact in snails by decreasing the prevalence of F. hepatica infection.
Field and laboratory studies on 11 populations of Galba truncatula were performed to determine the survival rate of snails after aestivation, to specify the range of their shell heights, and to establish the percentage of G. truncatula which burrowed in soil when summer drying occurred. These investigations were performed using six lowland populations living in central and five highland populations, located in the Massif central (alt. 806-900 m). The survival rate of G. truncatula after flooding dried soil with water was significantly higher in the highland than in the lowland populations (69.5-80.7 % in road ditches, for example, instead of 29.3-33.0%). The mean shell heights of surviving snails and the durations of snail re-activation in water did not show any significant variation, whatever the origin of snail population. The percentages of snails buried in drying soil were higher in the highland population (13.0 % and 15.0 % of juvenile snails, for example, instead of 4.0 % and 7.7 % in lowland snails). In the lowland population only young snails buried, whereas 5.8-8.3 % of adults in the highland population were partially burrowing. The local climate of the sites studied in the Massif central, and the ability of snails to burrow into the mud when stagnant water disappeared in July, might explain the higher survival rate during aestivation. Des études sur le terrain et au laboratoire sur 11 populations de Galba truncatula vivant en plaine ou en altitude ont été réa-lisées pour déterminer le taux de survie des mollusques après l'estivation, préciser la gamme de leurs hauteurs de coquilles et établir le pourcentage de G. truncatula qui s'enterrent dans le sol lors de l'assèchement estival. Ces observations ont été effectuées en utilisant six populations de plaine, vivant dans le centre de la France (ait. 206-282 m) et cinq colonies d'altitude, situées dans le Massif central (ait. 806-900 m). La survie des limnées après l'inondation du sol desséché est significativement plus éle-vée dans les populations d'altitude que dans celles de plaine (69,5 à 80,7 % dans lesfossés de route, par exemple, au lieu de 29,3 à 33 %). La hauteur moyenne des coquilles des mollusques survivants et la durée de leur reprise d'activité dans l'eau ne présentent pas de variation significative, quelle que soit l'origine des mollusques. Les pourcentages de limnées enterrées dans le sol en dessèchement sont plus élevés en altitude (13 et 15 % des juvéniles, par exemple, au lieu de 4 et 7,7 % en plaine). Chez ces dernières populations, seuls les jeunes mollusques s'enterrent alors que 5,8 et 8,3 % des adultes en altitude s'enfouissent partiellement. Le climat local des sites étudiés et l'aptitude des mollusques à s'enterrer dans le sol avant la disparition de l'eau stagnante en juillet peuvent expliquer la survie plus élevée des limnées pendant l'estivation.
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