Fasciola hepatica and Paramphistomum daubneyi: field observations on the transport and outcome of floating metacercariae in running water.

Experimental investigations in eight open drainage ditches and furrows from central France were carried out to analyse the dispersal of floating metacercariae of two digenean species by running water and to determine the outcome of larvae which settled on Nasturtium officinale (watercress). The frequencies of larvae found after their transport by water ranged from 33% to 49.7%, thus indicating that more than half of the metacercariae used in this experiment had fallen to the bottom of the water during this transport. The nature of the site (furrow, or ditch supplied by a spring) had a significant effect on the distribution of floating larvae, while the digenean species had no effect. Low percentages of metacercariae on watercress were noted in furrows (3.5-4.3% of larvae) and ditches (0.8-1.3%). When the watercress grew, most larvae that had settled on leaves and stems died but there were always several living metacercariae on this plant (0.7-1.5% of larvae for Fasciola hepatica and 0.2-0.5% for Paramphistomum daubneyi). The weak buoyancy of these floating cysts on running water limited their dispersal and, consequently, led to a real diminution of risks incurred by definitive hosts towards these metacercariae.

Metacercarial aggregation in Digenea (Fasciola hepatica and Paramphistomum daubneyi): environmental or species determinism?

Metacercarial aggregation of Fasciola hepatica and Paramphistomum daubneyi was studied under experimental conditions to determine if the formation of these aggregates was influenced by environmental factors, or it was a characteristic of trematode species. This process was studied using the confinement of infected snails on the bottom of Petri dishes (diameter, 14 cm) for 3 days. The formation of metacercarial aggregates of F. hepatica was not significantly modified by environmental factors such as intensity and duration of lighting, quality and volume of water. Metacercariae of F. hepatica were more numerous on the Petri dish walls and 63.9% of them constituted aggregates. In contrast, most metacercariae of P. daubneyi were found on the Petri dish bottoms and 78.3% of them were isolated or in groups of two metacercariae each. The mean number of metacercariae per aggregate ranged from 6.7 to 12.2 in the case of F. hepatica, and from 2.7 to 4.5 in the case of P. daubneyi. However, these mean numbers were independent of the site of cercarial attachment. The tendency of cercariae to form metacercarial aggregations was a characteristic of F. hepatica and was species determined.

Oviposition of Lymnaea truncatula infected by Fasciola hepatica under experimental conditions.

Experimental infections of Lymnaea truncatula by Fasciola hepatica (one, two, or three miracidia per snail) were carried out under laboratory conditions to analyze the oviposition of infected snails and determine the characteristics of their egg masses. In the infected snails from the three groups, egg-laying steadily decreased until week 4 postexposure and stopped afterward until the end of the experiment, except for the cercaria-shedding snails from the one-miracidium group, for which low numbers of egg masses were noted between weeks 9 and 12. In uninfected snails the number of egg masses decreased until week 4 and remained low during subsequent weeks. At weeks 11 and 12 postexposure the natality rate was 98.9% in controls, 56% in the cercaria-shedding snails from the one-miracidium group, and 59.2-68.5% in uninfected snails. In the cercaria-shedding snails from the one-miracidium group the restoration of reproduction activity after week 8 may be explained by a lower parasite burden in these snails than in those from the two- and three-miracidium groups.

Fasciola hepatica: characteristics of infection in Lymnaea truncatula in relation to the number of miracidia at exposure.

Experimental infections of Lymnaea truncatula by Fasciola hepatica were carried out in three snail populations to determine whether the number of miracidia used for each snail at exposure (1, 2, 5, 10, or 20 per snail) had any influence on the characteristics of Fasciola infection and metacercarial production. The number of miracidia had a significant influence on snail survival at day 30 postexposure and the frequency of infected L. truncatula that died without shedding (NCS snails). The frequency of NCS snails, the growth of cercaria-shedding snails throughout the experiment, the time between exposure and the first cercarial shedding, the duration of shedding, and the number of metacercariae were independent of the number of miracidia used for each snail. The highest metacercaria productivity for each miracidium was found in single-miracidium infections. Single-miracidium infections were the most effective, as the mean number of cercariae was the same as in other groups, whereas their survival rate was much higher.

Field observations concerning infection of Lymnaea palustris by Fasciola hepatica.

Fasciola hepatica miracidia were experimentally introduced into five sites colonized by Lymnaea palustris over a period of 4 or 6 years. In the first four ponds, a progressive increase in the prevalence of the spring-generation juvenile snails was observed (from 0.4 to 18.1%), with a corresponding increase in the shell height of infected snails (from 3.6 to 7.8 mm). In the fifth habitat, the pond dried in 1990 causing the prevalence to drop as compared with the initial values and to subsequently increase in 1991-1992.

The chemical and biological control of Lymnaea truncatula in natural watercress beds in the Limousin region (France).

The snails present in 17 natural watercress beds have been treated during 1 or 2 years by a control associating the use of a molluscicide (CuCl2) and the introduction of a predatory snail, Zonitoides nitidus. Lymnea truncatula was eliminated in only one year of treatment in 14 stations, and in two years in the three others. Elimination of other aquatic pulmonate snails was effected in two years for L. glabra, in 3 years for L. palustris, and in only one year for Physa acuta. Z. nitidus has disappeared from 12 watercress beds during the 4th year after treatment; in the five others, the numbers of this predator increased during 1 or 2 years and then decreased before their disappearance.

[Sensitivity of the prosobranch mollusk Potamopyrgus jenkinsi Smith to the action of metallic chlorides (ZnCl2, BaCl2, CuCl2) and to the synthetic molluscacide N-tritylmorpholine]. / Sensibilité du Mollusque prosobranche Potamopyrgus jenkinsi Smith à l'action de chlorures métalliques (ZnCl2, BaCl2, CuCl2) et d'un molluscicide de synthèse, la N-trityl-morpholine.

The toxicity of 3 metal chlorides (ZnCl2, BaCl2, CuCl2), and of N-trityl-morpholine (Triphenmorph or frescon) has been studied on the gastropod mollusc Potamopyrgus jenkinsi, in eucalcic and oligocalcic waters. The results showed the following order in the efficiency of the toxic compounds: triphenmorph greater than copper greater than barium greater than zinc. In eucalcic water, the toxicity of barium and decreased whereas the efficiency of triphenmorph and increased. The toxicity of the substances, at middle term, rose in relation to the time of the experiment. An exposure of 2 h to the toxic agents was sufficient to induce high mortality in P jenkinsi. In oligocalcic water, the juvenile snails were more sensitive than the adults to the action of metal chlorides, but they showed higher tolerance to Triphenmorph; in addition, the winter generation was more resistant than the summer one, except for barium.